As has been described, it is fairly simple to obtain a deviation table for your boat's compass by carrying out a swing. If the deviations obtained are fairly small (say less than 50 on any heading) it may be as well to let sleeping dogs lie and merely use the known deviations to correct your courses and bearings at sea. However, it may be that your compass is badly sited in your boat through being placed near some magnetic material which is part of the boat's structure. Deviations of more than 400 have been known!
These deviations can be removed by correcting the compass.
Compass correcting is a bit of an art which needs a good depth of knowledge and practice if a perfect result is to be achieved; but there are some things that the amateur can do with a good chance of improving matters and without much risk of a debacle. This chapter is therefore addressed to those whose compass points South when it should be pointing North, and who consequently would like to do something about it. To those in happier situations, whose compasses never tell a lie, a quick shift to the next chapter is recommended.
First study the deviation table produced as a result of the swing, as this will furnish some good clues to the cause of any trouble. The table previously given in Chapter i will be used as an example for this analysis—although the deviations given in that table are not too bad.
The various groupings of errors are normally broken down into co-efficients. Do not worry about this term—it is merely a convenient way of expressing the total error due to a particular cause or group of causes.
Coefficient A
This is found by taking the sum of all the deviations found during a swing and dividing the sum by the number of headings.
CORRECTING THE COMPASS 37
Easterly deviations are considered to be plus, and Westerly minus. So if you add up all the deviations (given in the table on page 31 being used as an example) you will find that:
Easterly (positive) total +28° Westerly (negative) total —281°
Sum =3°
.Z
Therefore Coefficient A = —1 = —r° (Insignificant)
i6 32
If, however, you do come out with a significant quantity for Coefficient A, this may be caused by:—
. The Lubber's Line of the compass not being truly fore and aft. (A fairly common complaint with offset and steering compasses where there is no direct method of checking).
2. Friction in the pivot of the compass.
(Deflect the card a few degrees with a magnet. It should, when the magnet is removed, return to its original settling position—providing the boat's heading is kept steady during this procedure.)
3. Swing conducted too fast.
(Various induction effects can cause trouble if the boat is swung too fast.)
~.. Other defects (e.g. a bubble) in the compass.
(."oeJcient B
hound by taking the sum of the deviation on East and West courses, changing the sign on West, and then dividing by two. ' Thus, from the table again:
Deviation on East 30 W = —30
Deviation on West 30 E = —30 (sign changed)
Sum = —6° Therefore Coefficient B = —6 = —3°
Coefficient B is caused by the boat's FORE AND AFT PERMANENT magnetism. The diagrams in Figures 6A and 6B illustrate this.
In these examples, the boat is shown as having a `Red' bow and `Blue' stern. This is no reflection on the owner! Merely that the boat was magnetised in this sense on building, and the polarity could well be the opposite in another case. The North (Red) end of the compass needle is attracted by the Blue pole of the Boat's magnetism and repelled by the Red. Therefore, on an Easterly course the compass needle is pushed away from Magnetic North towards the West causing Westerly Deviation. On a Westerly course, the needle is pushed in the opposite direction, towards the East. On North and South courses, there is no deviating force as the boat's magnetism is overcome by the Earth's field (the compass needle senses either a gain or loss of directive force). On intermediate courses, only a proportional effect will be felt.
To correct for any Coefficient B, Permanent Magnets must be placed with their ends pointing fore and aft in opposition to the boat's field. The general rules for placing corrector magnets are given in a later paragraph.
Coefficient C
Found by taking the sum of the deviations on North and South, changing the sign on South, and dividing the sum by two. Thus:
Deviation on North 40W = —g. Deviation on South 50 E = —5
Sum = —9 Therefore Coefficient C = =g = —4-°
!I}( Coefficient C is caused by the boat's ATHWARTSHIPS PERMANENT magnetism. Its effect is very similar to that caused by Fore and Aft magnetism (Figures 6c and 6n).
Athwartship permanent magnets are placed in opposition to the boat's permanent magnetism to correct for the latter.
"gym
Found by taking the deviations on the quadrantal points, i.e. NE--SE----SW--NW, changing the signs on SE and NW. The sum is divided by four. Thus:
Deviation on NE Deviation on SE Deviation on SW Deviation on NW
Therefore Coeffici~
This is a fortunate case where there is no Coefficient D, although there was a deviation value on most of the quadrantal points (probably due to a combination of Coeff. B. & C).
The magnetic effect responsible for the errors represented by Coefficient D is slightly more difficult to explain. The Earth's magnetic field induces a field in the soft iron in the boat. As the boat alters course, the direction and polarity of the induced magnetic fields in the metal also change. Both fore & aft and athwartship components are involved, but as the overall mass of metal on the beam is usually nearer the compass, the athwart-ship effect is normally greater than the fore & aft effect. The resulting excess of one over the other has to be taken into account and corrected. Figures 6F to r illustrates these effects. You will see that the Earth's North (Blue) pole induces a field in the boat's soft iron with a Red polarity on the side nearest the North pole; also, that the compass needle is much closer to the induced athwartship `poles'.
As the deviation is caused by magnetic induction in the soft iron of the boat, it should be corrected by soft iron with similar magnetic qualities. Soft iron spheres, or sometimes boxes containing soft iron chains, are placed on either side of the compass and level with its card. The size of the spheres and their distance from the centre of the compass can be varied to give the right amount of correction. Tables are available (Admiralty Compass Department pamphlet CD 13B is one)which show the correct size of sphere to be used and their distances from the centre of the compass for various values of Goctlicient D.
l figure 6Q illustrates the correction effect of such spheres on one particular heading. The North (Red) end of the Compass iiecdle is repelled by the `Red' Port side of the ship—from induced magnetism. This repulsion is balanced out by the side c ~f the spheres nearest the compass being induced with the opposite polarity.
Heeling Error
This has no related coefficient, but it is due to the combined influence of the boat's Permanent Vertical magnetism and induced Vertical magnetism. When the compass needles are horizontal, they are not affected by the boat's vertical magnetism. However, as the boat rolls, a component of its vertical magnetism has its effect on the needle. This is not easy to illustrate graphically, but the result is a pull on the compass needle one way when the boat rolls to port, and in the opposite direction on a starboard roll. The consequence is an oscillation of the compass in rough weather making it difficult to use for steering or for taking bearings.
Against the normal principles of correcting like with like, e.g. permanent magnetism with permanent magnets, heeling error is corrected solely with permanent magnets. The amount of correction applied is only, therefore, right for one magnetic latitude as the boat's induced magnetism will vary as the latitude changes (it has not been found possible to produce a satisfactory soft iron heeling error corrector due to induction effects from the other corrector magnets). Permanent vertical magnets are therefore employed, usually contained in a bucket on an adjustable chain hung underneath the compass.
There are two ways of finding the correct number of magnets to use and their optimum distance from the compass. The first (and most accurate method) is to use a Heeling Error Instrument—briefly, this consists of a magnetised needle balanced on a knife edge which allows it to see-saw freely in the vertical plane. The North seeking end is marked with an engraved circle near its tip. On the other end (in North latitudes) one or more small, adjustable collar weights are
6° W Io W
50 E
Nil Sum
:nt D =
=
= +i° (sign changed)
— o° (sign changed normally)
= 00
0 _0°
placed capable of being slid along the needle—which is graduated in divisions. The instrument is taken ashore, hung from some non-magnetic object at least three feet clear of the ground, and swivelled until the North seeking end is pointing approximately North. The weight is then adjusted at the other end until the needle is truly horizontal (a bubble at the bottom of the instrument indicates the level). A count is then taken of the number of divisions separating the weight from the centre of the needle. The instrument is then taken back on board, the compass bowl removed, and the H.E. Instrument installed in its place, again aligned with North. On board, the earth's vertical field will probably be weaker than ashore due to some screening by the vessel's structure. To allow for this, the weight must be moved in a small amount, an average factor being o.9. For example, if the weight was balancing the needle at 16 divisions ashore, when on board it should be placed at
16 X 0.9 = 13.4 divisions. Vertical magnets are then placed in the bucket and this is moved up and down, more magnets added etc., until the needle is once again horizontal.
However, you may not have a Heeling Error Instrument in your pocket or be expecting one for Christmas. Therefore, the second less accurate but probably more attainable method is to correct the Heeling Error at sea when the boat is rolling or pitching. This is best done on East—West courses and the drill is quite simple; taking bearings of some object, change the direction (Red up or Blue up), number, or position of the vertical magnets until the compass oscillation disappears and the bearing remains steady. It is best to use a bearing as a check, for the boat's heading may be altering without it being noticed amongst the oscillations. On the other hand, if the compass concerned is a steering compass, with no azimuth circle, the only solution is to watch the ship's head carefully, and by experiment, steady it as far as possible with the magnets.
Whichever way it is done, the Heeling Error correction must be done before any other corrections are made. This is because the vertical magnets may induce magnetism in any soft iron, including the spheres, which will have an effect on the final deviatiop1fthe compass. So—if you have to adjust the Heeling Error magnets at sea because the compass is oscillating too
badly to be f much use—you may well have affected the
deviations on all courses, and a subsequent swing to find the new deviations will be necessary.
The Corrector Magnets
These can be obtained in many strengths, lengths, and diameters, and it is a question of choosing the size appropriate to the compass concerned. Some compasses are mounted on a binnacle, and the latter normally contains compartments designed to take the appropriate size of magnet. Other types of compasses (particularly those originally designed for aircraft) have corrector magnets built into the compass bowl which are moved, for adjustment, by a screw and cantilever arrangement.
However, it may be that the compass in your boat has no built-in arrangement for correction, and magnet holders must be improvised to do the job. There are certain essential points to watch in doing this:
I. Once placed, the magnets must be held securely in place. (Brass clamps or drilled wood blocks.)
2. They should be level with the compass needle.
3. It is not recommended to put a magnet closer than twice its own length from the compass. Bigger magnets further away are better than small ones close in.
4. It is better to use two magnets placed symmetrically either side of the compass rather than a single magnet on one side.
Blocks of wood drilled to take the appropriate sizes of magnets are probably the best `home-made' corrector boxes. They must have a wooden or brass flap or plug to prevent the magnets moving or slipping out of their holes.
Summary of Compass Correction
i . Remove Heeling Error with vertical magnets, by either using a Heeling Error Instrument or by damping out the compass oscillation at sea. The boat should preferably be on an East—West heading when doing this.
2. Put the boat onto NE—SE—SW—NW headings in succession and observe the deviations to obtain a Coefficient D. If the sum—after changing the signs on SE and NW and dividing by four—is small (say less than I-2 degrees) there is no
need for any soft iron spheres or chains. However, if the sum is large, it means that correction is needed and spheres should be placed on each side of the compass before going any further with the correction process.
g. Put the boat onto an East or West course, and correct the deviation obtained there by placing Fore & Aft magnets on either ahead or astern of the compass. Then turn the boat until it is on the opposite heading (West if you started on East) and remove half the deviation remaining—if any.
4. Put the boat onto a North or South heading, check the deviation against a shore bearing, and remove the deviation with athwartships magnets placed on either side of the compass. Then turn the boat onto the opposite heading and remove half the deviation remaining.
5. Make sure that all corrector magnets are securely in place, spheres screwed down etc.
6. Finally, having done all these things, swing the boat and obtain a deviation table. If the deviation is now zero on all headings, open a bottle of champagne.
Notes
In (3) and (4.) above, you will see that on the final heading in each direction only half the deviation remaining is removed. This is because the deviations obtained on the Cardinal points may not be entirely due, in fact, to the boat's permanent magnetism; to simplify the issue, other possible causes have not been explained here. For instance, most ships and some large boats have a soft iron corrector, the Flinders Bar, placed in front of the compass to counteract the effects of induction in the funnel or other superstructure; although basically a vertical induction, it has a horizontal effect which can produce deviation on the Cardinal points, as well as on other headings. Detailed analysis is required to separate this effect from the boat's fore & aft and athwartships permanent magnetism. If you try to remove all the deviation on the final heading of each pair, you may find that you have produced a larger error on the opposite heading once again. And you can go on fiddling ad infinitum! Therefore be content and leave the small error remaining.
If as a result of your corrective efforts, the subsequent swingshows no deviations larger than 2 or 3 degrees, you may well be content. In big ships, it is possible with a well sited compass to get the deviation down to below 4°, but the majority of yachts and small boats pose greater problems and absolute perfection may not be achieved.
D
Monday, March 23, 2009
The Magnetic Compass
Since the earliest sailors set off in their tiny craft on long distance voyages, some method of establishing direction on the Earth's surface has been a necessity. It seems fairly certain that the early pioneers, such as the Phoenicians in Europe and the Polynesians in the Pacific, used their knowledge of the sun and the stars, the prevailing winds and currents to guide them. They had no instruments, but generations of accumulated experience enabled them to cover immense distances and eventually return from whence they came. By modern Standards, however, such methods as they used were bound to be crude and chancy, particularly in bad weather.
The discovery of a natural ferrous rock, lodestone, led to the first instrument that would give a visual indication of direction. Lodestone is magnetic and could therefore be seen to attract pieces of iron. Soon, no doubt, it was discovered that stroking a piece of iron with the rock would induce the iron in its turn to become magnetic. Then someone found out that a piece of iron or lodestone, suitably shaped and suspended, would point in a fixed direction approximately North or South. No-one knows, for certain, in which country a primitive magnetic compass was first used, but it is definitely recorded that compasses were used in both Europe and China between
A.D. 1000-1100
The earliest compasses probably consisted of a needle or double-bow shaped piece of iron or lodestone floated on a piece of wood in a water-filled vessel; alternatively a thin magnetised needle was pricked through a piece of straw which kept it afloat on the water surface. Whichever method was used, it must have been appreciated that the North-seeking force on the needle was weak and therefore an almost friction-free method of suspension was needed.
The Earth's Magnetism
The cause of the phenomenon which showed that a piece of freely suspended iron would point North remained a mystery to the early astronomers and explorers. At one time it was thought that the North end might be attracted by the Pole Star (Polaris) which lay close to the Earth's axis of rotation and was therefore known to give a good general indication of North. There were other theories, but matters became even more difficult to explain when it was discovered that a properly balanced needle not only indicated a different North in different areas of the world but also tended to dip its North end down from the horizontal as it travelled into higher Northerly latitudes.
In about the seventeenth century, it was suggested, correctly as we now know, that the Earth itself was a vast magnet whose poles attracted one end of a magnetised needle (in accordance` with the normal laws of magnetism, a magnet always has two poles; the North-seeking/Red end of one will always attract the South-seeking/Blue end of the other; or unlike poles attract: like poles repel) .
At that time, however, there was no explanation for the apparent difference between Magnetic North (as defined by compass) and True North, the direction of the geographical North Pole. Also, in different places the angle between the two was known to vary. In fact, in the middle of the sixteenth century, some European compass makers deliberately misaligned the needle when it was fixed to a graduated card so that when the needle pointed at Magnetic North the card indicated True North. This was all very satisfactory whilst the ship remained in home waters, but as soon as it ventured away considerable inaccuracies became apparent. It slowly became obvious that the magnetic poles could not be in the same place as the geographical poles, and furthermore that the magnetic poles were not stationary but changing their positions slowly over long periods of time. Not until 1831 was it definitely established that the Earth's Magnetic Pole was over i000 miles from the Geographical North Pole in the approximate position 700 North 97° West. The South Magnetic Pole has since been accurately located in one of the most inaccessible places in the world on the bleak snow plateau of Antarctica (about 720 30' South x55° East).
This concept of the Earth having its own Magnetic Poles explained the problems that had been worrying scientists over the years. As with an ordinary bar magnet, magnetic `lines of force' emanate more or less vertically out from the North Magnetic Pole, bend over the Earth's surface, and then descend vertically into the South Magnetic Pole. At any point on the Earth's surface, therefore, a simple magnet (or compass needle) suitably suspended will align itself to these lines of force. If it were held over the North Magnetic Pole, the North-seeking end of the needle would point vertically downwards, at the Magnetic Equator the needle would be horizontal and parallel to the Earth's surface, and at the South Pole its South-seeking end would point downwards (see Figure x).
At intermediate points North or South of the Equator, the needle tilts down at a certain angle. The Earth's magnetic field, therefore, has two components; a HORIZONTAL force (H) and a VERTICAL force (Z), the vertical force being maximum at the Poles and zero at the Equator; the horizontal force zero at the Poles and maximum at the Equator. If a magnet is suspended so that one end can tilt up and down—towards or away from the Earth's surface—it will (except at the Magnetic Equator) take up a certain angle from the horizontal called `the Angle of Dip'. It used to be thought that this could be used to measure Latitude (the angular distance North or South of the Geographical Equator) before the difference between the positions of the Magnetic and True Poles was known, but in any case it would be extremely difficult to observe this angle at sea in a rolling boat.
Variation—The Angle between Magnetic North and True North
The Horizontal component of the Earth's magnetic field is of greater importance as far as the magnetic compass is concerned. If a magnet or compass needle is suspended so that it will only rotate in the horizontal plane, it can only react to the earth's Horizontal magnetic force (H). As has been seen, this force will be a maximum at the Magnetic Equator and zero at the Poles; for this reason, a magnetic compass cannot be used at or near the Magnetic Poles. In a modern compass, the needles are carefully balanced and pivoted to gain the maximum effect from the horizontal force and to nullify the effects of the vertical component of the Earth's field. In normal latitudes, with one exception which will be mentioned later, the Earth's vertical force (Z) can be ignored.
Due partly to the physical difference in position between the Magnetic and Geographical Poles, at most places on the Earth's surface there is an angular difference in direction between the two as seen from the observer's position. This angle, between TRUE NORTH and MAGNETIC NORTH, is known as the ANGLE OF VARIATION.
In Figure 2, Position I the angle or Variation is zero, as the direction of the Geographical and Magnetic Poles is coincident. However, in Positions 2 & 3, the two Poles lie in different directions from the observer and the angle between these directions in each case is the Angle of Variation.
The early navigators had tried to associate Dip with Latitude; they also tried to associate Variation with Longitude, i.e. the angular distance East or West of an arbitrary meridian.*
* A Meridian is a Great Circle passing through both geographical poles. '1'hc Greenwich Meridian is that which passes through Greenwich, and is historically the one from which Longitude is measured.
It was established, for example, that as the Atlantic was crossed the variation first reduced to zero and then increased again in the opposite sense. They inferred that charts could be drawn joining places of equal variation and assumed that the distribution of these places would be regular, and therefore the
V lines joining them symmetrical. As time went on, more and more information on the variation in different parts of the world became available; it became very apparent that the construction of a Variation chart was no simple task, as not only was the distribution irregular, but the variation in any place was changing over the years. A number of variation charts were published in the eighteenth century, but the first Admiralty Variation chart was not printed until 1858. These charts enabled ships to obtain a very approximate longitude by observing variation, but when the chronometer was developed longitude could be more accurately defined by astronomical means. The variation charts then began to be used as a means of correcting the compass heading—the purpose for which they are used today.
Modern techniques, and the vast store of information garnered over the years, now enable Variation to be predicted accurately for any given place and time. The earth's magnetic field, in other words, has been precisely plotted and its yearly change established. At the present time, the variation in Britain is changing annually by about 8 minutes of arc. There are also small seasonal and daily changes in variation although these are not of significance to the practical navigator. The peculiar distribution of places having equal variation has been charted and places where local magnetic effects are strong (due to a large amount of ferrous rock near the surface) discovered.
To the modern seaman, variation is no mystery but allowance must still be made for it if an accurate TRUE COURSE is to be steered at sea. Remember it is the angle between TRUE NORTH and MAGNETIC NORTH and is defined as WESTERLY VARIATION if Magnetic North is to the West of True, and EASTERLY if Magnetic is East of True North. (See Figures 3A and 3B).
The value of the variatio#in any particular place is shown on the chart of the area; on ocean charts `isogonic' lines, that is to say lines joining places of equal variation, are drawn at
intervals (usually of one degree) . On coastal charts of larger scale, the variation is usually shown inside the `compass rose', together with the amount by which it is increasing or decreasing annually. The Compass Rose on a chart consists of two concentric circles graduated in degrees with the aid of which courses and bearings can be laid off on the chart. The North point of the outer (True) circle is aligned with the meridian and therefore indicates True North on the chart; courses and bearings laid off from this circle will be True. The North point of the inner (Magnetic) circle is aligned to Magnetic North at the date the chart was printed. Magnetic courses and bearings can therefore be laid off directly from the inner circle but it must be remembered that, owing to the annual change in variation, this will only give a reasonably accurate answer for a few years; if the chart was bought a long time ago, fairly large errors can result. Therefore it is usually preferable to look at the variation value written inside the compass rose, correct it for the annual change since the chart was published—the date and yearly change is shown next to the value—and then apply it arithmetically to obtain a True course or bearing. The method of doing this (with examples) is described in detail later in this chapter.
The Modern Magnetic Compass
There are many makes of magnetic compass now on the market, and it is not possible to describe them all here. The most common type is probably the liquid-filled compass with a horizontal compass card (designed to be viewed from above) suspended on a needle pivot within a sealed compass bowl. Another common small craft compass is contained in a transparent hemispherical bowl and has a vertically read card; the figures are engraved on the thick edge of the card and viewed from eye level.
In selecting a compass for a boat, the pocket will probably play a big part in the choice. However, some of the qualities to look for (or against) are listed below:
(a) It should preferably be gimballed, i.e. suspended in two rings which enable the bowl to keep level and comparatively still however much the craft pitches or rolls. If a compass is not gimballed, the card may foul the bowl as the boat moves and thus give an inaccurate and erratic heading. In a gimballed compass, the clearance between the card and the bowl allows a tilt of about 10 degrees which is quite adequate to cope with a sudden, sharp movement. Some compasses used in boats are of the un-gimballed type, similar to (or indeed may be) aircraft compasses. Most simple aircraft compasses are ungimballed, but sprung in some fashion underneath by felt pads, springs, rubber cushions and the like. They also have a greater clearance between the card and the cover glass; normally about 200. This is sufficient in an aircraft for as the aircraft banks steeply (say 90°), centrifugal force causes the compass card to take up a false horizontal, dipping one side down in the direction of bank. However in a boat, the motion is different and unless there is adequate clearance, the card may foul. An analogy can be made with a bucket of water; if this is swung round and round by the handle (aircraft turning) the water will remain in the bucket: put the bucket on a swing, however, where the direction of motion is reversed (boat rolling), and it will slop over.
(b) Dry Card or Liquid Compass
Dry Card. In this type, a graduated card made of paper or mica, with magnets attached beneath it, is suspended by a cap and pivot bearing. The lighter the card, the less friction and wear is imposed on the pivot. There is a limit to the amount the weight can be reduced, as the magnets must be strong enough to move the card physically. Secondly, more than one magnet must always be used, as a single needle, suspended at its centre, would tend to align itself in the direction of any motion imposed on it; for example, if the boat rolled, a single bar would try to swing in the direction of roll due to its moment of inertia.
Apart from the inevitable wear on the pivot, the main disadvantage of a dry card compass is its comparative instability, particularly in a small craft with a short period of roll. This wandering of the compass can be reduced to some extent by making the compass have a long period of oscillation—by attaching an aluminium or other alloy ring to the perimeter of the card, or by surrounding it with a copper bowl in which magnetic eddy currents set up by the magnets tend to oppose movements of the card.
20 SMALL BOAT NAVIGATION THE MAGNETIC COMPASS 2t
The main advantages of a Dry Card compass are its cheapness and simplicity.
Liquid Compass. If the bowl containing the compass card is made watertight and filled with a liquid, some immediate benefits accrue. The liquid has a frictional damping effect on the card thus reducing any unwanted quick oscillations. The card and magnets can also be made just sufficiently buoyant to reduce the weight and friction on the pivot to a minimum. These are overriding advantages, and it was for this reason that many large ships went over to liquid compasses in about 1906.
There are, however, certain difficulties which must be overcome in the design of a good liquid compass. Obviously the liquid must not be allowed to evaporate or freeze. The liquid normally used is a mixture of water and alcohol (about 47% alcohol). To. prevent evaporation, the bowl must be completely sealed, but must allow the liquid room to expand as the temperature rises and then contract when the temperature falls without any bubbles forming. In good compasses, these factors are allowed for by the fitting of expansion chambers or bellows, usually made of copper, which expand and contract as the pressure changes inside the bowl.
Another problem which the designers had to face was that of liquid `swirl'. As the boat alters course, some of the liquid is drawn round the bowl by friction. The effect is mainly felt by the liquid near the edge of the bowl. To prevent this swirl affecting the card, a good liquid compass has a card of considerably smaller diameter than the bowl. This also reduces the effect of another undesirable feature which could be encountered in bad weather. With a heavy roll or pitch, the card becomes tilted relative to the bowl (particularly in a compass with no gimbals) and liquid tries to pass from below the card to above it. If the gap between the bowl and card is adequate, this liquid movement will have no serious effect. Some compasses have domed, transparent top glasses and this shape of case tends to reduce the swirl effects, whilst at the same time providing some magnification of the card,
Occasionally, if a compass is old and its joints perished, or perhaps it has been damaged in some way, a bubble will appear in the liquid. It may be that the compass is still usable in this condition, but there is always a danger that it may readincorrectly, particularly if the bubble is large and has been allowed to remain in the compass for some time. There is no doubt that the best solution is to take the compass to a reputable compass maker for the removal of the bubble. However in emergency, the bellows cover or the top of some compasses can be removed by undoing the securing screws, lifting the top glass and rubber seal, and then refilling the bowl with alcohol and water. It is not easy to do this without trapping a certain amount of air. If alcohol cannot be had, distilled water is probably the best alternative additive, but dilution will, of course, raise the freezing point of the liquid. Neat alcohol, on the other hand, should not be used as too strong a solution will probably damage or raise the paints in the bowl or on the card.
Compass Graduations
Nowadays most compass cards are graduated from o degrees (North) right around clockwise to 859 degrees—the 360 degrees of a circle. Thus o° is North—ogo° East-18o° South-2700 West—and back again to North or 3600. But, partly as a tradition from sailing ship days, and partly because it is difficult to steer a boat to within i degree, many cards are still marked with some of the old compass points.
The old system was to divide the angles between the Cardinal points (North—East—South—West) first into quadrants and then further into points. A point was of i ii degrees, and there were therefore 32 points in the full circle. Each point was named—Boxing the Compass'—as shown in Figure 4.
Even on a modern compass, the Cardinal and Quadrantal points (NE—SE—SW—NW) are usually marked on the card inside the degree graduations. To set a particular course, the appropriate graduation on the card is brought opposite the Fore-and-Aft mark on the compass bowl—this is called the 'Lubber's Line'. For example, if North East is the course required, NE (0450) must be brought opposite the Lubber's Line by use of the helm.
Alignment of the Compass
Great care must be taken when the compass is first installed in a boat to ensure that the Lubber's Line is truly aligned fore and aft, i.e. parallel to the boat's keel. This is a comparatively
Even more difficult a problem is caused by the magnetisation of iron that is neither hard nor soft. This is sometimes called `Intermediate Iron', and the magnetic effect associated with it
SUB-PERMANENT magnetism. A piece of iron of this nature can
be magnetised quite easily, but only retains this magnetism over a limited period.
Deviation
The modern boat owner is therefore faced with the prospect that a good deal of iron, of one sort or another, may be within spitting distance of the compass. In a well designed craft, this factor will have been taken into account and the best possible position chosen for the compass, non-ferrous metals used near it, etc. However, there is a practical limit on what can be done, particularly in a craft with a metal hull. Tables, called `Safe Distance' tables, exist which list a wide number of objects—instruments, echo sounders, radio sets etc.—against their minimum safe distances from the compass bowl. These tables are principally of assistance to the boat builder, but may be of interest to the owner should he wish to add additional equipment or stow portable/potable objects on board.
Any magnetic material not outside these safe distances may cause a significant deflection or deviation of the compass needle. The amount of deviation caused will vary on different courses depending on the relative position of the compass and the object, and of the type of iron of which the latter is made.
The resulting `Angle of Deviation' on a particular heading is defined as the angle between Compass North and Magn1~tic North. An angle of Deviation described as 2 degrees West means that Compass North is 20 W of Magnetic North. This element of error is therefore defined in a similar way to Variation, although its cause is different and (unlike Variation) its value will vary as the boat alters course.
The TOTAL COMPASS CORRECTION is a combined allowance for the errors caused by VARIATION and DEVIATION. Examples
are shown in Figures 3C, 3D, 3E & 3F.
Deviation and `Swinging'
The best magnetic position for a compass in a boat would probably be on top of the mast, but this would be a little
THE MAGNETIC COMPASS 25
inconvenient for the owner. The boat builder therefore chooses the best practical position for the compass consistent with ease of use at sea. Boats vary a great deal in their magnetic properties, but even in the best circles some magnetic influences are likely to be present. The Deviation of the compass on ALL headings must therefore be found.
This is normally done by `Swinging Ship', i.e. swinging the boat slowly through North—East—South—West (but not necessarily in that order) through the whole 360° whilst checking the compass against a known True bearing.
Qualified Swinging Officers are employed for checking the magnetic compasses of big ships, but part of the job is well within the capabilities of the average boat owner. It is not difficult to find the deviation of a compass although a bit more knowledge of the subject is needed to remove the errors once they have been found. (The latter process, Compass Correction, will be described in Chapter 2). Once established, the Deviation values can be used to correct the compass heading so that a true course can be steered.
WHEN—Ideally, a boat should be swung to observe compass Deviation:
(a) On first delivery to the owner.
(b) At the beginning of a new sailing season.
(c) On a large change in Magnetic Latitude (say io°).
(d) If any new fittings or equipment are put into the boat ... remember that new outboard motor.
(e) If you have any cause to mistrust it!
(f) Finally, and most important—if any of the correctors have been changed or moved. (Note small son's movements) .
WHAT STATE—Before a craft is swung, it is as well to have everything on board in its normal seagoing position-the anchor stowed, engine (if outboard) in its normal place, Mum's knitting needles firmly entrenched in her knitting and not near the compass—in fact all portable articles such as knives, keys, cans of beer kept well away from the scene of operations. Similarly, keep the boat away from other iron
S SMALL BOAT NAVIGATION THE MAGNETIC COMPASS 29
How to take the Bearing
Whatever method is used, an accurate compass bearing of the charted distant object, transit etc. must be taken. The larger craft are often fitted with a compass from which a bearing can be taken directly; some form of sighting attachment is fitted on top of the compass for this purpose. The earliest designs took the form of a notched sight, not unlike the back-sight on a rifle. Nowadays, these bearing devices, normally called azimuth circles, have a prism (and sometimes a telescope) mounted on a ring fitted over the compass card. The object is viewed through the `V' notch on top of the prism, and at the same time the reflected image of the compass card is visible beneath the `V'. A hairline running down the face of the prism from the bottom of the notch appears to cut the card image at the bearing obtained. The advantage of the prismatic arrange- ment over the old open sight is that the observer's eye need not be exactly aligned with the centre of the card when the bearing is taken. Providing the object is seen through the notch, and the reflected image of the card in the prism, the bearing will be accurate.
If the compass in your boat is fitted with an azimuth circle, well and good. The process of taking bearings for ordinary navigation as well as swinging for compass adjustment is straightforward. However, an azimuth circle is not much use if the compass itself is not placed in the boat so that a good all round view of the horizon can be obtained fi 6m it. Many compasses, such as those used for steering alone, are sited well down below the gunwale. The point here is that if the boat has one compass with good horizon visibility, bearings can be taken with its azimuth circle and any other compasses compared with it. If the boat has no compass with a view, or azimuth circle, then life is more difficult. In this event, a bearing plate should be used for swinging. Briefly, this consists of a graduated non-magnetic card which can be turned manually by means of a knob on its face-plate, over which is fitted an azimuth circle. The instrument is placed high up on its tripod on the centre-line of the boat, and its Lubber's Line accurately aligned with ship's head (by taking a sight on the bow). The next boat's compass heading to be checked–e.g. North East (o45°—is pre-set on the bearing plate opposite the
Lubber's Line. As the boat reaches this heading (ship's head 045° by the compass being checked) the shore bearing is taken from the bearing plate. The bearing obtained at that instant is directly equivalent to that which would have been obtained, were it possible, from the compass direct.
Perhaps hand bearing compasses ought to be mentioned here, as they provide an alternative method of taking bearings, albeit a fairly unreliable method in this context. They are simple hand held magnetic contpasses with some form of sighting device for taking bearings. The marine variety are normally fitted on top of a wooden handle. The snag with them is that, being portable, they suffer a varying and unknown amount from any local magnetic influences in the boat and are therefore not really reliable as checking instruments. The hand bearing compass could suffer a worse deviation than the fixed compass, and certainly its deviation will alter as it is moved from place to place in the boat; it will always be an unknown quantity. However, if these limitations are fully appreciated, the hand bearing compass can be an invaluable aid to yachtsment for day to day use, and could be used, at a pinch, for checking another compass. When using it for this purpose, select a place in the boat where it is least likely to feel any magnetic influences. In a new boat, this will be largely a matter of guesswork, but in a known craft it will soon become apparent—from compass checks on passage and practical fixing-in which position the hand bearing compass can best be used and how good its results have been. Obviously, if it is known to perform accurately on board, there is nothing against using it for checking another compass.
The Swing
The business of swinging the boat round from one heading to the next should be done as slowly as possible. Ideally, one complete circle should take about an hour, but patience, or the call of less mundane things, may not permit this. Do not, however, ` be tempted to swing too fast as this may have an adverse effect due to certain induction influences on the compass. Forty minutes should be the minimum. This process has only to be done once, if it is done well, and you will sleep the sounder for it later on.
C
With a well placed compass, the taking of bearings should also be fairly simple. All you need is Qne bearing on every two points, or better still, every ten degrees; thus sixteen or thirty-six headings and their related bearings.
You may find that on a particular heading the shore object is `wooded', i.e. obscured from the compass by some part of the boat's structure. Therefore, if possible, have up your sleeve the true bearing of an alternative object. You will also find, with experience, that using the modern azimuth circle it is quite easy to take a good bearing `through' some obstruction, providing this does not have too large a spread across the field of view.
Procedure. In calm weather, and with the use of an engine, it is possible to carry out a swing alone and unaided. However, it is much simpler if there is another person on board to handle the boat whilst the `swinger' takes the bearings and notes down results. Before starting the swing, prepare a notebook listing in one column the sixteen (or thirty-six) ship's heads and in the other leave spaces for the compass bearing of the object against each ship's head. If using more than one distant object or interpolating between transits, another column will be needed for true bearings. It does not matter on which heading you start, nor does it matter in which direction, clockwise or counter-clockwise, the boat is swung. Steady the boat therefore on the nearest `s convenient ` heading and take an accurate bearing of your chosen object. Write this bearing down opposite the ship's head concerned. Then swing slowly on to the next heading and repeat the process, noting each time the compass bearing (and the true bearing if using more than one object), Continue the swing until all headings have been checked.
A few tips—you must take great care in getting accurate bearings but it is not vital that the ship's head be exactly on the chosen heading when the bearing is taken. A couple of degrees either way will not matter. Secondly, if you decide to use transits, and interpolate between them whenever necessary, it is as well to draw a small diagram in your notebook showing the true bearings of the transits so that you can interpolate between them on the spot and not have to keep referring to a chart.
Tabulating the Results. Having completed the swing, the results
THE MAGNETIC COMPASSy 31
must be tabulated for future use. An example is given below, followed by explanatory notes:
A B C D E
-- ,~~°raw)
True ~r
Bearing of ` Distant Magnetic Compass
Ship's Head Object/ Bearing of Bearing of
by Compass Transit Object Object Deviation/`
Deg. (°) Deg. (°) Deg. (0) Deg. (°) Deg. (°)`
N 0000 223° 228° 2320 40 W
NNE 0221 223 228 2331 51 W
NE 045 223 229 234 6 W
ENE o671 223 228 233 5 W
E 090 *279 284 z87 3 W
ESE II21 223 228 23o 2 W
SE 135 223 228 229 I W
SSE 1571 *2791k 284 282 2 E
S 18o 223 228 223 5, E
SSW 2021 223 228 22I 7 E
SW 225 223 228 223 5 E ,..
WSW 2471 223 228 224 4 E
W 270 223 228 225 3 E
WNW 2922 223 228 226 2 E
NW 315 223 228 228 Nil
NNW 3371 223 228 230 2 W
(I) * Denotes second distant object used, as the first was `wooded'
on these bearings.
(2) Variation—at place of swing-5° West.
Explanation of this table
COLUMN `A'—SHIP'S HEAD BY COMPASS
The successive Ship's heads—by the compass being checked—
are written in this column.
COLUMN `B'—TRUE BEARING OF OBJECT
Two distant objects have been used in the course of this swing. The True bearings of the objects from the swinging position .. are taken off the chart and entered in this column.
COLUMN `C'—MAGNETIC BEARING OF OBJECT
This is the bearing of the object after the Variation has been applied to the True Bearing. The Variation in this locality
LZ `_3
The discovery of a natural ferrous rock, lodestone, led to the first instrument that would give a visual indication of direction. Lodestone is magnetic and could therefore be seen to attract pieces of iron. Soon, no doubt, it was discovered that stroking a piece of iron with the rock would induce the iron in its turn to become magnetic. Then someone found out that a piece of iron or lodestone, suitably shaped and suspended, would point in a fixed direction approximately North or South. No-one knows, for certain, in which country a primitive magnetic compass was first used, but it is definitely recorded that compasses were used in both Europe and China between
A.D. 1000-1100
The earliest compasses probably consisted of a needle or double-bow shaped piece of iron or lodestone floated on a piece of wood in a water-filled vessel; alternatively a thin magnetised needle was pricked through a piece of straw which kept it afloat on the water surface. Whichever method was used, it must have been appreciated that the North-seeking force on the needle was weak and therefore an almost friction-free method of suspension was needed.
The Earth's Magnetism
The cause of the phenomenon which showed that a piece of freely suspended iron would point North remained a mystery to the early astronomers and explorers. At one time it was thought that the North end might be attracted by the Pole Star (Polaris) which lay close to the Earth's axis of rotation and was therefore known to give a good general indication of North. There were other theories, but matters became even more difficult to explain when it was discovered that a properly balanced needle not only indicated a different North in different areas of the world but also tended to dip its North end down from the horizontal as it travelled into higher Northerly latitudes.
In about the seventeenth century, it was suggested, correctly as we now know, that the Earth itself was a vast magnet whose poles attracted one end of a magnetised needle (in accordance` with the normal laws of magnetism, a magnet always has two poles; the North-seeking/Red end of one will always attract the South-seeking/Blue end of the other; or unlike poles attract: like poles repel) .
At that time, however, there was no explanation for the apparent difference between Magnetic North (as defined by compass) and True North, the direction of the geographical North Pole. Also, in different places the angle between the two was known to vary. In fact, in the middle of the sixteenth century, some European compass makers deliberately misaligned the needle when it was fixed to a graduated card so that when the needle pointed at Magnetic North the card indicated True North. This was all very satisfactory whilst the ship remained in home waters, but as soon as it ventured away considerable inaccuracies became apparent. It slowly became obvious that the magnetic poles could not be in the same place as the geographical poles, and furthermore that the magnetic poles were not stationary but changing their positions slowly over long periods of time. Not until 1831 was it definitely established that the Earth's Magnetic Pole was over i000 miles from the Geographical North Pole in the approximate position 700 North 97° West. The South Magnetic Pole has since been accurately located in one of the most inaccessible places in the world on the bleak snow plateau of Antarctica (about 720 30' South x55° East).
This concept of the Earth having its own Magnetic Poles explained the problems that had been worrying scientists over the years. As with an ordinary bar magnet, magnetic `lines of force' emanate more or less vertically out from the North Magnetic Pole, bend over the Earth's surface, and then descend vertically into the South Magnetic Pole. At any point on the Earth's surface, therefore, a simple magnet (or compass needle) suitably suspended will align itself to these lines of force. If it were held over the North Magnetic Pole, the North-seeking end of the needle would point vertically downwards, at the Magnetic Equator the needle would be horizontal and parallel to the Earth's surface, and at the South Pole its South-seeking end would point downwards (see Figure x).
At intermediate points North or South of the Equator, the needle tilts down at a certain angle. The Earth's magnetic field, therefore, has two components; a HORIZONTAL force (H) and a VERTICAL force (Z), the vertical force being maximum at the Poles and zero at the Equator; the horizontal force zero at the Poles and maximum at the Equator. If a magnet is suspended so that one end can tilt up and down—towards or away from the Earth's surface—it will (except at the Magnetic Equator) take up a certain angle from the horizontal called `the Angle of Dip'. It used to be thought that this could be used to measure Latitude (the angular distance North or South of the Geographical Equator) before the difference between the positions of the Magnetic and True Poles was known, but in any case it would be extremely difficult to observe this angle at sea in a rolling boat.
Variation—The Angle between Magnetic North and True North
The Horizontal component of the Earth's magnetic field is of greater importance as far as the magnetic compass is concerned. If a magnet or compass needle is suspended so that it will only rotate in the horizontal plane, it can only react to the earth's Horizontal magnetic force (H). As has been seen, this force will be a maximum at the Magnetic Equator and zero at the Poles; for this reason, a magnetic compass cannot be used at or near the Magnetic Poles. In a modern compass, the needles are carefully balanced and pivoted to gain the maximum effect from the horizontal force and to nullify the effects of the vertical component of the Earth's field. In normal latitudes, with one exception which will be mentioned later, the Earth's vertical force (Z) can be ignored.
Due partly to the physical difference in position between the Magnetic and Geographical Poles, at most places on the Earth's surface there is an angular difference in direction between the two as seen from the observer's position. This angle, between TRUE NORTH and MAGNETIC NORTH, is known as the ANGLE OF VARIATION.
In Figure 2, Position I the angle or Variation is zero, as the direction of the Geographical and Magnetic Poles is coincident. However, in Positions 2 & 3, the two Poles lie in different directions from the observer and the angle between these directions in each case is the Angle of Variation.
The early navigators had tried to associate Dip with Latitude; they also tried to associate Variation with Longitude, i.e. the angular distance East or West of an arbitrary meridian.*
* A Meridian is a Great Circle passing through both geographical poles. '1'hc Greenwich Meridian is that which passes through Greenwich, and is historically the one from which Longitude is measured.
It was established, for example, that as the Atlantic was crossed the variation first reduced to zero and then increased again in the opposite sense. They inferred that charts could be drawn joining places of equal variation and assumed that the distribution of these places would be regular, and therefore the
V lines joining them symmetrical. As time went on, more and more information on the variation in different parts of the world became available; it became very apparent that the construction of a Variation chart was no simple task, as not only was the distribution irregular, but the variation in any place was changing over the years. A number of variation charts were published in the eighteenth century, but the first Admiralty Variation chart was not printed until 1858. These charts enabled ships to obtain a very approximate longitude by observing variation, but when the chronometer was developed longitude could be more accurately defined by astronomical means. The variation charts then began to be used as a means of correcting the compass heading—the purpose for which they are used today.
Modern techniques, and the vast store of information garnered over the years, now enable Variation to be predicted accurately for any given place and time. The earth's magnetic field, in other words, has been precisely plotted and its yearly change established. At the present time, the variation in Britain is changing annually by about 8 minutes of arc. There are also small seasonal and daily changes in variation although these are not of significance to the practical navigator. The peculiar distribution of places having equal variation has been charted and places where local magnetic effects are strong (due to a large amount of ferrous rock near the surface) discovered.
To the modern seaman, variation is no mystery but allowance must still be made for it if an accurate TRUE COURSE is to be steered at sea. Remember it is the angle between TRUE NORTH and MAGNETIC NORTH and is defined as WESTERLY VARIATION if Magnetic North is to the West of True, and EASTERLY if Magnetic is East of True North. (See Figures 3A and 3B).
The value of the variatio#in any particular place is shown on the chart of the area; on ocean charts `isogonic' lines, that is to say lines joining places of equal variation, are drawn at
intervals (usually of one degree) . On coastal charts of larger scale, the variation is usually shown inside the `compass rose', together with the amount by which it is increasing or decreasing annually. The Compass Rose on a chart consists of two concentric circles graduated in degrees with the aid of which courses and bearings can be laid off on the chart. The North point of the outer (True) circle is aligned with the meridian and therefore indicates True North on the chart; courses and bearings laid off from this circle will be True. The North point of the inner (Magnetic) circle is aligned to Magnetic North at the date the chart was printed. Magnetic courses and bearings can therefore be laid off directly from the inner circle but it must be remembered that, owing to the annual change in variation, this will only give a reasonably accurate answer for a few years; if the chart was bought a long time ago, fairly large errors can result. Therefore it is usually preferable to look at the variation value written inside the compass rose, correct it for the annual change since the chart was published—the date and yearly change is shown next to the value—and then apply it arithmetically to obtain a True course or bearing. The method of doing this (with examples) is described in detail later in this chapter.
The Modern Magnetic Compass
There are many makes of magnetic compass now on the market, and it is not possible to describe them all here. The most common type is probably the liquid-filled compass with a horizontal compass card (designed to be viewed from above) suspended on a needle pivot within a sealed compass bowl. Another common small craft compass is contained in a transparent hemispherical bowl and has a vertically read card; the figures are engraved on the thick edge of the card and viewed from eye level.
In selecting a compass for a boat, the pocket will probably play a big part in the choice. However, some of the qualities to look for (or against) are listed below:
(a) It should preferably be gimballed, i.e. suspended in two rings which enable the bowl to keep level and comparatively still however much the craft pitches or rolls. If a compass is not gimballed, the card may foul the bowl as the boat moves and thus give an inaccurate and erratic heading. In a gimballed compass, the clearance between the card and the bowl allows a tilt of about 10 degrees which is quite adequate to cope with a sudden, sharp movement. Some compasses used in boats are of the un-gimballed type, similar to (or indeed may be) aircraft compasses. Most simple aircraft compasses are ungimballed, but sprung in some fashion underneath by felt pads, springs, rubber cushions and the like. They also have a greater clearance between the card and the cover glass; normally about 200. This is sufficient in an aircraft for as the aircraft banks steeply (say 90°), centrifugal force causes the compass card to take up a false horizontal, dipping one side down in the direction of bank. However in a boat, the motion is different and unless there is adequate clearance, the card may foul. An analogy can be made with a bucket of water; if this is swung round and round by the handle (aircraft turning) the water will remain in the bucket: put the bucket on a swing, however, where the direction of motion is reversed (boat rolling), and it will slop over.
(b) Dry Card or Liquid Compass
Dry Card. In this type, a graduated card made of paper or mica, with magnets attached beneath it, is suspended by a cap and pivot bearing. The lighter the card, the less friction and wear is imposed on the pivot. There is a limit to the amount the weight can be reduced, as the magnets must be strong enough to move the card physically. Secondly, more than one magnet must always be used, as a single needle, suspended at its centre, would tend to align itself in the direction of any motion imposed on it; for example, if the boat rolled, a single bar would try to swing in the direction of roll due to its moment of inertia.
Apart from the inevitable wear on the pivot, the main disadvantage of a dry card compass is its comparative instability, particularly in a small craft with a short period of roll. This wandering of the compass can be reduced to some extent by making the compass have a long period of oscillation—by attaching an aluminium or other alloy ring to the perimeter of the card, or by surrounding it with a copper bowl in which magnetic eddy currents set up by the magnets tend to oppose movements of the card.
20 SMALL BOAT NAVIGATION THE MAGNETIC COMPASS 2t
The main advantages of a Dry Card compass are its cheapness and simplicity.
Liquid Compass. If the bowl containing the compass card is made watertight and filled with a liquid, some immediate benefits accrue. The liquid has a frictional damping effect on the card thus reducing any unwanted quick oscillations. The card and magnets can also be made just sufficiently buoyant to reduce the weight and friction on the pivot to a minimum. These are overriding advantages, and it was for this reason that many large ships went over to liquid compasses in about 1906.
There are, however, certain difficulties which must be overcome in the design of a good liquid compass. Obviously the liquid must not be allowed to evaporate or freeze. The liquid normally used is a mixture of water and alcohol (about 47% alcohol). To. prevent evaporation, the bowl must be completely sealed, but must allow the liquid room to expand as the temperature rises and then contract when the temperature falls without any bubbles forming. In good compasses, these factors are allowed for by the fitting of expansion chambers or bellows, usually made of copper, which expand and contract as the pressure changes inside the bowl.
Another problem which the designers had to face was that of liquid `swirl'. As the boat alters course, some of the liquid is drawn round the bowl by friction. The effect is mainly felt by the liquid near the edge of the bowl. To prevent this swirl affecting the card, a good liquid compass has a card of considerably smaller diameter than the bowl. This also reduces the effect of another undesirable feature which could be encountered in bad weather. With a heavy roll or pitch, the card becomes tilted relative to the bowl (particularly in a compass with no gimbals) and liquid tries to pass from below the card to above it. If the gap between the bowl and card is adequate, this liquid movement will have no serious effect. Some compasses have domed, transparent top glasses and this shape of case tends to reduce the swirl effects, whilst at the same time providing some magnification of the card,
Occasionally, if a compass is old and its joints perished, or perhaps it has been damaged in some way, a bubble will appear in the liquid. It may be that the compass is still usable in this condition, but there is always a danger that it may readincorrectly, particularly if the bubble is large and has been allowed to remain in the compass for some time. There is no doubt that the best solution is to take the compass to a reputable compass maker for the removal of the bubble. However in emergency, the bellows cover or the top of some compasses can be removed by undoing the securing screws, lifting the top glass and rubber seal, and then refilling the bowl with alcohol and water. It is not easy to do this without trapping a certain amount of air. If alcohol cannot be had, distilled water is probably the best alternative additive, but dilution will, of course, raise the freezing point of the liquid. Neat alcohol, on the other hand, should not be used as too strong a solution will probably damage or raise the paints in the bowl or on the card.
Compass Graduations
Nowadays most compass cards are graduated from o degrees (North) right around clockwise to 859 degrees—the 360 degrees of a circle. Thus o° is North—ogo° East-18o° South-2700 West—and back again to North or 3600. But, partly as a tradition from sailing ship days, and partly because it is difficult to steer a boat to within i degree, many cards are still marked with some of the old compass points.
The old system was to divide the angles between the Cardinal points (North—East—South—West) first into quadrants and then further into points. A point was of i ii degrees, and there were therefore 32 points in the full circle. Each point was named—Boxing the Compass'—as shown in Figure 4.
Even on a modern compass, the Cardinal and Quadrantal points (NE—SE—SW—NW) are usually marked on the card inside the degree graduations. To set a particular course, the appropriate graduation on the card is brought opposite the Fore-and-Aft mark on the compass bowl—this is called the 'Lubber's Line'. For example, if North East is the course required, NE (0450) must be brought opposite the Lubber's Line by use of the helm.
Alignment of the Compass
Great care must be taken when the compass is first installed in a boat to ensure that the Lubber's Line is truly aligned fore and aft, i.e. parallel to the boat's keel. This is a comparatively
Even more difficult a problem is caused by the magnetisation of iron that is neither hard nor soft. This is sometimes called `Intermediate Iron', and the magnetic effect associated with it
SUB-PERMANENT magnetism. A piece of iron of this nature can
be magnetised quite easily, but only retains this magnetism over a limited period.
Deviation
The modern boat owner is therefore faced with the prospect that a good deal of iron, of one sort or another, may be within spitting distance of the compass. In a well designed craft, this factor will have been taken into account and the best possible position chosen for the compass, non-ferrous metals used near it, etc. However, there is a practical limit on what can be done, particularly in a craft with a metal hull. Tables, called `Safe Distance' tables, exist which list a wide number of objects—instruments, echo sounders, radio sets etc.—against their minimum safe distances from the compass bowl. These tables are principally of assistance to the boat builder, but may be of interest to the owner should he wish to add additional equipment or stow portable/potable objects on board.
Any magnetic material not outside these safe distances may cause a significant deflection or deviation of the compass needle. The amount of deviation caused will vary on different courses depending on the relative position of the compass and the object, and of the type of iron of which the latter is made.
The resulting `Angle of Deviation' on a particular heading is defined as the angle between Compass North and Magn1~tic North. An angle of Deviation described as 2 degrees West means that Compass North is 20 W of Magnetic North. This element of error is therefore defined in a similar way to Variation, although its cause is different and (unlike Variation) its value will vary as the boat alters course.
The TOTAL COMPASS CORRECTION is a combined allowance for the errors caused by VARIATION and DEVIATION. Examples
are shown in Figures 3C, 3D, 3E & 3F.
Deviation and `Swinging'
The best magnetic position for a compass in a boat would probably be on top of the mast, but this would be a little
THE MAGNETIC COMPASS 25
inconvenient for the owner. The boat builder therefore chooses the best practical position for the compass consistent with ease of use at sea. Boats vary a great deal in their magnetic properties, but even in the best circles some magnetic influences are likely to be present. The Deviation of the compass on ALL headings must therefore be found.
This is normally done by `Swinging Ship', i.e. swinging the boat slowly through North—East—South—West (but not necessarily in that order) through the whole 360° whilst checking the compass against a known True bearing.
Qualified Swinging Officers are employed for checking the magnetic compasses of big ships, but part of the job is well within the capabilities of the average boat owner. It is not difficult to find the deviation of a compass although a bit more knowledge of the subject is needed to remove the errors once they have been found. (The latter process, Compass Correction, will be described in Chapter 2). Once established, the Deviation values can be used to correct the compass heading so that a true course can be steered.
WHEN—Ideally, a boat should be swung to observe compass Deviation:
(a) On first delivery to the owner.
(b) At the beginning of a new sailing season.
(c) On a large change in Magnetic Latitude (say io°).
(d) If any new fittings or equipment are put into the boat ... remember that new outboard motor.
(e) If you have any cause to mistrust it!
(f) Finally, and most important—if any of the correctors have been changed or moved. (Note small son's movements) .
WHAT STATE—Before a craft is swung, it is as well to have everything on board in its normal seagoing position-the anchor stowed, engine (if outboard) in its normal place, Mum's knitting needles firmly entrenched in her knitting and not near the compass—in fact all portable articles such as knives, keys, cans of beer kept well away from the scene of operations. Similarly, keep the boat away from other iron
S SMALL BOAT NAVIGATION THE MAGNETIC COMPASS 29
How to take the Bearing
Whatever method is used, an accurate compass bearing of the charted distant object, transit etc. must be taken. The larger craft are often fitted with a compass from which a bearing can be taken directly; some form of sighting attachment is fitted on top of the compass for this purpose. The earliest designs took the form of a notched sight, not unlike the back-sight on a rifle. Nowadays, these bearing devices, normally called azimuth circles, have a prism (and sometimes a telescope) mounted on a ring fitted over the compass card. The object is viewed through the `V' notch on top of the prism, and at the same time the reflected image of the compass card is visible beneath the `V'. A hairline running down the face of the prism from the bottom of the notch appears to cut the card image at the bearing obtained. The advantage of the prismatic arrange- ment over the old open sight is that the observer's eye need not be exactly aligned with the centre of the card when the bearing is taken. Providing the object is seen through the notch, and the reflected image of the card in the prism, the bearing will be accurate.
If the compass in your boat is fitted with an azimuth circle, well and good. The process of taking bearings for ordinary navigation as well as swinging for compass adjustment is straightforward. However, an azimuth circle is not much use if the compass itself is not placed in the boat so that a good all round view of the horizon can be obtained fi 6m it. Many compasses, such as those used for steering alone, are sited well down below the gunwale. The point here is that if the boat has one compass with good horizon visibility, bearings can be taken with its azimuth circle and any other compasses compared with it. If the boat has no compass with a view, or azimuth circle, then life is more difficult. In this event, a bearing plate should be used for swinging. Briefly, this consists of a graduated non-magnetic card which can be turned manually by means of a knob on its face-plate, over which is fitted an azimuth circle. The instrument is placed high up on its tripod on the centre-line of the boat, and its Lubber's Line accurately aligned with ship's head (by taking a sight on the bow). The next boat's compass heading to be checked–e.g. North East (o45°—is pre-set on the bearing plate opposite the
Lubber's Line. As the boat reaches this heading (ship's head 045° by the compass being checked) the shore bearing is taken from the bearing plate. The bearing obtained at that instant is directly equivalent to that which would have been obtained, were it possible, from the compass direct.
Perhaps hand bearing compasses ought to be mentioned here, as they provide an alternative method of taking bearings, albeit a fairly unreliable method in this context. They are simple hand held magnetic contpasses with some form of sighting device for taking bearings. The marine variety are normally fitted on top of a wooden handle. The snag with them is that, being portable, they suffer a varying and unknown amount from any local magnetic influences in the boat and are therefore not really reliable as checking instruments. The hand bearing compass could suffer a worse deviation than the fixed compass, and certainly its deviation will alter as it is moved from place to place in the boat; it will always be an unknown quantity. However, if these limitations are fully appreciated, the hand bearing compass can be an invaluable aid to yachtsment for day to day use, and could be used, at a pinch, for checking another compass. When using it for this purpose, select a place in the boat where it is least likely to feel any magnetic influences. In a new boat, this will be largely a matter of guesswork, but in a known craft it will soon become apparent—from compass checks on passage and practical fixing-in which position the hand bearing compass can best be used and how good its results have been. Obviously, if it is known to perform accurately on board, there is nothing against using it for checking another compass.
The Swing
The business of swinging the boat round from one heading to the next should be done as slowly as possible. Ideally, one complete circle should take about an hour, but patience, or the call of less mundane things, may not permit this. Do not, however, ` be tempted to swing too fast as this may have an adverse effect due to certain induction influences on the compass. Forty minutes should be the minimum. This process has only to be done once, if it is done well, and you will sleep the sounder for it later on.
C
With a well placed compass, the taking of bearings should also be fairly simple. All you need is Qne bearing on every two points, or better still, every ten degrees; thus sixteen or thirty-six headings and their related bearings.
You may find that on a particular heading the shore object is `wooded', i.e. obscured from the compass by some part of the boat's structure. Therefore, if possible, have up your sleeve the true bearing of an alternative object. You will also find, with experience, that using the modern azimuth circle it is quite easy to take a good bearing `through' some obstruction, providing this does not have too large a spread across the field of view.
Procedure. In calm weather, and with the use of an engine, it is possible to carry out a swing alone and unaided. However, it is much simpler if there is another person on board to handle the boat whilst the `swinger' takes the bearings and notes down results. Before starting the swing, prepare a notebook listing in one column the sixteen (or thirty-six) ship's heads and in the other leave spaces for the compass bearing of the object against each ship's head. If using more than one distant object or interpolating between transits, another column will be needed for true bearings. It does not matter on which heading you start, nor does it matter in which direction, clockwise or counter-clockwise, the boat is swung. Steady the boat therefore on the nearest `s convenient ` heading and take an accurate bearing of your chosen object. Write this bearing down opposite the ship's head concerned. Then swing slowly on to the next heading and repeat the process, noting each time the compass bearing (and the true bearing if using more than one object), Continue the swing until all headings have been checked.
A few tips—you must take great care in getting accurate bearings but it is not vital that the ship's head be exactly on the chosen heading when the bearing is taken. A couple of degrees either way will not matter. Secondly, if you decide to use transits, and interpolate between them whenever necessary, it is as well to draw a small diagram in your notebook showing the true bearings of the transits so that you can interpolate between them on the spot and not have to keep referring to a chart.
Tabulating the Results. Having completed the swing, the results
THE MAGNETIC COMPASSy 31
must be tabulated for future use. An example is given below, followed by explanatory notes:
A B C D E
-- ,~~°raw)
True ~r
Bearing of ` Distant Magnetic Compass
Ship's Head Object/ Bearing of Bearing of
by Compass Transit Object Object Deviation/`
Deg. (°) Deg. (°) Deg. (0) Deg. (°) Deg. (°)`
N 0000 223° 228° 2320 40 W
NNE 0221 223 228 2331 51 W
NE 045 223 229 234 6 W
ENE o671 223 228 233 5 W
E 090 *279 284 z87 3 W
ESE II21 223 228 23o 2 W
SE 135 223 228 229 I W
SSE 1571 *2791k 284 282 2 E
S 18o 223 228 223 5, E
SSW 2021 223 228 22I 7 E
SW 225 223 228 223 5 E ,..
WSW 2471 223 228 224 4 E
W 270 223 228 225 3 E
WNW 2922 223 228 226 2 E
NW 315 223 228 228 Nil
NNW 3371 223 228 230 2 W
(I) * Denotes second distant object used, as the first was `wooded'
on these bearings.
(2) Variation—at place of swing-5° West.
Explanation of this table
COLUMN `A'—SHIP'S HEAD BY COMPASS
The successive Ship's heads—by the compass being checked—
are written in this column.
COLUMN `B'—TRUE BEARING OF OBJECT
Two distant objects have been used in the course of this swing. The True bearings of the objects from the swinging position .. are taken off the chart and entered in this column.
COLUMN `C'—MAGNETIC BEARING OF OBJECT
This is the bearing of the object after the Variation has been applied to the True Bearing. The Variation in this locality
LZ `_3
Distance, Course, Speed and Direction on the Earth's
Distance, Course, Speed and
Direction on the Earth's
Surface
A good compass is an essential piece of equipment. However, it is not much use having a dependable compass but no idea of the proper course to steer to reach a particular destination or how long it will take to get there.
We are fortunate these days in that practically the whole of the Earth's surface has been either charted or mapped, and therefore the business of finding the course between two places is a simple matter. It was the British Admiralty that gave the major impetus to the science of cartography (map-making) in the 18th century by commissioning such men as Cook to carry out explorations and meticulous surveys and to record the observations they made. Nowadays, many nations produce charts from their own or other nation's hydrographic information, but the Hydrographic Department of the British Admiralty alone produces over 6000 different charts and diagrams in common use by seamen all over the world.
Distance on the Earth's Surface
The basic unit for the measurement of distance at sea is the nautical mile. This is a distance of 6o8o feet. Why this awkward figure ? It is in fact the distance subtended at the Equator by an angle of i minute of arc at the Earth's centre. The significance of this chosen distance is that I minute of Latitude = I Nautical Mile and therefore, as will be seen later, distance can simply be measured from the latitude scale on a chart without conversion or a separate scale being necessary.
For general purposes, the mile is considered to be 2000 yardsDISTANCE, COURSE, SPEED AND DIRECTION 47
long, divided into io cables of zoo yards apiece. The old fashioned term `league' was a distance of just under three miles but this is seldom used at sea today; nor are the statute mile of 1760 yards, furlongs etc.
Speed
A knot is defined as a speed of one nautical mile per hour. Therefore, do not fall into the old landsman's trap of saying in airy fashion `Oh! I was doing 20 knots per hour' as any bloke around might presume you were serving your time. Twenty knots means 20 nautical miles per hour.
If you have difficulty converting time and distance into speed and vice versa, remember this simple formula:
Distance (miles) = Speed (knots) x Time (hours) D=ST
Therefore, if you have covered I o miles in 40 minutes your speed must be:
I o= S x 60 or S= 6 0= 15 knots. 4
Another very useful dodge to remember is the `six minute' rule. Six minutes is, of course, one tenth of an hour. Therefore, if you know or can estimate the distance you have covered in six minutes, a straight multiplication of this distance by ten will give your speed direct.
e.g.
You travel 0.2 miles ( 2 cables) in six minutes. Speed 2 knots You travel 0.5 miles ( 5 cables) in six minutes. Speed 5 knots You travel I.2 miles (12 cables) in six minutes. Speed 12 knots You travel 2.5 miles (optimist!) in six minutes. Speed 25 knots
Obviously you can adapt this rule in your head to cover different times, e.g. for a three minute run multiply by 20, and for twelve minutes' run multiply by 5, etc.
Direction on the Earth's Surface
This is not quite as simple as it may seem at first sight as the Earth's is not a plane surface; nor is it a sphere but a spheroid
this figure the curve is exaggerated for illustrative purposes, but the courses are correct for an uninterrupted Great circle track.
Thirdly, the fact that the Earth's spherical surface has to be translated onto a flat sheet of paper sets a problem for cartographers and some distortions are inevitable.
Before discussing the practical solution of these problems it is necessary to explain the geographical reference framework that is superimposed over the Earth's surface to define position —the Latitude and Longitude grid that has been in use for centuries.
Latitude
As the Earth is almost spherical, distances between a point on its surface and a reference axis must be measured in angular units. The reference used for Latitude is the Equator, the great circle midway between the poles. Latitude is measured North and South from o° at the Equator to go° N at the North Pole and go° S at the South Pole. The Latitude of intervening places is defined by the angle that it subtends at the Earth's centre from the Equator. e.g. in Figure 7B places X, Y, and Z all subtend the same angle to the Equator and are therefore at equal Latitude. The small circle (not passing through the centre of the Earth) joining these places is called a Parallel of Latitude, in this case that of 50° N.
The Latitude of a particular point is given in degrees, minutes and seconds; for instance 500 00' oo" North (60 minutes in a degree-6o seconds in a minute). Remember also that I minute of Latitude = I Nautical Mile. Therefore, the parallel of 50° N must be 5o X 6o =3000 miles North of the Equator.
Difference of Latitude D'Lat.
The difference of latitude between two places both North of the Equator or two places both South of the Equator is the straight arithmetic difference between them, e.g.
First point 56° 29' 35" N Second point 31° 53 2711 N
D'Lat. = 24° 36' o8"
Longitude
This is also measured in angular units. As the Earth is circular, it could have been defined from an arbitrary place on the surface from o0-3600 in one direction. However, the terms East and West had been in use for centuries, and as England was in the forefront of the maritime powers in the 18th century, the position of the Greenwich Observatory was chosen as the datum line at Longitude o° or zero. From this place, Longitude is measured i8o° East and 1800 West. The `join' at i 8o° Longitude occurs in a line that runs through the Pacific (the Date Line). Lines joining places of equal Longitude are called MERIDIANS—these circles (Great circles) start from one pole, cut the Equator at right angles and pass down to the other pole. Figures 8A and 8B illustrate the Longitude grid superimposed on the Earth's surface.
It can easily be seen that a person standing at the Pole can travel from the Greenwich Meridian through I8o° East and
back through West merely by turning on his heel, but he would
Direction and Bearing
iIf have to walk around the whole Earth's circumference to change
Direction is basically measured from the meridian, i.e. the
longitude by the same amount at the Equator. In other words,
•
direction of True North if referring to the geographical
the meridians converge at the poles, and the distance between
meridian. Both True Course—and the True Bearings of
one meridian and another on the Earth's surface varies with
objects—are measured clockwise from the meridian and
flatitude. Also, if you travel up or down one meridian, you must
expressed as an angle between o° and 3600.
North
be travelling Nor or South.
III
Course
THE TRUE COURSE
Diference of Longitude
The angle between the boat's fore & aft line and the meridian
In much the same way as D'Lat., difference of longitude is
'
is her course. So if the boat is steering 260° True she will be
measured in degrees and minutes (and seconds if necessary) as
heading through the water in a direction I o° South of West.
an algebraic difference, e.g.:
8c.
(Figure)
First place 123° 00.2 E
I
If there is no tidal stream, current, wind, or compass error,
Second place 25° 43.6 E
she will also be making good this course over the ground.
D'Long. = 970 16.6
COURSE MADE GOOD
Life, however, is seldom so simple that one or more of these
or
disturbing effects does not exist in practice. Therefore though
First place 22° 10.4 E
the boat may be heading 260° through the water, a current
Second place 02° o8.6 W
setting to the South may result in the boat making a `course
made good' of 250°. This `course made good' can only be
D'Long. = 240 I9.0
precisely established by fixing, i.e. finding the geographical
position of the boat at a number of successive times, or it can be
Aslight trap arises when the two places in question are
estimated from knowledge of the currents and the amount of
l 8 i l i d the
neary 1o° apart in longitude, maybe across te Date ne.
Line.
`::
leeway the boat makes in a wind of a given strength and
The shortest way round must be taken—thus the D'Long
direction.
between
The course steered and the `course made good' are, therefore,
I O° o5.o E T
more often than not different directions, the first related to the
water and the second to the ground.
and
$ 70° 00.0 W
Compass Course and Magnetic Course
As explained in the last chapter, the Magnetic course is the
is NOT 1800 05.o but 1790 55.0' (18o° —I Io° 05.0 +180°70°)
angle between the Magnetic Meridian and the boat's fore &
aft line. Similarly, the Magnetic Compass course is the angle
Departure
between the direction of Compass North and the boat's fore
I: This is the distance in miles travelled in an East or West direction
& aft line. To arrive at a True Course, both variation and the
over the Earth's surface.
compass deviation (on that particular heading) must be
applied.
Departure = D'Long. x Cosine Mean Latitude.
The following example may help to summarise this:--
Bearings
True Bearings and Compass Bearings are defined in very much the same way. The True Bearing of an object is its angle measured clockwise from the meridian.
In Figure 8F, the lighthouse bears Ioo° True from the observer. The True Course to steer for the light is also I oo°.
As a reminder—when using a magnetic compass for taking
4i the bearing of an object, the compass bearing must be corrected for deviation and variation to obtain True Bearing (again remember to use the deviation for the boat's heading and NOT the direction of the object). Example:
Compass Course 300°—(Deviation on this course 30 E) Compass Bearing of object 125°
Deviation (30 E) + 30
Variation (12° W) —12°
TRUE BEARING = i16°
Relative Bearing
Relative bearings are commonly used for indicating the position of another object or craft relative to one's own vessel.
A relative bearing is measured in degrees from one's own ship's head, 18o° in each direction from the bow. The Starboard (right) side is designated GREEN and the Port (left) side RED.
In Figure 8G, the relative bearing of the other boat is Green 30°, i.e. 30° on the Starboard bow. As the boat's course is 025°, the True Bearing of the other boat would be 055°
DISTANCE, COURSE, SPEED AND DIRECTION 55
(025+30). Were the other vessel 30° on the Port bow, the Relative Bearing of it would be Red 300, and the True Bearing 355° (360 +25-30).
The terms Bow, Beam, and Quarter are also commonly used. Strictly speaking, an object said to be `on the Port Bow' without any number of degrees specified should be bearing Red 45°, i.e. 450 to port of right ahead—'on the Port beam' Red go°—and `on the Port Quarter' Red 135°. However, these terms are often more loosely used to indicate a general area, `,Port Quarter' meaning anywhere on the port side from right astern to Red 135°, `Port Beam' meaning anywhere from Red 135° to Red 45°, and `Port Bow' anywhere from Red 45° to right ahead.
7lie Great Circle and the Rhumb Line
Earlier in this chapter it was stated that the Great Circle route is the shortest distance between two places on the Earth's surface, and that steering a Great Circle course means a constantly changing heading. In practice, on a leg of less than about 200 miles, the distance lost in steering a steady course said not a Great Circle is not significant. Even when following a Great Circle route, it is not a practical proposition to alter the boat's heading continuously and gradually to follow the curve, as the rate of change of heading is so small even at high speed.
The Rhumb Line
I u order to steer a steady course over the Earth's curved surface, the boat must follow a track that cuts successive meridians at the same angle (otherwise it would be altering course). Such a track is called a RHUMB LINE—by definition a linc that cuts all the meridians at the same angle. On the Earth's surface it is, in fact, a spiral (Figure 8x).
In this example, a Rhumb Line course of 070° is shown. In I,ractice, the navigator always steers a succession of Rhumb Line courses, even when following a great circle track. This is explained in Appendix II.
Charts
The chart is to the seaman what a map is to the landsman—a pictorial representation of the Earth's surface. The chart, however, shows features both above and below the water line.
Different projections, or methods of transferring the Earth's curved surface onto a flat piece of paper can be and are used in chart construction. But by far the most common projection used for charts is the Mercator's projection. This is used for practically all charts of a natural scale smaller than i I inches = i mile.
Mercator's Projection
The simplest way to explain the Mercator's projection (which came into use at sea in about 1630) is to imagine a paper cylinder wrapped around the world. Imagine also that the Earth is fairly transparent and that a lamp is burning at the Earth's centre. The various land and sea features would then be shown up as shaded images on the surrounding paper.
Figure 9A is a representation of a standard Mercator projection where the cylinder touches the Earth at the Equator.
A Transverse Mercator projection is similarly constructed but in this case the paper cylinder is turned through ninety degrees and touches the Earth at the poles.
In fact, the simple explanation so far given is not entirely accurate, as the projection is mathematically adjusted to give the chart certain properties. The principal ones are:
i. Rhumb lines are straight lines on the chart. Therefore courses can be laid off on the chart or read off without
difficulty.
2. The Equator, which is both a Rhumb line and a Great circle on the Earth, is a straight line on the chart.
Longitude ' 111°'°"
of Greenwich
Long —
Longitude sc le C
IS C
10 ~.
0 5 io is
Latitude stale (S'to lin.) Long.
180°
Figure 9
3. The Meridians (great circles converging at the poles on the Earth) are straight, equidistant parallel lines on the chart, cutting the Equator at right angles.
4. The Parallels of Latitude (small circles on the Earth) are straight, but not equidistant, lines parallel to the Equator on the chart.
5. Great circles, on the chart, are curves whose apexes lie towards the poles.
Thus the Latitude and Longitude grid (or graticule as it is sometimes called) appears on the chart as a lattice of straight lines, with the meridians equally spaced, but with the parallels of latitude increasingly far apart the further they are from the Equator.
You will see in Figure 9B that the length of the line BC is greater than AB on the chart although both correspond to a difference of io° in latitude. The latitude scale on a Mercator chart is thus variable, steadily expanding the further North or
South the area covered, until at the Poles it is infinitely large. As this scale is also used for the measurement of distance, it is not possible to measure distances from anywhere on the scale.
Distance MUST BE MEASURED FROM THAT PART OF THE LATITUDE SCALE IN THE MARGIN OPPOSITE THE POINTS CONCERNED. For
example, the distance AD would be measured with a pair of dividers at EF on the scale.
The Longitude scale (across the top and bottom of the chart) must never be used for the measurement of distance.
Disadvantages of the Mercator's Projection
On the Earth, the parallels of latitude remain a constant distance apart and the meridians converge at the Poles. On the Mercator's chart, the Longitude scale is constant whilst the Latitude scale is made to expand with Latitude (it is actually related to the Secant of the Latitude). The result is that the land masses on the Mercator chart become increasingly `blown up' in size (although they retain their correct shape) the further North or South they are, and the ordinary Mercator cannot be used at the Poles where the latitude scale would be infinite. Except in the Polar regions this distortion does not matter practically. For instance, although Iceland and an Equatorial island of the same actual size on the same Mercator chart would appear totally out of proportion in size (Iceland seeming to be three times bigger than the other), the Latitude scale opposite to Iceland would also be three times bigger. Thus shape, distance, and bearing are preserved, but areas are distorted.
Constructing a Latitude and Longitude grid on a plain
sheet of paper
In ocean areas well away from land, it quite often happens that the scale of the only chart available is very small—a pencil point may be the equivalent of one mile. In such situations it is impossible to plot an astronomical fix accurately. It is therefore necessary to draw a Latitude/Longitude grid on a plain plotting sheet using a much larger scale, for instance 5 miles equals one inch.
This is easily done (as illustrated in Figure ga) by following this procedure:—Draw your Latitude scale horizontally across the paper.
(i) From the zero of the Latitude scale, draw a line at an :angle equal to the Latitude of your position—in this case 600.
(ii) Drop perpendiculars to this line from the Latitude graduations. Your Longitude scale is then given along the inclined line.
It is fairly easy to see from this that at Latitude go°, the longitude scale would be zero, whilst at the Equator (Latitude o°) the two scales would be the same This is the `mirror Picture' of the Mercator chart graduations (Longitude scale constant and Latitude scale infinite at the Poles). The main thing to remember is that, except at the Equator, the latitude graduations will always be further apart than the Longitude uses. (Note: This construction is only reasonably accurate over a comparatively small area—about 200 miles each way.)
[he Gnomonic Projection
The main property of this projection is that Great Circles appear on a gnomonic chart as straight lines.
The chart is drawn by projecting the Earth's surface onto a plane touching the Earth's surface at a chosen tangent point. II' the tangent point selected is a Pole, the Latitude and Longitude grid produced is quite simple, as shown in Figure n and E.
The Parallels of Latitude are concentric circles round the Pole, and the Meridians straight lines radiating from it.
Unfortunately, Rhumb lines are not straight lines on a Great Circle chart, and therefore course and direction cannot I determined directly from a small scale gnomonic projection.
On a much larger scale they are, however, used for harbour plans, where the small area covered can safely be assumed to be `flat'. Courses can be laid off on these charts, and a separate distance scale is normally given on gnomonic charts of this t yl>c, together with the Latitude and Longitude of a given `sl4Ot' on the chart.
The other uses of Great Circle charts and diagrams are for I Iar charts (where the Mercator projection becomes useless) and for Great Circle sailing.
British Admiralty Charts
The ordinary navigational charts published by the British Admiralty cover all the areas of the world. They are grouped in Folios, each folio covering a particular geographical area and containing charts of both large scale (harbour plans) and small scale (passage and oceanic charts). Each chart is given an individual distinguishing number (e.g. Chart No. i824a shows the East Coast of Ireland together with the Irish Sea and St. George's Channel).
The numbers of all the charts published by the Admiralty, their folio groupings, and their prices are given in the Catalogue of Admiralty Charts, which is itself re-published annually.
These charts are fairly expensive, averaging about 83 pence for an ordinary chart to CI .2o for one that is overlaid with a radio aid grid—such as a Decca lattice. The average yachtsman will not want to be too lavish when buying charts from a chart agent. The best way, therefore, to see what charts are available and to obtain the necessary area coverage without excessive cost, is to consult the diagrams in the chart catalogues showing the individual limits of each chart and then to choose accordingly. A reputable chart agent will hold copies of the catalogues and help with the selection, so a personal purchase of a catalogue is not essential. References to charts (and their limits) are also contained in the Admiralty Sailing Directions (or Pilots). However, the catalogue is the better reference, being republished every year whereas the Sailing Directions are not.
Yachtsman's Charts
Until recently, the Admiralty also published a series of yachtsman's charts and these are now produced commercially by firms such as Stanford and Imray. They have the big advantage for small boat use of being much smaller in size, which allows them to be used on small chart or saloon tables—or even on a portable board. Their current price is about go pence per chart.
Sizes of Charts
The ordinary Admiralty charts are designed to fold into a uniform size for convenient stowage in a folio, but they are not all identical in overall size; for instance some charts (mainly harbour plans) are printed on a single unfolded sheet; others (passage charts for example) are folded double and must be opened for use. If the space is available, an important point in a boat is to have a chart table of large enough size to allow a comfortable working surface—the minimum area needed to take an ordinary Admiralty chart unfolded is about 38 inches X 25 inches. Some Yachtsman's charts are 13 inches long X 8 inches deep, other about 21 inches X 15 inches.
Selection of Charts
To sum up—there are these two basic types of chart: the Admiralty charts in use by seamen, in ships large and small, all over the world and the commercial adaptations of these charts specifically designed for use in small craft in limited areas. Charts are not cheap and you will not want to spend more than is necessary. However, when planning a voyage, it is essential to have sufficient charts to cover all eventualities—for example, if going on passage down the Spanish coast it would be unwise to rely merely on a small scale passage chart; in the event of a blow, you might have to run for cover and it might prove difficult to trip your way through the rocks on the back of a small postage stamp. Therefore, you would need the coastal charts and harbour plans on that coast. Combine prudence with thrift!
Chart Titles and Numbers
Each chart has its own individual title and number. These chart numbers are geographically somewhat haphazardly distributed, so in ships carrying complete folios it is customary to give the charts another number—the consecutive number. The top chart of the folio would be given consecutive number I, and so on. Most boats will not be carrying complete folios, but it is not a bad idea to adopt a similar numbering system for the charts carried. A folio list on the front of the chart cover showing these consecutive numbers against each title much
simplifies the task of finding or stowing away a particular chart.
Chart Scale and Type of Projection
The natural scale of the chart and the type of projection on which it is drawn is shown just underneath the title. The natural scale is a straightforward comparison of distance on the chart against distance on the Earth's surface. With a scale of I/150,000, an inch on the chart therefore represents 150,000 inches on the Earth's surface, or about 2.08 miles.
This is of general interest, but of more practical importance is the scale of distance. Remember that on Mercator projections one minute of LATITUDE equals one mile, and that when using dividers to measure distance on the chart, that part of the scale (at the side of the chart) opposite the distance being measured must be used. NEVER use the Longitude scale, running across the top and bottom of the chart, for distance measurement.
On harbour plans—quite often drawn on the Gnomonic projection—a special distance scale is provided.
INFORMATION ON CHARTS
It is impossible to list here all the conventional symbols that are used on charts and getting to know them is largely a matter of experience. However, a special chart is published by the Admiralty (No. 5011) which gives all the symbols used on British charts and this is an easy means of reference.
Land Features
The amount of information given on land features is limited to that which is of practical use to the navigator at sea. Thus a considerable amount of detail is shown of the coastal areas, whilst inland areas are largely left blank—except where, for instance, a hill or mountain can be seen from seaward. In recent years, with the increased use of radar for navigation, contour lines and spot heights (e.g. ggo—a hill 990 feet in elevation) have become of more direct interest and greater detail is shown of these.
A B C D E
,.ry
O Chy o9 O Orr
Bn Bn
CHURCH LIGHTHOUSE CHIMNEY BEACON TOWER F
Figure r o
.just a few of the more common land symbols are shown in 'Figure io (A–E).
An object that stands out boldly when viewed from seaward may have the notation (conspic.) written on the chart against its symbol—a conspicuous object.
Lighthouses
These are a minor subject in theriselves. They are shown by the conventional symbol for a lighthouse surmounted in the case of a major light with a magenta coloured flash (Figure I OF) so that they readily catch the eye. Obviously on small scale ocean charts it would be impossible to include all the navigational lights on a seaboard, so only the biggest and best are shown.
The British Admiralty publishes a book called the Admiralty list of Lights (in a number of different volumes covering different parts of the world) which gives complete details on all the navigational lights. Their characteristics, heights, power etc. are all listed and also whether they are attended (lighthouse keeper) or unattended (nobody around!). In some places, a light may only be switched on at particular times or for a particular event. Do not therefore expect a light that is marked (Occas.)—occasionally—on the chart or in the Light Lists to lbc always switched on when you desperately need it—it may he that the local luminary has gone home, and one is not
rmally handily placed to ring him up and ask that it be lit.
' I'he lighthouses along a particular stretch of coastline all have different characteristics, so that one may not easily be confused with another. The main varieties of lights are Fixed, l- lashing or Occulting. A fixed light, self-evidently, is one that is on all the time; a flashing light is one whose characteristics
64 SMALL BOAT NAVIGATION CHARTS 65
are such that the period of darkness exceeds the period of light; and an Occulting light is one that is ON more than it is off.
If no colour is specifically mentioned, then it is white. The letters (W) (R) (G) are used on charts to indicate colour whenever necessary; white, red and green respectively. There is a complication in the existance of Alternating Lights (marked Alt. on the chart) . These successively flash the colours indicated.
obscured I white
1
green red
Figure ii
Thus Alt. W.G. means that a light will successively show first a white flash followed by a green flash throughout its visibility range.
These alternating lights must not be confused with Sectored lights, which show different colours continuously in different sectors. These sectors are normally marked on the chart by dotted lines, and the Red sector of a light is usually chosen to cover some particular hazard in that area. One other thing to remember about sectors—in the Light Lists, the sectors covered by a particular colour are always given in BEARINGS FROM SEAWARD. For example, a light showing White from 18o° to 29o0, Red from 29o0 to 000°, Green from 000° to 030°, and
Obscured from 030° to 18o0 would be shown on the chart as shown in Figure i I.
Some other points on lights that need to be mentioned. The `Period' of a light is the time it takes to complete one whole cycle of its operation, measured from the start of its first flash. Some examples again—a light that is shown on the chart as:
(/.Fl. (2) is a light that gives two white flashes in quick
I o sec. 29M succession—followed by an interval of darkness —every I o seconds. The 29M means that it is visible on a clear night 29 miles away from an observer whose height of eye is 15 feet above sea level.
0cc. W.R. This is a sectored light (white shown in one
I o sec. sector, red in another) which goes out for a
brief period once every io seconds.
Gfi.Fl. (3) This lighthouse shows two lights. The major
Io sec. 125 ft. light flashes three times in quick succession
17 M F.R. every io seconds. It is at an elevation of 125
I 12 ft. IoM feet above sea level, and visible 17 miles.
The second is a Fixed Red light at a slightly
lower elevation and only visible io miles.
(p.Occ. (q.) This light goes out four times in quick suc‑
W.R. 16 sec. cession every 16 seconds. The white light is
I 7,14M visible for 17 miles and the red for 14. Again
it is a sectored light.
All. W.R. This gives a white flash followed by a red
20 sec. 15M flash every 20 seconds all round the horizon
and is visible from 15 miles.
These are but a few of the characteristics of lights that may )c met with. The majority of the markings against lights on the charts are fairly self evident. If further information on a I).trticular light is required, a glance at the Light List will give the full details, for example how long each successive flash is in a Group Flashing light. In practice it is not often necessary to consult the Light List unless you do not possess a good chart of the area concerned.
Providing the visibility is good, the range at which a light is
66 SMALL BOAT NAVIGATION CHARTS 67
U
sighted can give a good indication of your distance from it. The loom of very powerful lights can normally be seen long before the light itself is sighted, but it should be remembered that lights at very high elevation may be obscured by cloud; and weak lights may be obscured by haze.
Depths on a Chart
At the present time, the depths on existing British Admiralty charts are normally shown in fathoms and feet (a fathom =6 ft.) On some large scale charts such as harbour plans, the depths may be shown in feet alone. It is normally self evident which depth scale is being used, as with the fathom scale, the odd number of feet between whole fathoms are shown as a small figure beside the fathom digit. A depth of 52 on a chart indicates that the depth in that particular spot is 32 feet i.e.
5 x 6+2=32). However, if there is any doubt, check the notation under the chart Title-it is always stated whether the depths are given in fathoms, feet or soon (perish the thought) metres.
For at the present time British charts are being converted to the metric system already in use on many foreign charts. The soundings will then be shown in metres and points of a metre. A conversion table for putting fathoms and feet into metric quantities is given below.
CONVERSION TABLE-FATHOMS AND FEET INTO METRES
Feet 6 12 18 24 30 36 42 48 54 6o
Fathoms I 2 3 4 5 6 7 8 9 10
Feet - i.8 3.6 5.5 7.3 9.1 10.9 12.8 14.6 16.4 18.3
1 0.3 2.1 3.9 5.8 7.6 9.4 11.3 13.1 14.9 16.7 18.6
2 o.6 2.4 4.2 6.1 7.9 9.7 11.6 13.4 15.2 17.0 18.9
3 0.9 2.7 4.5 6.4 8.2 10.0 11.9 13.7 15.5 17.3 19.2
4 1.2 3.0 4.9 6.7 8.5 10.3 12.2 14.0 15.8 17.7 19.5
5 I.5 3.3 5.2 7.0 8.8 io.6 12.5 14.3 16.1 18.o 19.8
Chart Datum
The depths shown on a chart cannot be taken as absolute depths, i.e. ones that never change, because of the rise and fall of the tide; indeed, meteorological conditions in some parts of the world can alter the level of a whole sea area at different seasons of the year.
When a survey is carried out, therefore, the depths found at all the different points on the sea bottom have to be corrected for the tidal conditions appertaining at the time and referred to a common datum before they are plotted and the chart drawn. This datum is known as Chart Datum, and for safety reasons is so chosen that the sea level seldom falls below it (it approximates to the lowest predictable tide level under average Incteorological conditions).
height‑
charted elevation
of lighthouse
__m_eanH~hWaterSprings____~
sea surface
mean sea level
heightoftide- i5ft. mean lnw Water Springs _ , _ _ _ _ , I _ _ _ _
actual depth beneath
charted depth 32+ 15 .- 47ft.
51I.31ft 35.23ft
bed " depth m 32ft.
Figure 12
From a practical point of view, it is not necessary to know the actual datum level chosen-i.e. 52.9 feet below an ordnance survey bench mark set into the Vicarage wall at Little Belchillglon; it is, though necessary to appreciate that the depth of water over a patch marked 21 (two fathoms one foot) will seldom be exactly 13 feet. It is likely for the majority of the iiine to be more than that, but there may be a few occasions when it is less. To find the actual depth over that patch at a particular time, the height of the tide must be calculated and added to it. Again, to err on the side of caution, the elevation of land features on a chart are referred to a different plane-Mean High Water Springs. (An explanation of Spring and
68 SMALL BOAT NAVIGATION CHARTS 6g
Neap Tides is given in the next chapter.) This reference is chosen so that their charted elevation will rarely be less than the figure shown. This point is perhaps of less importance these days in large ships when ships' navigators use radar instead of vertical sextant angles for taking the range of an object—but a vertical sextant angle can still be useful to the yachtsman not possessing any other ranging device.
Figure I2 may help summarise the difference between the various levels and datums:
The yacht is shown over a spot marked 52 on the chart; however the actual depth of water under her keel at that particular moment in time is 47 feet, the height of the tide being 15 feet.
Mean sea level is also shown; this can be taken as the average level of the sea surface over the whole year.
Fathom Lines
These lines, joining points of equal depth, are drawn on the chart with each different line having a conventional symbol
B yY'
-b-. 61 6L /
61
62 , 6ij
54
44 S 53 55 55` 54 ~oa +
D
F G H E
~ K `~ 1 ' ~7rcul
,~
PD Eb .~
Figure 13
C Rocky patch which does D Individual dangerous rock
not uncover—dotted line with less than 6 feet of water
encloses danger area over it
E Rock awash at the level of
Chart Datum
F Position of a wreck that is NOT G Wreck that is considered
dangerous to surface navigation dangerous to surface navigation
H Dangerous wreck—Position J Wreck—Existence Doubtful
Doubtful
K Foul
composed of dots or dashes (sometimes combined with colour shading) to indicate its value. However, it is not necessary to remember the individual symbol for each fathom lines as the soundings on either side of the line indicate its numerical value. For instance, the 6 fathom line might appear as shown in Figure 13A.
Drying Heights
Certain features are covered and uncovered by the sea, depending on the state of the tide; reefs or rocks near the shore, [r example, or sandbanks out to sea. A patch that dries 2 feet above Chart Datum is shown by the figure 2 with a line drawn underneath it, e.g. 2. Sometimes features are marked with the words `Dries ... feet' written alongside. The latter method is normally used with individual features—a rock for example, and the former to indicate a small area or patch.
An above water rock that does not cover at all would be marked as in Figure 13B. The numeral alongside gives its elevation above M.H.W.S.
1)angers
Some of the most important chart symbols for the dangers to navigation that lie beneath or on the surface are given in Figures 13c J.
1%oul Ground
The remains of a wreck or other debris on the bottom which is not dangerous to surface navigation, but which should be avoided by vessels anchoring. (Figure 13K.)
Type of bottom
The nature of the bottom is indicated by various letters written alongside the soundings. These are primarily of interest from the point of view of anchoring, as the nature of the sea bed will affect the holding power of the anchor. The best holding ground is thick mud or sand and the worst, rock and weed. The types of holding ground are (with the chart symbols) in approximately the correct descending order of preference:
70 SMALL BOAT NAVIGATION CHARTS 71
S......Sand
M ...... Mud Good Holding Ground Cy...... Clay
G...... Gravel Sh ...... Shingle
P ...... Pebbles Moderate
St...... Stones Sh ...... Shells
R ...... Rock Poor
Wd...... Weed
Buoyage
Navigational buoys are used where it is not possible or economic to build permanent marks such as lighthouses or beacons. Some dangers, such as sandbanks, shift their position quickly and therefore the marks showing the limit of these hazards have to be moved quite frequently. The major hazards in busy shipping lanes out to sea are sometimes marked by Light Vessels instead of Buoys.
The charted position of a buoy is the position of the small circle in its base. However, it must always be remembered that buoys, which are moored to the bottom by a length of chain, may be shifted by heavy weather, and therefore total reliance should not be placed in their position.
Two systems of buoyage are used—called the LATERAL and CARDINAL systems. The former is used in United Kingdom waters (and in the tidal waters of many other countries) and is the one described here. The other, CARDINAL system, is more commonly used in areas where there exist a large number of offshore islands, rocks or shoals, and the shape of a particular buoy is governed by its true bearing from the danger.
The Lateral System of Buoyage
With the Lateral system, the character of the buoys used is governed by the direction of approach, i.e. a port hand buoy (of a particular shape) must be left to Port WHEN ENTERING
A CHANNEL OR GOING with THE FLOOD STREAM.
The Flood Stream, as its name implies, is the Tidal Stream associated with a rising tide (the EBB stream with a fallingtide). Around the British Isles, the Flood Stream flows up the Channel as far as Dover. The Flood also flows up the Irish Sea around the North of Scotland and down the East coast of England. There is thus a meeting point near Dover where the water coming from two directions—up Channel and down the North Sea—meet.
The offshore buoyage round the British Isles is governed by this pattern of water movement. Coming up Channel from the West, a buoy marking the limit of a shoal extending from the Sussex coast would therefore have Port Hand Characteristics. A buoy in a similar position off Newcastle would have a Starboard Hand characteristic, and be left to Starboard.
The most important buoys used in the Lateral system are shown in Figure 14.
MIDDLE GROUND BUOYS
(Used to mark a shoal in the middle of a channel)
The topmarks on the buoys at the inner end of the channel are different from the outer marks. The lights shown by these buoys are distinctive whenever possible but follow the normal rules, e.g. red flashing or an even number of white flashes for the `Main channel to the Right or Channels of Equal Importance buoys' and white flashing with an odd number in the group for `Main channel to the Left' buoys.
Mid Channel Marks
These buoys are sometimes used to mark the centre of a channel —particularly at the entrance to some harbours where the position of the channel cannot otherwise be marked effectively. They can be of any shape, but must be easily distinguishable from the Port, Starboard, and Middle Ground buoys. The characteristics of their lights must also be distinctly different from neighbouring lights. See Figure 14.
Spoil Ground and Quarantine Anchorage Marks
Spoil grounds may either be the result of the dumping of refuse in an area or may be made unhealthy by the outflow of sewers, etc. Quarantine anchorages are used by ships with cases of infectious disease on board. In either event, the area is not
74 SMALL BOAT NAVIGATION CHARTS 75
altogether satisfactory as no indication is given of the varying rates at which the tidal stream increases in strength or slackens. This depends on the interval in time from the local High or Low water and whether it is a Spring or Neap tide.
The modern way of showing tidal information on a chart is by the use of Tidal Diamonds. Inscribed on various places on a chart are capital letters enclosed in a diamond, for example
~. Elsewhere on the chart, normally near the edge, will be found a block of tables, each with its alphabetical indicator above it. A typical example is:
490 52.2 N
5° 10.9 W Dim. Rate
Sp. Np.
Hours
6 256 r.8 o.9
0
5 254 1.2 o.6
A 4 234 0.4 0.2
3 045 0.4 0.2
0 2 054 1.0 0.5
V
1 059 i.8 0.4
H.W. 067 2.3 I.!
1 075 i.8 0.9
0
p 2 082 o.8 0.4
3 203 0.4 0.2
4 233 1.3 0.7
5 247 2.3 1.1
6 257 1.9 0.9
All that is necessary for the use of this table is to look up in Tide Tables the time of High Water on that particular day.
Let us assume that H.W. Devonport was at 1315 (GMT) and it was Spring Tide period. The tidal stream at 0715 would be setting in the direction 256° at I.8 knts and at 15io–o82° at o.8 knts.
Chart Corrections
When originally bought, the charts should normally have been corrected up-to-date for any details that have changed since the chart was published, e.g. a buoy moved, or newly discovered shoal inserted etc.
The date of publication of a chart is shown outside the bottom margin. However, if a lot of detail has changed in the course of time, a new edition of the chart may be printed—again the state of this new edition is shown near the date of publication. Then, if important information comes to light which is too complex to insert by hand on the chart, a Large Correction may be made. This, once again, involves the re-printing of the chart by the Admiralty—the date of the Large Correction being shown under the date of the new edition.
Outside these reasons for reprinting the chart in toto and discarding the old copies, a small correction system is used for keeping charts fully up to date. Every week, Admiralty Notices to Mariners are issued which give all the important corrections that need to be made to all charts. The corrections are suitably indexed so that each individual chart affected by a correction can be seen at la glance. These permanent corrections are normally made in pen and waterproof violet ink on the chart copy held by the individual owner. Sometimes, if the detail is sufficiently complicated to merit it, `blocks' are enclosed in cash copy of the Notice to Mariners; these can be cut out and pasted on the chart. It is a good tip to align these blocks into the correct position on the chart before applying any glue, and tick the corners with a pencil mark—then apply the glue and stick them on.
Some corrections are classed as Temporary or Preliminary corrections when a (T) or (P) is shown against the particular notice affected. These corrections should only be applied in pencil for, as their names imply, they concern impermanent changes—a light vessel being removed temporarily from its station for maintenance would warrant a temporary notice to
76 SMALL BOAT NAVIGATION
Mariners, Preliminary notices may later need to be inked in. The numbers of these small corrections are listed at the bottom of the chart concerned outside the bottom margin on the left, against the year concerned, e.g. 1968-22. 156. 273. means that in 1968, Notices to Mariners 22, 156 and 273 affected that particular chart and the necessary corrections to it have been made.
Chart correcting can be a tedious business, particularly in a small boat with limited space, but for general navigational safety in strange waters, it is a very necessary chore.
Admiralty Notices to Mariners can be obtained via any chart agent.s
Tides and Tidal Streams
or practical purposes, it is not necessary to have a detailed knowledge of the complex astronomical factors that give rise to the tide generating forces. The Moon and Sun, in that order, have a differential attractive or repellent effect on the water covering the Earth. It is largely the relative position of these bodies to points on the Earth's surface which determines the amount of attraction they impose on the water surface at a particular place and time.
As you know, the Earth rotates anti-clockwise about its Polar axis once every 24 hours. The Moon orbits around the Earth—in the same direction as the Earth's rotation—once every 291 days or so. Thus the lunar day—from an earthworm's point of view—is longer than the ordinary day by a period of about 50 minutes. If a spot on the Earth was directly underneath the moon at 1400 on the 17th July, it would not be directly underneath the Moon again at 1400 on the i 8th July, because the Moon would have moved on in its orbit and the transit will occur at a later time.
The Earth is also in orbit round the sun, completing a full revolution once every 365.3 days (hence the Leap Year every four years). To an observer on the Earth, the Sun appears to be • rotating around the Earth once every 24 hours; this is due to the Earth's rotation. But it is obvious that the Sun does not rise or set at the same times or on the same bearings every day of the year. This is principally due to the fact that the plane of the Earth's orbit round the Sun is not aligned to our Equator, but makes an angle of about 231 degrees with it. The apparent orbital path of the Sun around the Earth (the reverse way of looking at it) is called the ECLIPTIC. (See Figure 16A).
Twice during the year, on about March 21st and September 21St, the sun is directly over the Equator and the period of light
F
78 SMALL BOAT NAVIGATION
it
equals the period of darkness—I2 hours each. These are called the EQUINOXES (equal day and night). At other times of the year, the Sun's Declination—its angle above or below the Equator—varies until it reaches a maximum of 23 Z South on December 21st (the Winter Solstice) and a maximum declination North on about June 21st (the Summer Solstice). This is speaking as a Northern Hemispheric man! Down under, no doubt, they would want to arrange things differently.
Similarly, the Moon's Declination, or its position above or below the plane of the Equator, is not constant but varies between 28+ North to 281 South approximately every 13 days.
More important still, as far as the tides are concerned, is the phase of the Moon, its position relative to the Sun as seen from the Earth. A new moon—only a thin crescent visible—occurs when the Moon approaches a position between the Earth and the Sun. (When the Moon and Sun have almost the same declination, an Eclipse of the Sun may occur as the Moon gets between Sun and Earth.)
About 7 days later, the Moon will be at go° to the Sun—as seen from the Earth—and half of it will be illuminated. The Moon is then said to be in its First Quarter and it will be waxing —the illuminated portion increasing each night with the horns of its crescent pointing to the left.
Another 7 days and it will be a Full Moon on the opposite side of the Earth from the Sun. A full Moon will always, therefore, start rising over the eastern horizon near sunset.
For the next seven days, the Moon wanes until it reaches its Last Quarter—the other half illuminated—and in a further seven days it will be a New Moon once more.
When. the Moon is in its New or Full phases, the Sun's and Moon's gravitational forces combine to exert the maximum pull on the Earth's water surfaces.
Supposing Figure 16B shows the situation at a particular instant in time. The Moon and Sun—drawn wildly out of scale—are in `conjunction' and on the Earth's Equator, i.e. both have zero declination. In accordance with the normal gravitational laws, both the Moon and the Sun will exert an attractive force on the Earth. This force is considered as being directed from the centre of one body to the centre of the other and is inversely proportional to the square of the distance
80 SMALL BOAT NAVIGATION TIDES AND TIDAL STREAMS 81
between them. Therefore, although the Sun has a far greater mass than the Moon, the Moon's attractive force in this context is the greater.
Obviously, these attractive forces must be balanced by other forces due to the bodies' orbital speed around one another or a collision would result, but these other forces need not be considered here.
The Moon and Sun, then, exert an attractive gravitational force of a certain value at the centre of the Earth (C). At Point (E) in the Diagram they will exert the equivalent force PLUS a certain quantity, as that point is nearer to them. At Point (W), they will exert the equivalent force MINUS a certain quantity.
These differential forces have their greatest value on the Earth's surfaces nearest the body and farthest away—at points N and S they would be zero as those points lie at the same relative distance as the centre of the Earth. The effect of the forces on the solid land surface is negligible, and as far as the Earth's atmosphere is concerned they merely cause a slight change in atmospheric pressure. But the water on the Earth's surface, like the air, is free to respond, and being fairly dense, reacts to them.
As a result, water is drawn towards E and away from the heavenly bodies at W. At other points, e.g. between N and E and N and W, the attractive force is not directly towards or away from the local vertical.
At these intermediate points, the total force can be resolved into vertical and horizontal components, and the horizontal components will move the water towards E and away from W. The result of all these forces is to cause a `bulging' of the water surfaces at E and W which progressively reduces towards the points go° away—N and S as shown in Figure 16c.
The Earth's Rotational Effect
Clearly, if the Earth did not rotate, nor the Moon and Sun change their declination, there would be a permanent high water at E and W, and a permanent low water along the Meridian of N to S.
But the Earth does rotate and the Moon likewise around the Earth. In one Lunar Day (of about 24 hours 50 minutes) any particular point on the Earth's surface will therefore feel (Moonand Sun having zero declination) two High Waters and two I ow Waters. This is called a SEMI-DIURNAL Tidal Pattern, i.e. in ordinary time, a High Water followed by a Low Water about 6 hours 12 minutes later, another High Water at 12 hours 25 minutes, a Low Water at i8 hours 37 minutes, and the cycle completed by the following day's High Water at zq. hours 50 minutes.
,Spring Tides
The earlier diagram illustrated the situation with the Sun and Moon working in conjunction on the same side of the Earth—at the period of a New Moon. The tide raising forces are pulling together and are at a maximum. These are occasions when SPRING TIDES, those that rise highest and fall lowest, will occur. '1 The example also illustrated the particular time of the year when the Sun had zero Declination, at the Equinox, when the highest and lowest tides of a semi-diurnal nature are likely to occur in the whole year—hence the expression EQUINOCTIAL
SPRING TIDES.
Spring tides will also occur when the Sun and Moon are in `opposition' at the opposite sides of the Earth at the time of a full moon, as shown in Figure 16D.
In this context, the term `opposition' is misleading as, in fact, the tide raising forces of the two individual bodies are still additive. The Moon's gravitational pull exerts Force M at the Earth's centre, M +m at W and M +m at E. The Sun exerts Force S at the centre, S +S at E and S—s at W. They both exert therefore a differential force towards E and W as before. SPRING tides result, and when a FULL Moon occurs at the Equinox, EQUINOCTIAL SPRING TIDES will occur.
.Neap Tides
'These are tides which have the smallest rise and fall in height. They occur when the Moon is in its Quarters, i.e. Half-Moon. The differential forces of the Sun and Moon are now applied at c)o" to each other.
As a result, the Moon is trying to raise High Water at N and S, whilst the Sun is trying to raise High Waters at E and W as shown (diagrammatically only) in Figure I6E. The Moon, being closer, will win, but smaller amplitude tides will follow.
82 SMALL BOAT NAVIGATION TIDES AND TIDAL STREAMS 83
The Elect of Declination
So far, the Moon and Sun have been shown at zero Declination. However, in the case of the Sun this occurs but twice a year, and for the Moon every 6-' days. To consider the Moon alone for a moment, and the diagram in Figure 16F.
Having a certain Northerly Declination, the Moon will raise the maximum `humps' in the water at points A and B. A point `D' as it rotates with the Earth's rotation will experience no Tide Raising force at D, but a maximum at D1. Thus only one High Water and one Low Water per day will occur. These are called DIURNAL TIDES.
The Sun's Declination will have a similar, but smaller effect. The Semi-Diurnal Tide Raising effect is therefore greatest when the Sun/Moon's Declination is zero, and the Diurnal Tide Raising effect greatest when both have maximum declination.
Each ocean area has a natural period of oscillation—the `slop' in a basin. The Atlantic responds readily to semi-diurnal tide generating forces, whereas the Pacific is most affected by those of a diurnal nature. The tides round the British Isles, therefore, are mainly semi-diurnal in character.
Summary
To summarise these effects: At New and Full Moon, the attractive forces of the two bodies combine to produce SPRING tides, i.e. tides with the greatest difference in height between High and Low Water. When the Moon is in its Quarters, NEAP tides result, tides that have the smallest difference in height between High and Low water. (The Spring tides actually occur usually about I day after the Moon is New or Full as there is a time lag in the force taking effect.)
At certain times of the year, all the factors attracting the tide generating power of the bodies—such as Sun's declination, Moon's declination, phases of the Moon, combine to produce the highest tides of the year. This occurs about the time of the Equinoxes—hence the expression Equinoctial Spring Tides.
In European waters, the tides are almost invariably semi-diurnal in character; two high and two low waters a day, with the time of the first high water becoming about fifty minutes later each successive day. However, although the astronomicalforces are the basic cause of tides, their effect in particular areas is much affected by the shape of the sea or ocean, the gradient of the sea bed, etc. If a particular ocean has a natural oscillatory frequency near to the period of the tidal forces, then high tides will result. The North Atlantic is subject to some of the highest tides in the world; on the other hand, there is very little tidal movement in the smaller enclosed basin of the Mediterranean.
In some other areas, notably in the Pacific, the tide is diurnal in character, i.e. only one high water and low water a day.
In other places, local anomalies may cause a stand of the tide; in Southampton water, for example, there may be as many as four high waters a day, the shape of the Solent and Spithead causing double high tides.
Over the years, a great deal of information has been gathered to enable the tides, particularly in European waters, to be predicted with considerable accuracy. However, these predictions only apply when the normal meteorological conditions for the time of year pertain. Large falls or rises in barometric pressure can alter the height of the sea to quite a marked extent, and strong, persistent winds can build up the sea level against a coast line (remember the floods on the East Coast). Any tidal prediction is subject to unforeseen factors such as this, and cannot therefore be infallibly correct.
Tide Tables
The British Admiralty publishes annually three volumes of Tide Tables, and based on much the same data, a good number of other tide tables are produced, either for local use, or for inclusion in Almanacs, such as Reed's.
Part I of the Admiralty Tide Tables is published in three volumes:
Vol. i—covers European Waters (including the Mediterranean)
Vol. 2—covers the Atlantic and Indian Oceans
Vol. 3—covers the Pacific Ocean
These tables give the times and heights of High and Low waters for a large number of the major ports—called Standard Ports. They also provide methods of working out the heights
84 SMALL BOAT NAVIGATIO `.
and times of tides at an even greater number of smaller place (Secondary Ports) based on a Standard Port in their vicinity.
Tables are also provided for working out the height of the'. tide at times between high and low water. The method of doing these calculations is not identical in each volume of the Tide Tables. It is not intended to go into these calculations here, as a full explanation of the calculations and examples of their practical solution are given in each volume.
Apart from the Admiralty Tide Tables, there exist a large number of other tables produced by publishers and local authorities. These are normally perfectly satisfactory for finding out the heights and times of high and low water, but do not always give a facility for calculating the height of a tide at intervening times. There is not much doubt that the Admiralty Tide Tables will give the widest range of ports and the most accurate predictions, but there are others, notably those contained in Reed's Nautical Almanack, which may suffice for the average yachtsman's needs.
Whichever Tide Tables you use, always be careful to check the Zone Time of the predictions. For example, a lot of the U.K. tables are published in G.M.T. and therefore in England, when Summer Time (Zone —r) is being kept, one hour needs to be added to the predicted times, e.g.:
10.00 G.M.T. = I I.00 B.S.T. (Zone-I or A) (I IooA)
DEFINITIONS
A few simple definitions may be of help in de-cyphering Tide Tables.
Height of a Tide—is its height above or below Chart Datum. If the tide falls below chart datum, the height is shown in the tables as a minus quantity, e.g.—x.2 (feet).
Range of a Tide—is the difference in feet between the heights of successive High and Low waters, e.g.:
High Water Height 16.5 feet 19.5 feet
Low Water Height o.2 feet —o.6 feet
Range of Tide = 16.3 feet 20.1 feet
TIDES AND TIDAL STREAMS 85
Duration of a Tide—is the time interval between successive High and Low waters.
The Interval—is the time difference between High (or Low) water and the time at which it is desired to predict the height of tide. Spring Tides—Occur roughly once a fortnight—normally in the day succeeding a New or Full Moon. They are tides with the greatest range, and consequently produce the strongest Tidal Streams.
.Neap Tides—occur at the intervening weekly intervals. Smallest range, and therefore weakest Tidal Streams.
Mean High Water Springs and Mean High Water .Neaps—are the
average heights of high water at springs and neaps, taken over a long period. Mean Low Water Springs and Mean Low Water Neaps are calculated on the same basis.
Other Methods of Tidal Prediction
The Tide Tables are produced by the careful analysis of a large number of harmonic constituents related principally to the movements of the Moon and Sun, but this sort of work needs the services of a tidal machine or computer, and only the results are of interest to the seaman. Next down the ladder is a method of prediction using a few of these harmonic constituents to produce a graph of a particular tide at a particular place. This gives a very accurate result in working out the height of a tide at a secondary port, but it is fairly complex and slow and would not normally be used by small boat sailors. (Those interested will find it in Part III (not Volume III, Part I) of the Admiralty Tide Tables.) Next in accuracy come the predictions for the Secondary ports given in the Tide Tables.
There are, however, a couple of rough and ready, and not very accurate, methods of predicting tides where these are semi-diurnal in character. These are by using the non-harmonic constants shown on some charts. The constants are:
Mean High Water Lunitidal Interval (terrible mouthful—so
M.H.W.I.)—This is the interval between the Moon's transit at Greenwich and the next following High Water. To find the time of the Moon's Meridian passage you will need to look in a Nautical Almanac; to this time you must add M.H.W.I. to find the time of High Water. To find the time of Low Water, add or subtract 6 hours I2 minutes from the High Water time.
86 SMALL BOAT NAVIGATION ' TIDES AND TIDAL STREAMS 87
High Water Full and Change (H. W.F. & C.)—This term is sometimes given on the older charts. It means that when the moon is full or changing, the next high tide in the area concerned will take place at the stated interval after the Moon's meridian passage. It is therefore similar to M.H.W.I. above.
Both these constants can be made use of, without reference to the time of the Moon's meridian passage, if the chart you are using states the constant and you know the time of High Water at another place along the coast—and the constant there. The difference between the two constants, added or subtracted from the time of High Water at the other place, will give a reasonable approximation for the time of High Water in your own area.
There is one other useful approximation concerning the likely height of a tide at intermediate times between High and Low Water, assuming that there is a regular semi-diurnal tidal pattern. The level is likely to change in the following way:
During the First hour (after H.W. or L.W.) By 7%
During the Second hour (after H.W. or L.W.) By 18%
During the Third hour (after H.W. or L.W.) By 25%
During the Fourth hour (after H.W. or L.W.) By 25%
During the Fifth hour (after H.W. or L.W.) By 18%
During the Sixth hour (after H.W. or L.W.) By 7%
So, assuming that the Range of a particular tide is zo feet, and the height at High Water was 24. feet, the expected heights of tide would be:
At the end of the First hour after H.W. 22.6 feet
At the end of the Second hour after H.W. Ig.o feet
At the end of the Third hour after H.W. 14.0 feet
At the end of the Fourth hour after H.W. 9.o feet
At the end of the Fifth hour after H.W. 5.4 feet
At the end of the Sixth hour after H.W. 4.0 feet
Note: This method is only very approximate. The decimal points
are entirely unrealistic in practice.
Tidal Stream Tables
Apart from the Tidal Stream and Current information shown on charts by means of tidal diamonds and arrows, other books exist which give additional information. For Tidal Streams,
some of the most useful are the Pocket Tidal Atlases covering the waters around the British Isles. There is one, for example, for the Channel, another for the Solent, and another for the Irish Sea. They display, by means of arrows, tht* expected set and rate of the tidal stream for each hour before or after High Water at the port on which they are based (in most cases—Dover). These are particularly useful books for yachtsmen as they are small, comparatively cheap, easy to use and contain a lot of information in condensed form.
Unfortunately their equivalents do not exist, to my knowledge, outside U.K. waters. However, The Admiralty Tide Tables for foreign waters (Vols. II and III) include some Tidal Stream tables amongst the other tidal predictions.
i rr-‑
This is the nub of the business of navigation and pilotage; messy and unmethodical chartwork can quite quickly lead to that `indefinable feeling of impending doom' and the hire of a prayer mat. So decide that you are not going to wear out the latter from the start by being accurate and neat in your workings on a chart. Speed is sometimes also necessary, but should not normally be sacrificed for accuracy.
The simple instruments required for use on a chart are:
I. A Soft pencil-2B or softer. Hard pencil marks are difficult to erase and the chart will not remain usable for long if it is deeply scored with hard pencil marks. A clutch pencil, with a box of spare leads, is ideal as it can be sharpened easily without spraying the chart table, boat, and the soup with chippings.
2. A good soft rubber.
3. A pair of dividers—these can be bought in various shapes (bow or straight legged) and sizes; a pair with 6" legs is convenient for general use, although you may occasionally need a longer pair. Personally, I prefer the straight type as they seem more easily handled with one hand. They should be workable with one hand, but not so slack that they will not stay fixed at a set spread.
4. A pair of compasses—any pair will do providing they work satisfactorily and will hold pencil or lead.
5. A Station Pointer or Douglas Protractor. The former may be
quite expensive unless you can pick up a second-hand one cheaply but is extremely useful for plotting horizontal sextant angle fixes. It consists of a central circular scale, engraved in degrees, to which is attached three long legs; the central leg is
CIIARTWORK, FIXING AND PILOTAGE HINTS 89
fixed to the scale but the outer legs can be set and clamped at any angle. Good instruments of this type are normally made of brass or gunmetal and have a vernier arrangement to allow the angles to be set accurately to within a few minutes of arc.
You will probably be familiar with the Douglas Protractor—a large square plastic protractor engraved with parallel lines across its face and in degrees around the edge. This is cheap, and also useful for plotting fixes; its limitation lies in its size for when plotting fixes on a large scale chart you may `run out of protractor' whereas the Station Pointer will still cope. Also, of course, angles cannot be marked very accurately on a Douglas Protractor.
6. A Parallel Ruler. This is essential. It is used for transferring courses or bearings from one part of the chart to the compass rose, so that they can be read off, and for the plotting of position lines. Parallel rulers are available in two types, roller or hinged. The roller type is much the quickest and easiest to use if the chart table is sufficiently large to allow the chart to be laid out flat upon it. A sketch of this type of ruler is shown in Figure 17A.
It simply consists of a rectangular strip of brass, wood, or plastic. At each end is a roller projecting slightly beneath the under surface, the two rollers being joined to a common axle (in some newer types with nylon rollers there is no axle but a double roller arrangement which serves the same purpose). These rollers only allow the ruler to move in one direction unless they are lifted off the chart surface, and therefore parallel lines can be transferred from one area of a chart to another. This type of ruler does not, however, work satisfactorily on uneven surfaces.
The second type is the hinged variety (Figure 17B) wherein two rectangular strips are moved by two pivoted metal arms. The ruler is moved across the chart by successively keeping one strip held to the paper whilst pivoting the other. It is much slower and more fiddly to use than the roller on a flat surface, but it is lighter and easier to operate when the chart is folded or laid on a bumpy surface.
For laying of a course or bearing with a parallel ruler, see Figure 17c.
With either type in hand, suppose that you want to lay off a course of o30° from Point X on the diagram above. The parallel
eIIARTWORK, FIXING AND PILOTAGE HINTS 91
ruler is laid on the nearest `compass rose' on the chart and one edge aligned through the centre of the rose to the 0300 graduatlurr. The ruler is then slid across until an edge passes through I'4)irnt X. A pencil line is then drawn from this edge and can be extended as necessary.
Additionally, it will be necessary to find the bearing between WI) points on a chart, or the course between them. The ruler in t lien first aligned with one edge through both points and su I I )sequently moved across to the nearest compass rose. The edge joining the centre of the rose to its circumference will give (lie course or bearing required. (Point Y bears 2100 from Point X i r i the diagram above.)
7. A Notebook for `keeping the reckoning'. A lay-out for this
r rt ~ I (,book is discussed at the beginning of Chapter 9.
The easiest way of getting oneself lost at sea is to keep no II u ord of the boat's position on the chart, and no written details If course alterations, fixes, etc., in a notebook. The starting
I r~ psi lion is always known (presumably!) but a plot must be kept oil t lie chart of the boat's subsequent movements.
'l lie Dead Reckoning Position
The Dead Reckoning or D.R. position is arrived at by allow‑
J u ge only for the boat's true course and speed through the water nce leaving a known point. Looking at Figure 17D, let us
s u y a boat left Point `S' at iioo. It steered ogo° for I hour, :r l i ered course to 1300 at 1200, and altered course again to ogo° it i400; then its D.R. position at 1700 allowing for a boat's nIuc•ed of four knots is Point `T':
There are certain conventions in plotting the D.R. The 4u„rses steered are sometimes written against the track as rullurwn, and the D.R. position is always shown as a small `tick' o r ross the line with a time against it. The distances (bracketed l're) would not normally be written on the chart, but merely measured off with a pair of dividers.
It is also conventional to use a single arrow to indicate the u Irse being steered through the water.
1 he Estimated Position (E.P.)
'Tile Estimated position allows for the best estimate of tidal rit ream and leeway, etc., being applied to the D.R. position
92 SMALL BOAT NAVIGATII
In other words, it is the best estimate of the boat's position o~ the ground allowing for course, speed, current or tidal stream as leeway, since the last known position of the boat.
The symbol for Estimated position is a triangle with a dot it, viz. p 1300. (The time is also written alongside—as with D.R. position.)
Figure I 7E shows much the same example as before; ti D.R. position at 1700 is Point `T'—but the E.P. at 1700 Point `T1' allowing for an estimated set to the North West i
over the six hours since i ioo. Note that the direction of set is normally indicated by three arrows along the line.
A Fix
A fix is the positive establishment of the boat's position at given time (although some fixes may be more positive an honest than others). The conventional sign for a fix is a circl with a dot in it—and a time against it, viz. 0 1400.
A fix may be obtained by a variety of means and its accurac' will depend on many factors. The methods of obtaining a fi: given below are those mostly commonly used in a small boa without radio aids or radar.
I. THE TERRESTRIAL FIX
(Visual bearing of charted objects)
This is the simplest form of fix and one of the most accurat( under normal circumstances with a good compass.
If a bearing is taken from the boat of a single charted objet on shore and this bearing (corrected for compass errors) i plotted on the chart, it has been established that the boat mus lie somewhere on a line reciprocal to that bearing. For example in Figure 18A, the compass bearing of a lighthouse at i000 wa: 085°. The Variation was io° W and the Deviation (for th( boat's heading) was 2°E.
Compass Bearing 085°
Variation —Io
Deviation +2
True Bearing 077°
94 SMALL BOAT NAVIGATION CHARTWORK, FIXING AND PILOTAGE HINTS 95
This bearing (077° True) can now be laid off on the chart (using a parallel ruler from the nearest compass rose) as a single position line. The boat must lie somewhere along that line, but how near or far from the lighthouse is not known. Note also that the lighthouse bears 077° from the boat, but the line, with its arrowhead, is drawn away from that bearing, and the time is written along the line.
A single position line is not, in itself, a fix as the distance off the object has not been established. However if TWO or more almost simultaneous bearings can be taken of two or more different charted objects, a fix is obtained (Figure 18B).
The lighthouse again bore 085° compass, but a bearing of the beacon was taken a few seconds later and turned out to be 350° compass. The True bearing of the two objects (Variation Io° W and Deviation z° E) was therefore 077° and 342° respectively. Using a parallel ruler, the two lines are laid off on the chart from the objects and the boat must lie at their point of intersection. A circle is drawn round this point and the time written alongside the fix.
A word of warning about a two bearing fix. The boat need only be at the point of intersection of the two lines if:
a. The compass is accurate—its errors known and properly allowed for.
b. The charted objects have been correctly identified and the bearings taken accurately.
c. The plotting is also correct.
Factors (b) and (c) are largely a matter of practice and experience and (c) is not the most common cause of error. The accuracy of the compass or object identification are more often the doubtful factors. For these reasons alone, it is unwise to put unwarranted faith in a single two point fix, particularly where the angle of cut of the two position lines is less than 30°. In such a situation, any compass errors (or any other errors for that matter) have a greatly magnified effect. Suppose that bearings were taken of two different objects—the bearings were o85° and 105°. These were then plotted on the chart—assuming for simplicity that both the variation and deviation were nil (Figure 18c).
By all appearances the boat should be at Point Y. But there isonly zo° difference between the two bearings, and suppose that the compass was a few degrees in error ... say 5° when the bearings were taken. The plotted bearings should have been ogo° and I Io°. The boat is, in reality, at Point X, a considerable distance away from its supposed position.
On the other hand, a go° cut between the two bearings taken is ideal; whatever the compass error, its effect is minimised. Assuming the same error ... 5° and bearings of 355° and o85° (Figure I8D) .
Point S is the supposed position and Point T the true position. T is much closer to S than in the previous example.
Therefore, when taking a two-point fix:
I. Try and obtain bearings of objects about go° apart.
2. Do not use objects less than 30° apart if this can possibly be avoided—if you must, treat the resulting fix with some suspicion.
It is usually far better, if sufficient objects are visible, to use a 3 or 4 point fix. Again, with a three-point fix the optimum choice is three objects each 6o° apart, and for a four-point 45°, viz. Figures 18E and F.
However, these separations are not critical; the aim must be to keep a difference of at least 30° between position lines.
The `Cocked Hat'
The great advantage of using three or more position lines in a fix is that the quality of the fix is immediately apparent when it is plotted on the chart. The three lines should pass through a point, and if they do, you can be pretty sure that the boat was in the position shown at the time of the fix. (There is one exceptional circumstance when this does not hold true and this is mentioned below.)
Perhaps though, the three lines do not pass through a single point when they are plotted. In this event, the area of uncertainty produced is called a `Cocked Hat' (see Figure i8G).
All that can be assumed initially in this case is that the boat is somewhere within the area enclosed by the three lines. In open water it is normal to assume that you are in the centre of the area of doubt. But close inshore or in the proximity of
g8 SMALL BOAT NAVIGATION CHARTWORK, FIXING AND PILOTAGE HINTS 99
III. FIXING BY RANGE AND BEARING
This is a common method of fixing in use at sea today; a compass bearing and simultaneous range is taken of a charted object. In ships, the range is normally obtained by radar— outside the capabilities of the normal small boat. Some yachtsmen may possess a small optical range finder but these are comparatively expensive instruments. The majority of the larger cruising boats will, however, have a sextant on board. The principal use of this instrument is in finding the altitude of the sun, moon, stars and planets above the visible horizon but it can also be used for measuring the vertical and horizontal angles subtended by charted objects. Vertical sextant angles of objects of a known height, with the appropriate corrections applied, can be looked up in tables listing angles against range. There are several books which include tables of this nature, but one of the most compact and useful, containing tables specifically for this purpose, is Lecky's "The Danger Angle and Offshore Distance Table'.
The procedure is quite simple. For instance, a lighthouse is shown on the chart to have an elevation of 130 feet. The sextant, held vertically, is used to measure the angle between the centre of the lantern of the lighthouse (the charted elevation) and the sea surface beneath it. Suppose the angle measured was o° Ig.8'. The first correction that must be applied is the Index Error of the instrument itself (see Chapter 8).
Sextant Altitude of lighthouse o° I9.8'a
Index Error +01.2
Observed Altitude = o° 21.0'
Consulting Part I of Lecky's tables—Heights of objects from 50 to I I oo feet and distances from o. I to 7.5 miles—the range of the lighthouse can be read straight out as being 3.5 miles. The only correction to be applied when using this part of the tables is that for Index Error, as shown. A range is therefore obtained very simply.
Part II of these tables covers heights of objects from 200 to
I 8,000 feet and distances from 5 to I 10 miles. This section is
normally used when trying to find the range of the land whilst you are still a considerable distance away from it. The peak of the island of Fogo in the Cape Verde Islands has an elevation of 9281 feet and may be seen at great distances. Approaching from the South, from your D.R. position you estimate that you are 48 miles off it but wish to find the range accurately for a fix. Using Part II of Lecky's tables, there are two additional+corrections which must be applied to the Sextant Altitude before entering the tables to obtain the range. These are the Angle of Dip (for the observer's height of eye above the water) and the other (I/12th of the estimated distance) for refraction—the bending of the light rays in the atmosphere. The Sextant Angle is therefore corrected for:
a. Index Error.
b. Dip (from another small table in Lecky's)
c. Refraction—I/12th of estimated distance (correction in minutes of arc) .
FOGO, elevation 9281 feet. Your height of eye in the boat is io feet, estimated distance off, 48 miles. Having taken the Sextant Altitude between the peak and the horizon between you and the island, this turns out to be 10 20.0'.
Sextant Altitude I ° 20.0'
Index Error +01.2 (same as before)
Observed Altitude 10 21.2
Dip —03.4 (always minus—this for height of eye of I o feet)
10 17.8
Refraction —04.0 (always minus—this for
(1/12th of estimated 48
distance) 12 =4 minutes of arc) Angle = I0 13.8'
Looking up I ° 13.8' in Part II of the tables against an altitude of 9200 feet gives a range of 52.1 miles for Fogo's peak. Note
I00 . SMALL BOAT NAVIGATION CHARTWORK, FIXING AND PILOTAGE HINTS I01
that Part II of Lecky's tables are intended for use with objects lying beyond the sea horizon.
The plotting of a visual range and bearing fix on the chart is very straightforward. The bearing must, of course, first be corrected for compass errors to obtain the true bearing before it is plotted. Theoretically also, when using bearings of objects at very long range there is a discrepancy between observed bearings and those taken from, or plotted on, the chart, due to distortions involved in the construction of the Mercator projection. However, for practical purposes, this is of no significance except in high latitudes. It is important, though, to plot `long range' bearings very carefully.
Therefore, plot your true bearing first, draw the position line from the object, and then with a pair of compasses or dividers set to the range obtained, draw the range circle from the object on the chart. The point of intersection of the bearing position line and the range circle is the fix position. Then write the time against it. This fix is shown in Figure I8M.
IV. FIXING BY TWO OR MORE RANGES
If the ranges of two or more objects are taken almost simultangeously, using vertical sextant angles or any other means,the position circles from these objects intersecting on the chart will give a fix. This is a good method of fixing if suitably defined objects of known height are available. (Figure I 8N.)
V. FIXING BY USING HORIZONTAL
SEXTANT ANGLES
This is a highly accurate method of fixing which can be used when precision is vital, e.g. for laying a buoy or mooring in the correct spot or in charting the position of a navigational mark. It is aethod that has been used by surveyors for many years in the normal course of their business. Its main disadvantage is that it is a comparatively slow method of fixing and the plotting of horizontal sextant angles is a somewhat pernickety job.
Horizontal sextant angles are taken by holding the sextant on its side, handle underneath; one object is viewed directly through the clear glass section of the horizon glass, and the reflected image of the other object is superimposed on it by moving the scale micrometer or vernier. The angle between the two can then be read off.
A minimum of three charted objects must be used to obtain a fix, as the angles on each side of the centre object are needed. If there is a large height difference between any of the objects as viewed from the observer's position, it may be difficult to superimpose the objects correctly; therefore objects of more or less equal height are preferred. The objects chosen should also be more than 300 apart to give a good cut. Ambiguity can again exist if the objects and the boat all lie on the circumference of the same circle (as commented upon earlier when discussing `cocked hats'). It is therefore safer to choose a centre object nearer to you than the other two—or objects in a straight line—so that such a situation cannot arise.
Once the angles have been observed, they can be plotted onto the chart with the aid of a `Station Pointer' or `Douglas Protractor'. The Station Pointer is the more accurate of the two.
To use the Station Pointer, set the two angles on the outer arms. Supposing that the central object was a church, the left-hand object a lighthouse, and the right-hand object the right-hand edge of a cliff (Figure I9A) .
The Station Pointer, with its arms set to the appropriate angles, is laid on the chart and moved around until the bevelled edges of each leg pass exactly through the charted position of the objects observed. The boat's position is then the centre of the instrument, normally marked by a small `V' notch through which a pencil mark can be made on the chart.
The result is a good, accurate fix achieved independently of any compass bearing.
The Douglas Protractor is designed for use on exactly the same principles, but it merely consists of a transparent plastic square, with an engraved, matt upper surface graduated round the edge in degrees, and with a small hole at its centre. ,.
The left and right angles are marked with a pencil on the matt surface, and lines drawn from the edge to the centre of the protractor. It is then placed on the chart and moved until the
F.
I04 SMALL BOAT NAVIGATION
cannot choose, so a less accurate method of fixing has to be
4 used. The Running Fix is that most commonly used in these circumstances. A bearing is taken of the only object visible, the time noted and the position line from this bearing plotted on the chart. Later when the bearing of the object has altered sufficiently—due to the boat's movement—a second bearing is taken and plotted. The first position line is then `transferred' up to the second line, allowing for the boat's course and speed over the period between the two bearings. This should be the boat's course and speed `over the ground' if it can be estimated from past history. If not, the boat's course and speed through the water must be used. It is this `distance run'—or the distance travelled in the time between the two bearings—which is the weakness of this type of fix. Any inaccuracy in the estimation of the distance run will show up as fix inaccuracy, without, of
course, the navigator being aware of it.
An example again. Variation 8° E, Deviation 3° W for a course of 2700 by compass, speed 6 knots. A wireless mast bears 3400 at 1030 and 0500 at 1245. What is the boat's position at this time? (See Figure IgD.)
Compass bearing 3400 Compass bearing 0500
Total correction +5 +5
True bearing 345 True bearing 055
True Course 275°
As you do not know precisely where you are at 1030, it is normal to take the point on the position line nearest the D.R. position—thus Point X. From Point X, allow for a run of 2'J hours at 6 knots = 13.5 miles. Plot this, Point Y, on your presumed track since 1030. Then draw a parallel to the 1030 position line through this point. The intersection of the transferred position line (conventionally shown with two arrows) and the 055° (1245) bearing is the fix position at 1245•
The more accurate the run between fixes, the better the fix itself. If you had been moving along a coastline prior to obtaining this running fix and had been able to take two three-point fixes in the previous few hours, you might estimate your distance run for the running fix differently (Figure I 9E).
Between o600 and o800, although steering a course of 275°CHARTWORK, FIXING AND PILOTAGE HINTS I05
True, the course made good over the ground (measured with a parallel ruler between Points X and Y) is found to be 2710. Also, by measuring XY with a pair of dividers, the distance made good in the 2 hours was 14 miles=a Speed Made Good of 7 knots. From Fix Y, now lay off a course of 2710 and allow 21 hours at 7 knots = 17.5 miles to bring you to a 1030 Estimated Position—Point Z.
The beacon has now disappeared, but the transferred position line from the Radio Mast at o800, combined with the 1030 bearing of the same object, gives you a fix at 1030 to the North of your Estimated position but South of the course steered.
Doubling the Angle on the Bow
An old dodge, but this in fact is really a form of running fix. If the time is taken when an object is at a certain angle on the bow—say 30° on the port bow—and it is taken again when the object is twice this angle on the bow (6o°), the distance run between the two times will be equal to the boat's distance off the object at the later time. For example, in Figure i 9F, a lighthouse was 30° on the Port bow at i000, course ogo° speed 6 knots, and was found to be 600 on the bow at I03o. The boat should then be 3 miles off the object at 1030.
This check is not much use if any tidal stream or current is present, i.e. the course and speed made good is different from that steered. If a set is expected, it is better to use a Running Fix allowing for an unknown tidal stream. Finally, in the case of 45° (doubling to go°) on the bow—called a `four point bearing' —the distance run will be equal to the distance off the object when it is abeam.
Running Fix with unknown Tidal Stream (Figure I 9G)
Suppose that a fix was taken previously at Point W at ogoo. The lighthouse bore 308° at i000 and 346° at 1030. Presuming that you were at Point X at 1000, the speed made good is
WX - =4 knots. Then insert Point Y allowing for half an Time I
hour's run at the speed over WX, i.e. 4=2 miles for the distance 2
XY. Your fix position at 1030 is then point Z, the course made good WZ, and the current YZ, setting towards Z.
Io6 SMALL BOAT NAVIGATION CHARTWORK, FIXING AND PILOTAGE HINTS I07
Other Dodges
Some less positive methods of obtaining position are given below. Strictly speaking, these are not `fixing' methods in the accurate sense, but may nevertheless be very useful in giving a good indication of the boat's probable position.
Using Soundings
A sharply defined fathom line can, on many occasions, be a good navigational aid, particularly when approaching land. Perhaps you might be approaching Cape St. Vincent from the West in thick weather and be uncertain of your exact position. The bottom shelves steeply in this area and the Ioo fathom line is sharply defined about 5 miles off the Cape. If your boat is luxurious enough to be fitted with an echo sounder that will reach that far down, well and good. Providing you keep outside this depth, the boat cannot come to any harm from grounding—the hazard is more likely to come from large ships rounding the Cape. But not all boats have echo sounders nor a lead line much more than 20 fathoms in length, so it is a question of getting the best out of the equipment held on board. A sharply defined 20 fathom line is an obvious target in these circumstances for the gaining of a position line at the least risk, and a glance at the chart will soon show whether this is a practical proposition.
In many places, the seabed slopes too gently or is too undulating for the crossing of a particular fathom line to immediately ring up three red cherries. However, unless the bottom is absolutely flat, there is a technique which can assist in the use of soundings to establish a position. This is to take soundings regularly, noting the time and depth, and adjusting the latter for the likely height of tide. At the same time, keep a careful record of the course and speed. After a long enough time to allow a reasonable picture to be built up, place a piece of tracing paper on the chart, marking on it a line to indicate North. Then estimate the boat's probable course and speed made good over the period, laying off this course on the tracing paper. Finally, plot and mark the depth of each sounding along the track, using the chart scale to measure the distances between soundings. The resulting sounding pattern on the tracing paper is then moved around on the chart, keeping the North/South line roughly parallel with the chart meridians,until the pattern fits the sounding marks on the chart in the vicinity of the probable position.
Horizon Range
At night, the approximate range of a lighthouse can be estimated from the moment when it just appears above, or dips below, the horizon. Visibility must be good, of course, and the light of sufficient power or intensity to reach the horizon and some distance beyond it. The intensity of all lights is given in the Admiralty Light Lists, but a glance at the chart usually gives sufficient clue on this point as the major lights will show long visibility ranges. It is the actual raising of the light itself above the horizon or disappearance below it that must be taken. The Loom—the beam only visible sweeping around the horizon— may be seen long before or after the actual light is seen.
A table in Lecky's, mentioned previously under vertical sextant angles, can be used for calculating the range; there are similar tables in each List of Lights, and in other marine navigational tables, such as Inman's Nautical Tables and Reed's Almanac/c.
Taking Cape St. Vincent as an example; the light there is described as Fl. 5 sec 264 ft. 23 M—i.e. it flashes white once every 5 seconds, has an elevation of 264 feet, and is visible to an observer, whose height of eye is 15 feet, at 23 miles. If your height of eye is I o feet, the light should appear above the horizon at the following range:
Range of light for its 264 feet elevation 18.7 miles
Range of light for your I o feet elevation 3.63 miles
Light appears or dips at 22.33 miles
Range from Sound Reflection
Only of practical use really when one is confronted with the situation of being stuck in the middle of a Scottish Loch or Norwegian Fjord with fog coming down, depth too great for anchoring, and the beer running out.
The echo of any noise you care to make (which might be considerable) will return to you, so that:
I08 SMALL BOAT NAVIGATION
Distance in feet of boat from cliff = Time in seconds X 1130
2
or if this sum is too difficult to work out in your head, the rough distance is:
Distance in Cables = Time interval in seconds x o.9
SOME PILOTAGE HINTS
`Shooting Up' Objects
In an unfamiliar area, it is important to be able to identify objects quickly and accurately; otherwise a lot of time will be wasted trying to plot position lines that do not meet and close cocked hats that refuse to shut. Or, there may be a very conspicuous object in the area which is not charted but would otherwise be useful for fixing.
There are several ways of `shooting up' an object. The quickest method is to get the unknown object in transit with a charted object, and at the same time take a bearing of the two. The bearing, laid off with a parallel ruler on the chart from the known object, will often identify the doubtful one.
This system can also be used in another way. For example, going up a narrow channel, it may be necessary to identify buoys quickly and there is not much time for plotting fixes (see Figure 2oA) :
The target is the mooring buoy at X, passing between the Port hand buoy at Y, and the Starboard hand buoy at Z. From the chart, it is obvious that there are a lot of buoys in this channel and it may be difficult, on rounding the corner, to identify one from the other. To save the embarrassment of either going aground or pinching someone else's mooring, before reaching this point find out from the chart the true bearings of these three buoys when they are in transit with the pier-head on the South shore. Convert these bearings to compass bearings by applying the variation, and the deviation for a heading of o65° Then, quick bearings of o8o°, I io° and 115° on the pier-head on the approach will help identify these buoys.
Getting an uncharted object marked on the chart; if the
a
Tidal Stream and Current
Problems
Tidal streams and currents vary greatly in strength not only from one ocean or sea to another but also between adjacent local waters. The coasts of the North Atlantic are uncomfortably placed in this respect, the tidal range being generally large, and therefore strong tidal streams abound. Tidal streams of over 5 knots are not unknown round the British Isles.
Due respect must therefore be paid to these streams and currents, particularly in small craft where the boat's speed in poor sailing conditions may well be less than the rate of the stream.
With respect to chartwork, there are several individual problems—where tidal streams are concerned—that need to be solved:
I . To shape a particular course to a given point allowing for a known tidal stream; and as a refinement to this, to arrive at a particular place at a particular time allowing for the set.
2. To estimate the boat's position after a given time allowing for a known tidal stream.
3. To find the rate and direction in which an unknown tidal stream is setting.
4. To use this information to arrive at a destination at a given time.
i. To shape a course to reach a given point allowing for a known tidal stream (Figure 21 A)
In this case, you know your starting and end positions, the direction and rate of the tidal stream, and the boat's speed through the water. Wanted—the course.
You are at Point X and wish to reach Point Z, i.e. the line XZTIDAL STREAM AND CURRENT PROBLEMS I13
A x
3 w
B X
--. 3 u
Y
2'S 1300
y 1800
D M
L Z x 1930
2400 K 6'
C Y
z x
1200 0 _ --- K
x E
Y
Figure 21
must be the course made good (C.M.G.). The tidal stream is setting (towards) 2500 at 3 knots, and your boat's speed is 5 knots. The construction is quite simple. Draw XZ and XY, the set and rate of the tidal stream-25o° length 3 miles (if using a I hour triangle on the chart). Set your boat's speed (5 miles) on a pair of dividers or compasses and with one point on Y, cut off along the line XZ. Then:
YW = The course to steer
XZ = The course made good
XW = The speed made good along XW/WZ
Measuring XW off the same scale as that used previously for XY and YW, you can find its value in knots (about 2.3 knots) .
XZ (distance to go) Then XW (speed made good)
allows the calculation of the time of arrival at Point Z.
I14 SMALL BOAT NAVIGATION TIDAL STREAM AND CURRENT PROBLEMS I15
The example shown above has assumed a fixed boat's speed‑
5 knots. However, you may wish to arrive at Point Z at a particular time, and must therefore calculate not only the course to steer but also the boat's speed through the water to achieve this. The basic construction is the same, but in this case you must first work out what the speed made good has to be over the distance XZ. For example: You are at Point X at 1200, and would like to arrive at Point Z at 1600 (Mother-in-law's waiting).
The distance between the two, from the chart, is I2 miles—I2 miles to be covered in four hours. Therefore the speed made good has to be 3 knots. (Figure 2IB.)
XW must therefore be 3 (miles/inches/units of any sort). XY, the tidal stream is also 3; the triangle can only be closed by joining YW, and this will give the course to steer and the boat's speed required. About 087° at just under 6 knots. (This is the True course—not compass.)
2. To estimate the boat's position after a given time allowing for a known tidal stream (Figure 21 c)
As has been explained in Chapter 5, the characteristics of the streams around the British Isles are now fairly well known and information about them for particular times and places is given in the Tidal Diamonds on the charts. Let us say that the Diamond off the Isle of Wight at 1200 on October 2nd states that the stream is setting o65° at 2.5 knots. Your position is south of St. Catherine's Point Light and making for the Needles, steering 305° True at 6 knots. Wanted—your E.P. at 1300.
Lay off XY your course and speed through the water-305°
6 knots. Then lay off YZ, the direction and rate of the tidal stream, which was 065° 2.5 knots. Your E.P. at 1300 is then Point Z, your course made good the direction XZ, and the speed made good is the distance XZ.
With any of these problems, you can use any units you like in the speed triangle, i.e. I knot= I inch, mile, centimetre or what have you. But as you are working on the chart, it is more convenient to- use the chart scale and make, when you can, I knot = i mile. On some charts of small scale, this would not be satisfactory and then you could use I knot =5/10 miles. Nor is it necessary solely to use one hour's run for each vector of thespeed triangle. Providing the same time is allowed for each vector, any period can be used—j- hour, 2 hours' worth, etc. These comments only apply to the speed triangle. Distances must, of course, be measured on the scale of the chart and it is easy to forget this if using different units for speed.
In the example above, it is obviously simpler to use an hour triangle at i knot = I mile. Note also the conventional arrow symbols on the sides of the triangle:
Course and speed through the water One arrow
Course and speed made good Two arrows
Tidal stream direction and rate Three arrows
3. To find the set and rate of an unknown Tidal Stream, and
4. to use this information to reach a destination at a given time (Figure 2ID).
In this example, a fix is obtained at Position J at I 800. The second fix is obtained at Position K at 1930, the course and speed steered in the interval being 24o°-3 knots. You wish to reach Position L by midnight if possible, and the distance between K and L on the chart is 15 miles. What course and speed should be steered assuming that the tidal stream or current remains the same?
JK must be the course made good between 1800 and 1930. The course and speed steered during this time are then laid off from J-240° at 3 knots=24o° for 4.5 miles over II hours. This gives Point M. MK must be the direction and distance by which the boat has been set to the South-East over the II hours, i.e. 120° for 2 miles. Therefore the current must be setting
towards 12o° at a rate of 2 X 2 = 1.3 knots.
3
You wish to make good a course between K and L and have qj hours in which to cover the 15 miles. That means a speed
made good of 15 X 2=33 knots.
9
Anywhere along the line KL, lay off the tidal stream vector XY-12o° at 1.3 knots. Then from X along XL, lay off the speed to be made good—XZ—which is 3.3 knots. Then YZ is the course to steer, and the length YZ the boat's speed required —about 4.5 knots.
116 SMALL BOAT NAVIGATION
Alternatively, if insufficient wind or power is available to make this speed, you might want to find out at what time you will reach Position L at your present speed through the water.
Lay off the current vector as before—then with the dividers set to your speed, cut off along XL at Z. XZ is then the speed made good (Figure 2IE).
XZ turns out to be about i.8 knots. Therefore it will take
I$ = 8 hours 20 minutes to reach Position L at your present speed, and you will not be there until 0350. Bad luck!
._ 1 'l ,
Magnetic Compasses and chart table instruments have been described in earlier chapters, but there are other instruments and navigational aids in common use which are of interest to the small boat owner.
Perhaps one ought to start with the Gyro Compass. This is not generally used in the smaller boats, mainly no doubt because of its cost, but also because it needs a continuous power supply of one sort or another and a certain amount of maintenance. Small North-seeking gyro compasses are fitted in some fairly small craft, and it is fair to say that they are generally more accurate than a magnetic compass—and, of course, no irritating corrections for variation and deviation to worry about. They do suffer from errors, but unless something is radically wrong, these are normally of a fixed variety. Incidentally, gyro error is normally classified as `High' or `Low'. A gyro error of 10 High means that the compass is over-reading by i°, and therefore one degree must be subtracted from the compass reading to obtain True course or bearing. They have another advantage in that a number of repeaters can normally be driven off the master compass, and therefore a good all-round view of the horizon can be gained for the taking of bearings, and the steering repeater sited in an ideal position.
However, all these advantages must be set against the price. Before leaving the subject, though, it might be worth mentioning an interesting (and cheap) adaptation of a gyro compass which I came across a few years ago. The owner of this boat had on board a small portable air-blown free gyroscope, which had probably been originally fitted in an aircraft. This was connected by a rubber pipe, through a reducing valve, to a
II8 SMALL BOAT NAVIGATION NAVIGATIONAL INSTRUMENTS AND AIDS I19
compressed air bottle. The procedure was then quite simple—when a bearing was required, the gyro was fished out of its stowage (it only weighed about a pound) the card aligned to the True boat's heading, and the air switched on. It ran up to full speed in a few seconds and could then be used anywhere in the cockpit area for the taking of bearings. Its advantages over a hand bearing magnetic compass were that the card was absolutely steady, however much the boat was moving about, and it was not, of course, affected by any local magnetism in the boat.
A free gyro, as was this instrument, is not North-seeking and suffers from the normal apparent drift due to the Earth's rotation. This is 15° per hour, so four minutes would have to elapse before the gyro drifted one degree. This is more than enough time to take the bearings for a three-point fix. If anybody wishes to copy this idea, then I should refer them to the owner, Wing Commander Crammond, but I regret that I do not know his whereabouts at the present time. Perhaps, also, similar instruments are now available commercially.
Sextants
There are two main types; those that use the horizon as a reference (marine) and those that use a bubble (aircraft). The marine type is the more accurate—naturally!—but to dispose of the opposition first. Bubble sextants are difficult to use at sea —bubble acceleration caused by the boat's or ship's movement makes it very difficult to hold the bubble steady. This is vital, of course, to the accuracy of the readings obtained as the bubble is the horizon. It is unfortunate that they cannot be used satisfactorily at sea, for they would give the facility for taking astronomical sights throughout the night, which is not possible with the normal marine sextant.
Marine sextants have remained largely unchanged in design for many years, although the method of taking readings from the sextant scale has been altered, the `micrometer' drum replacing the now obsolescent `vernier' scale. A standard marine sextant, capable of being read to an accuracy of about o. I of a minute of arc, now costs about £75 new. A second-hand vernier sextant can probably be picked up for about £50. Such an accuracy is desirable for astronomical sights, but is not reallyneeded for horizontal and vertical sextant angles. There are a few smaller marine sextants about, which may be picked up second-hand for about £Io to £2o, that only read to the nearest minute of arc but are good enough for sextant angle fixing.
A marine sextant has two mirrors, the top one (called the Index glass) being fully silvered and the bottom one (the
incoming Index I ss
for
from object '~--- ~ Screw V
A Horizon (adjustment perpendicularity)
glass
Screw W Frame
Eyepiece
of telescope
incoming from /
horizon or I
another Handle
r
Index bar
e es
r ~/
Screw X Scale
Scale window
IMicrometer ~'*—Spring clamp drum r'" 340 33° 32° 31° 30°
Main scale { B
on arc— I
Vernier- t 2' I'0-8 0.604 02 0'
Y Z
Figure 22
Horizon glass) half silvered and half clear glass. An observer can thus see the horizon (or any object) directly through the clear part of the Horizon glass. At the same time, he can see the image of another object reflected from the top mirror to the silvered portion of the bottom mirror. If the two mirrors are exactly parallel, the reflected and direct images of a single object should be coincident and the scale reading o° oo.o'.
Figure 22A gives a rough idea of what a sextant looks like when held vertically (as it would be when taking sights) and
I20 SMALL BOAT NAVIGATION NAVIGATIONAL INSTRUMENTS AND AIDS I2I
viewed from the side. This is only for those who are not familiar with the instrument, and it does not show various details, such as frame supports, shades, etc.
The Index Bar is movable over the whole arc of the scale, the spring clamp grips holding it locked at roughly the desired angle once they are released by the thumb and forefinger. Fine adjustments, also affecting both the reflected image and the scale reading, are made by turning the micrometer drum. The sextant can be set, therefore, to about the right angle by moving the Index Bar until the pointer in the scale window shows the correct number of degrees and the micrometer drum will click into place near this reading once the spring clamps are released. It is important to refrain from trying to move the Index Bar without, or only partially, releasing the spring clamps as this may cause damage to the worm on the micrometer drum or to the rack on the sextant scale.
Suppose that the instrument is to be used for the taking of a vertical sextant angle. First make sure that the smaller (star) telescope is fitted. (The sextant box normally contains two telescopes; the longer (sun) telescope gives greater magnification but produces an inverted image.) Then focus the telescope using the horizon or any other convenient object. Holding the sextant by its handle in the right hand, point it at the object keeping the horizon in view through the clear glass. Release the clamps with your left hand, and move the index bar until the top of the object heaves into view in the silvered part of the horizon mirror. Let the clamps go, and move the micrometer drum until the horizon and the top of the object are exactly level with each other. Now read ofd and on!
Reading off an angle is much easier with a micrometer than a vernier sextant, particularly when the light is not very good. Micrometer first; the degrees in the angle are read off opposite the pointer in the scale window—simple! The minutes are read off a scale inscribed around the drum opposite a pointer attached to the frame—also simple. The points of a minute are dealt with on the vernier principle; beside the frame pointer near the micrometer drum, you will find marked divisions—o.2, 0.4, o.6, o.8 of a minute. Whichever of these divisions is most nearly opposite one of the divisions on the micrometer drum, that division gives the points of a minute.
The vernier sextant is used in the same way, except that fine adjustments are made by a tangent screw attached to the bottom of the index bar, and the method of scale reading is different. In the first place, the main scale on the are is more finely graduated than on a micrometer sextant, so a movable magnifying glass is attached to the frame to assist. Secondly, the degrees, minutes, and points of a minute have to be deduced from the relative positions of the main scale on the are and the vernier scale outside it.
The vernier scales used on sextants can be read to 0.2 minutes of arc. In Figure 22B above, it is shown as a straight scale for ease of illustration although, in practice, it would be curved.
Each of the small divisions on the main scale is the equivalent of I o minutes of arc. Therefore the arrowhead indicating the zero of the Vernier scale is showing a main scale reading of between 300 Io' and 300 20' (Point Z). The starting point is, then, 300 io'. To find the additional minutes and points of a minute, look along both scales until a mark on each exactly coincides. This occurs at the point marked Y. There, the reading on the vernier scale is 2.2'. Thus the whole angle is 300 10' plus 02.2 =300 12.2'.
When taking horizontal sextant angles, the sextant is laid on its side, handle underneath, and the left-hand object viewed through the horizon glass. The actual image of one object is then brought into coincidence with the reflected image of the other on the dividing line between the clear/silvered parts of the horizon glass. This is easier to do than it sounds and should cause no problems.
Sextant Errors
Some errors, to which a sextant is susceptible, can be removed by its owner, and these are listed below in the order of treatment (potential doctors please note) :
a. Perpendicularity. The index glass must be perpendicular to the plane (frame) of the instrument.
Remedy: Put the index bar roughly in the centre of the arc. Hold the sextant horizontally and look into the index glass from outside—towards the arc of the scale. The reflected image of the arc should join (be in line with) the direct view of the arc. If it is
I22 SMALL BOAT NAVIGATION NAVIGATIONAL INSTRUMENTS AND AIDS I23
not, take the small cover off Screw V (see diagram) and turn this screw until all is as it should be.
b. Side Error. The horizon glass must be perpendicular to the plane of the instrument.
Remedy: Set the scale reading to about zero. Hold the sextant vertical and point it at a star. When the index bar is moved by the micrometer, the reflected image of the star should pass directly over the direct image. If it does not, but passes to one side or other, take the cover off Screw W (this is the one on top of the glass away from the sextant frame) and adjust it until the correct answer is achieved.
c. Index Error. Mirrors not exactly parallel when the scale reading is zero.
Remedy: This is the most important error, but the one which must be dealt with last as adjustments of the other two errors may well affect the index error.
By day, choose a section of the horizon which is clear and sharply defined, hold the sextant vertical, and adjust the micrometer until the reflected and direct images of the horizon are exactly in line horizontally. Then read off the scale. At night, select a bright star, point the sextant directly at it with the scale reading set near zero, and superimpose the two images. Read off the scale as before.
The Index Error readings thus obtained may either be `on' or 'off' the scale. Supposing that when the images or the horizon, or a star, were brought exactly into coincidence, the scale showed a reading of o° 02.3'. This is `on' the scale and indicates that the sextant was over-reading by that amount. The Index Error was then—o2.3' of arc. If, on the other hand, when image coincidence was obtained, the scale reading was 58.8', the sextant would be under-reading and the error `off the scale' by o1.2'. The Index Error was then +I.2 minutes.
Either way, this Index Error can be corrected by adjusting Screw X—at the side of the frame of the horizon glass. Watching the horizon or star through the sextant, and with the scale set precisely to o° oo.o', turn the screw until the images are coincident. Finally, re-check the Side Error, and have a last Index Error check.
Note: Particularly with vernier sextants, it may not be worthtrying to remove Index Error if it is less than about 3 minutes. It can always be allowed for by adding or subtracting its value to/from observed altitudes or bearings.
Care of a Sextant
Sextants are fairly expensive instruments and should there- fore be looked after properly. One or two points:
a. Always pick a sextant up by its frame, and carry it either by the frame or handle. Never carry it by the mirrors, or put it down on a surface upside down so that the mirrors are taking the instrument's weight. Obviously it should not be knocked about, and its replacement in its box after use is a good safety precaution.
b. Do not stow the sextant away for long periods encrusted with salt—wipe the mirrors and frame over carefully with a soft cloth and fresh water. Then oil the scale and drum.
INSTRUMENTS FOR MEASURING DEPTH
The Echo Sounder
Quite a few boats are now fitted with echo sounders, so perhaps a few remarks about these instruments would not come amiss. The basic principle on which they work is very simple—A sound pulse is transmitted from an oscillator attached to the boat's hull below the waterline, reaches the seabed, and is reflected back to a receiver in the boat. As the speed of sound in water is known (although it varies between waters of different densities—sea and fresh for example), if the time is measured between transmission and reception of the pulse, a depth can be obtained.
In shallow water, the time interval between transmission/ reception is very short and can only be measured by electronic means. Most echo sounders, too, work on a supersonic frequency so that one cannot actually hear the transmissions. The main difference between the various echo sounders on the market is in the way the information received is displayed.
Some of the more sophisticated echo sounders use a recorder-type display where the incoming bottom echoes are marked on
I24 SMALL BOAT NAVIGATION NAVIGATIONAL INSTRUMENTS AND AIDS I25
a moving trace of impregnated paper by an electric stylus. This has the advantage of enabling a permanent record to be kept of the seabed over which the boat has passed. This can be very useful in constructing a sounding trace—as explained in Chapter 6—and it is more or less essential for surveying work. However, most yachtsmen are only interested in knowing what their depth is at the time, and there are other types of display which show this more simply. Some take the form of a circular gas-filled tube, marked with an appropriate depth scale. When a transmission is made, a spot of light starts rotating around the tube, and this brightens when any echo is received from the seabed. This is quite a good, practical display. The choice of an echo sounder really depends, once again, on your pocket. There should not be a great deal that can go wrong with an echo sounder, as it is a comparatively simple piece of electronic equipment, but on the whole the more expensive—the more reliable.
One point before leaving this subject—be careful to check whether your echo sounder is set to read from the waterline or the keel. In certain circumstances, this might make quite a difference!
The Lead Line
For those working on a limited budget, the luxury of being able to take a depth at will merely by pressing a switch is probably unattainable. So, one must resort to the bracing life and time-honoured handraulic methods; the lead line, or possibly in very shallow water, a suitably marked stave or boathook.
A lead line should be made out of very pliant rope that does not kink or retain any loops or bights once it is uncurled. Ordinary stranded rope is no good for this purpose but special plaited rope is sold. The rope is spliced through a ring on the top of the lead weight. The lead itself has a hollowed-out section at the bottom which can be `armed' with tallow. When the lead is recovered, it should be therefore possible to identify the nature of the bottom from whatever is sticking to the tallow. If it comes up with a bit of shell attached—shells. If it comes up with a fly-button ... ?
There is a standard way of marking a boat's lead line, although this will presumably be changed when the charts all
become metric. There is no necessity to use this method if you
do not wish to do so—any method will do providing it is clear
to the user—but it has the advantage of being familiar to a lot
of people. The fathoms—starting from the lead end—arc
marked as follows:
One fathom One strip of leather (One Fathom
Two fathoms Two strips of leather =6 Feet)
Three fathoms Three strips of leather
Five fathoms Piece of White bunting (cloth)
Seven fathoms Piece of Red bunting
Ten fathoms Piece of leather with a hole in it.
Thirteen fathoms Piece of Blue bunting
Fifteen fathoms Piece of White bunting
Seventeen fathoms Piece of Red bunting
Twenty fathoms Two strips of leather with holes in them
These are known as `Marks'. So if the lead line came out of the water with a piece of blue bunting resting on the surface, the leadsman should, in his best Metro-Goldwyn-Mayer voice, call out `By the Mark—Thirteen'. The intervening fathoms, between the marks, are estimated and called `Deeps'. `Deep Four', `Deep Six' and so on.
When taking a sounding with the lead line, the rope is coiled up in the leadsman's inboard hand, leaving a small bight between it and where the other hand is grasping the lead line a few feet above the lead. The lead is then swung and released to land in the water a sufficient distance ahead to allow for the boat's speed—the object being to take the sounding when the lead line is up and down in the water right underneath the leadsman. In most boats, with a fairly low freeboard, this can be achieved with an under-arm swing, but in ships the man has to be more ambitious and whirl the lead overhead with consequent hazard to the rest of the community. However, if when first practising you instruct your friends to go below, there should be no need for the first-aid box.
Logs
These either measure speed directly or distance directly; no matter, for one can easily be deduced from the other. There are basically two types:
I
I26 SMALL BOAT NAVIGATION NAVIGATIONAL INSTRUMENTS AND AIDS I27
a. A bottom log that works on pressure differences between static pressure orifice and an impact pressure orifice—The Pitometer Log.
b. A bottom or towed log that is driven by a small propeller, or impeller, whose rotation is measured by electrical or mechanical means—The Chernikeef Log, Walker's Patent Log and others.
The Pitometer Log
This measures speed directly, and is generally used in large ships. It works on very much the same principle as an aircraft's pitot tube. A rodmeter projects below the hull, one shielded pipe in it measuring the static pressure at the depth of the rod-meter and the other pipe facing forward to sense the full pressure due to the ship's speed through the water. These two pressures are compared in a mercury (or other) differential, which also acts as a damping device, and a speed produced. This can be converted into distance for display on another dial.
Advantages. No moving parts outside the hull which can get clogged with sea-weed, etc. Very accurate, particularly at low/ medium speeds.
Disadvantages. Comparatively expensive and rather bulky for small craft. Needs an opening in the hull for the rodmeter.
The Chernikeef Log
This measures distance directly, and has been used in all sizes of craft. A small propeller is sited at the end of a shaft projecting through the hull (this can normally be raised and lowered through a gland—as can the rodmeter of a Pitometer Log). As the vessel moves through the water, the impeller rotates and this rotation is transmitted electrically to a distance recorder. This is converted to speed for display on another dial.
Advantages. Possible to measure very slow speeds as the impeller is quite sensitive. The log can also be adjusted (calibrated) to a certain degree by bending the impeller blades.
Disadvantages. Expense, and the impeller can become quite easily fouled, or perhaps damaged, by debris, seaweed, etc. Needs an opening in the hull for the impeller shaft.
The Walker's Patent Log
This measures distance directly, and is the log (or variety of log) most used in small craft—and in big ships too, sometimes as an emergency log. It is a towed log, where a small screw propeller, called the rotator, is towed at the end of a long piece of log line. The boat's passage through the water causes the rotator to turn, the rotation being transmitted via the line to a mechanical register fitted on a bracket in the stern of the boat. A metal wheel, called a governor, is sometimes used in the line just astern of the boat, clear of the water, to smooth out the rotation. The distance travelled can either be read off the mechanical register or, with electric logs, transmitted to a receiver sited elsewhere in the boat.
Streaming the log is quite simple. The rotator is paid out over the stern and the inboard end attached by its hook to the ring of the mechanical register. If the governor wheel is going to be used, this should be attached by its short length of line to the register—before the rotator is paid out—and kept inboard. The rotator is then paid out and the inner end of the log line clipped to the governor. This is then eased over the stern.
Recovering the log is a bit more difficult, particularly if the boat is making sternway—so haul in the log in good time! First recover the governor, unclip it, and if the boat is still making headway, pay out the inner end of the log line over the other side of the stern whilst hauling in the rotator. When the latter is inboard, the rest of the line can be brought in free of turns.
The log line is made specially for this purpose. Generally speaking, the longer the length used the better for the purposes of accuracy, but do not forget that if the boat's speed is very low, the rotator may trail across the bottom and possibly get snarled up in something down there. The recommended lengths* are:
Maximum speed 10 knots —40 fathoms 15 knots —50 fathoms 18 + knots — 65 fathoms
Advantages. Cheap and simple. No holes in the hull. Spares easily carried.
Disadvantages. The main disadvantage is that the log has to be
* A lot of boats only use about 75-100 feet of line with fairly good results.
1;2$ SMALL BOAT NAVIGATION I NAVIGATIONAL INSTRtUMENTS AND AIDS 129
hauled in whenever the boat goes very slowly, stops, or goes astern (unless in deep water—or in powered boats you want a rope round the screw). The rotator is liable to be fouled with seaweed; in foreign parts, also, it seems to have a strong fascination for sharks—with somewhat violent results. It is also inaccurate in a heavy following sea.
Other methods of estimating speeds, without the use of a log, are mentioned in the next chapter.
Radio Aids
There are quite a number of Radio Aids in constant use by both ships and aircraft, but not all of these, mainly for reasons of equipment expense or complexity, are available to the average yachtsman.
Those aids which require sophisticated equipment on board include the following:
a. Radar. Very widely used at sea by ships and an invaluable aid to navigation. Some large yachts and power boats have been fitted with radar, and there are, in fact, fairly small and portable radar sets now being produced commercially. However, the vast majority of small craft are without it.
b. Loran. This is an ocean aid, and Loran transmitting stations are sited in the North Atlantic, Pacific, and other areas. There are two systems, one of which is very accurate, but an expensive receiver is required.
c. Decca. A very well known, and widely used, coastal aid of high accuracy. Decca chains surround the coasts of the British Isles, the North Sea, the Channel and a growing number of localities abroad. The chains are generally usable within about 250 miles of the master station. However, again a special receiver is required. .
The Admiralty publishes special charts, overlaid with the Decca and Loran lattices. These can be obtained from any chart agent, together with small booklets showing the worldwide coverage of these aids.
Coastal Radio Direction Finding Stations
This is an aid which falls into an intermediate category as far as yachtsmen are concerned. RDF stations are spread around the
coasts and will, on request and on the booking of a small fee (free, I believe, in an emergency), take a bearing of the vessel asking for it and then transmit the bearing back to the boat. This, of course, means that the boat must be fitted with both a transmitter and receiver which can be tuned to the correct frequencies. Details of the services provided by these coastal RDF stations can be found in the Admiralty List of Radio Signals. These stations should not be confused with Radio Beacons (q.v. below). The station takes the bearing, not the boat. With morse (continuous wave) transmissions from the boat, bearings can be taken over very long ranges and this really is probably most useful as an ocean aid.
Consol
This is an ocean radio aid, freely usable by all who care to read the signals on an ordinary radio receiver. The range of Consol
Figure 23
is about 1500 miles and there are stations at Bushmills (Northern Ireland), Stavanger (Norway), Ploneis (France) and Seville (Spain). Consol charts are published, and details of frequencies, together with Consol position tables, are given each year in the Admiralty List of Radio Signals, and in other almanacks and publications.
The system works on the principle of phase-differences, and was developed from the radio aid the Luftwaffe used to find targets in Britain during the night bombing raids of the war. A very simple explanation of phase-difference; drawn diagrammatically a radio wave can be considered, not as a straight line, but as a Sine wave (see Figure 23).
The wave starts at phase o°, rises to a maximum value in one
Phase
130 SMALL BOAT NAVIGATION I NAVIGATIONAL INSTRUMENTS AND AIDS 131
direction go°, sinks again to zero 180°, and rises to a maximum in the other direction 27o0 before returning to zero again. During this time, depending on the wave length (frequency) of the transmitter, the wave will have covered a certain physical distance over the land or the water. If a boat was precisely half-way between two transmitters of the same wave length, the two signals would reach it at exactly the same phase, e.g. both at the go° point. If, on the other hand, the boat was further away from one transmitter than the other, the signals would arrive out of phase, each with its own phase value, e.g. one at go° the other at 270°.
The Decca receiver uses this principle by actually measuring the phase differences with considerable accuracy. In Consol, the transmissions are modulated in such a way that a different sound is produced in an ordinary radio receiver as the phase-difference changes. For example, a continuous note at zero phase-difference, 3o dots and 3o dashes at maximum phase-difference, and so on.
If on a chart or a map a line is drawn joining all the points equidistant from two fixed points, this line will turn out to be a hyperbola. A line joining places of equal phase-difference will therefore produce a hyperbolic position line. Thus the Consol lattice on a chart is a lattice of hyperbolic position lines identified by the number of dots and dashes each represents.
The fixing procedure is fairly simple—if you have a good ear! (I haven't, and have never yet achieved a really satisfactory Consol fix.) Tune into a Consol station. The frequency is given on the chart as well as in the books. Listen until you hear the call-sign and then start counting the dots and dashes. The total received until the start of the next cycle should add up to 6o. It is not always easy to discern when the dashes start or the dots end as they merge into a continuous note (called the equisignal) . If, for example, only 4 dots and 5o dashes are heard, i.e. 6 symbols are missing, the numbers should be adjusted to read 7 dots and 53 dashes. On the chart, pencil in the position line equivalent to this Consol reading nearest the D.R., and then tune to the next station and repeat the procedure. This should give a Consol fix. Remember that this is an ocean aid and that therefore great accuracy cannot be expected in the fix—as a rough estimate 5 to 10 miles might be taken as a guide, but alot depends on the radio conditions, distance from the transmitters, and the efficiency of the count.
Radio Direction Finding (D/F)
Quite a number of boats these days are fitted with a Radio Direction Finder—or D/F set as it is commonly called. These sets usually have some form of rotatable circular `loop' aerial. By tuning the receiver to a particular shore station and moving the aerial around, a maximum or minimum received signal is found. The signals are then amplified, and compared aurally or on a meter. When the minimum signal is reached, the plane of the boat's aerial must be at right-angles to the bearing of the transmitter. The aerial bearing at this time will, of course, be relative to the boat's heading, but if the aerial bearing is related to the compass, the compass bearing of the transmitter can be established.
D/F sets of this nature can give quite good results over comparatively short ranges—in the order of I oo miles. The accuracy of the bearing obtainable depends on a number of factors. The character of the set being used is one, range from the transmitter another. A principal cause of trouble is the interference between radio waves (from the same transmitter) that follow a ground-wave path and those following a sky-wave track. The latter pass out through the Earth's atmosphere but are reflected back by an ionospheric layer, to return to Earth at varying distances from the transmitter. A pure ground-wave signal will give the best bearings, but particularly at dusk and dawn, and also at night, sky-wave signals may intrude. With loop aerials, in sky-wave interference conditions, serious errors may result at ranges over 25 miles from the transmitter. As far as reception is concerned, there is no indication whether a sky- or ground-wave is being received.
Another possible cause of trouble in some ship or boat-borne D/F sets is that a receiprocal bearing may be obtained (i.e. a bearing that is 18o0 out). A simple loop aerial cannot differentiate between the signal strengths received when it is turned through exactly 18o°. Some sets are fitted with a sensing device which removes this difficulty, and the prospective purchaser of a D/F set would be wise to check this point.
Another cautionary note: depending on the position of the
132 SMALL BOAT NAVIGATION
x
Figure 24
aerial in the boat, other fittings such as masts and shrouds may deflect the incoming signals on certain relative bearings. A D/F set should, therefore, be calibrated `in situ' to discover whether any of these local errors are important enough to be added to or deducted from the received bearings.
Over ranges of less than I oo miles, the fact that radio waves travel on Great Circle paths can be ignored and little error will result if the bearing of the transmitter is plotted directly on a Mercator chart. But over long ranges—IOO-2000 miles or so—the difference between the Great Circle and Rhumb Line bearings must be taken into account.
In Figure 24, the boat at Point A takes the D/F bearing of a transmitter at Point B. The radio wave from the transmitter follows the Great Circle course BZA; the tangents to this curve from A and B meet at D. The bearing of B from A on the chart is the Rhumb Line bearing AB. A correction has therefore to be applied to the observed bearing equal to the angle DAB. This angle is called `Half convergency' as it is equal to half the convergency angle ADX.
The correction for this angle is always applied towards the Equator as the Great Circle curves towards the Pole. Its value can be found from the following formula:
D' Long Sine Mean Lat. Half-Convergency (in minutes) =X
(minutes) 2
(The D' Long. and Mean Lat. between boat and transmitter)
NAVIGATIONAL INSTRUMENTS AND AIDS 133
An easier way to find the value of Half-Convergency is to use the Traverse Table (the use of this table is described in Chapter 9).
Enter the D' Long. as the Distance and Mean Latitude as the Course.
Departure = Half-Convergency in minutes.
2
There are now many Radio Beacons, a lot of them fitted in lighthouses, sited around the coasts. They transmit at fixed intervals, normally sending their identification signal in Morse Code first, followed by a long dash to facilitate D/F bearings. The Radio Beacons (Ro. Bn. on the chart) and Coast Radio stations transmitting for D/F purposes are ringed with magenta circles. However, the charts give no details on callsigns, frequencies and times of transmission. Complete details of these are given in the Admiralty List of Radio Signals, from diagrams available at chart agents or from the makers of the D/F sets. (Coast Radio Stations may only transmit on request.)
A D/F set is therefore very useful to the yachtsman. Two plotted D/F bearings will give a fix, and many an owner must have been grateful for his D/F set on a foggy night in mid Channel. However, the limitations and possible causes of error in D/F bearings must always be remembered. It is an Aid—not God!
E
Practical Navigation in a Boat
The preceding chapters have described some of the tools and techniques used in the art of navigation; the time now approaches when theory has to be put into practice. The real work should start well before you slip from your moorings or leave that nice, comfortable jetty where the boat has been lying whilst you have been busy with other things.
Planning the Passage
The first thing to ensure is that all the charts, tables, books, instruments, are on board to cover the whole route and any possible diversions it may be necessary to make should the weather turn foul or one of the crew go sick; precautions first, fun later. Having done this check, start to plan your passage on the charts. Even though you may not be able to stick to them rigidly because of weather conditions, it is always a good plan to draw in the intended tracks on the chart; whilst doing this, mark back from your destination the distance to go so that, on whichever chart happens to be in use at the time, you know how far away the target lies. On long passages, it will probably be necessary to draw out the general track initially on a fairly small scale chart, and then transfer it to larger scale charts, amending, it as necessary to skirt any dangers that may show up more clearly on the latter. If in any doubt as to how wide a berth to give a particular point or headland, always consult the Pilot for the advice it may offer. This will also give pretty detailed information on the tidal streams and currents to be expected even if the chart is a bit vague on the subject. Look out for any unhealthy spots which it would be unwise to approach too closely at unfavourable moments, e.g. unlit rocks or sandbanks in the middle of a pitch-dark night. There are a host
PRACTICAL NAVIGATION IN A BOAT 135
of things to look for, and they cannot all be listed here, but it is largely a matter of common sense.
When transferring positions or courses from one chart to another, it is usually easiest to do this as a range and bearing from a particular point common to both charts. It can also be done by Lat. and Long., but this is usually slower and in a few areas still the Lats. and Longs. vary slightly between charts.
When you have got the general track marked on the chart, perused any doubtful places or corners, got a general idea of the tidal streams or currents likely to be met, the final considerations should be planning for leaving harbour and the weather.
To take the weather first. In U.K. waters, it is very simple to get a weather forecast. They are given frequently on the ordinary radio programmes, and the Met Office will give a forecast on request by telephone. Abroad it is not quite so easy, as the local radio (damn it) may be incomprehensible. However, in harbour it is usually possible to get a local forecast from the yacht club, port authorities, or even a chap stroking a piece of seaweed on the beach; it is a question of where English is spoke, if you do not know the country's mother tongue, rather than a lack of available forecasters—which are legion everywhere. At sea, details of area forecasts in English are given once again in the Admiralty List of Radio Signals. General weather information is also contained in the Pilots. Your final decision on when to leave harbour must rest, obviously, on the local and area weather forecast and the state of the tide. Even if the wind is fair, it may be prudent to wait until the worst of the flood tide has passed before making out an estuary. On the other hand, it may be worth beating out against the tide for a while if greater advantage can be gained later. It is impossible to generalise on this.
Finally, the plan for leaving harbour. If it is a familiar place, there should be no problems in getting away to sea from it. A strange place, though, may require a little more careful study, and perhaps a written-down plan for leaving harbour with courses on leading marks, turning points, and clearing bearings taken off the chart and noted down for quick reference as you proceed to sea. It is a good thing to keep one section of your `navigator's notebook' (see next paragraph) reserved for leaving harbour plans of this nature. Once the problem has been
136 SMALL BOAT NAVIGATION I PRACTICAL NAVIGATION IN A BOAT I37
studied and the plan made, it can be used again at any time in the future with adjustments for different states of the tide, wind, etc. In power driven boats, the same plan can be used time and again without any alterations.
The Navigator's Notebook
It is a good practice when making a passage to keep a small notebook in which all important navigational data can be entered. It serves as the place in which alterations of course and speed, bearings taken for fixes, compass checks, etc., can be written. It is better to write such data down in a notebook rather than scribbling it on odd scraps of paper or on the chart where it stands in grave danger of being erased or lost or confused with other detail. One good reason for recording these facts on a systematic basis is because of the necessity sometimes to go back and check previous fixes, courses, etc. Suppose a mark is not sighted at the expected time; the wise sailor then goes quickly back over his previous work to make sure that no glaring errors have been made. If the facts have been recorded, an error may be found and corrected in time. If no obvious error has been made, then there is at least some comfort in that knowledge.
The notebook need not be an elaborate affair. A small pocketbook with a pencilled margin down the left-hand side of each page is all that is required ... time being inserted in the margin and relevant details across the rest of the page opposite the appropriate times.
An example of a typical page of a navigator's notebook is given opposite.
Taking Bearings for Fixes
Whether taking bearings by using the Azimuth circle on a compass or by means of a hand-bearing compass, it is necessary to observe them accurately—and preferably quickly. The best routine in this respect is:
a. Identify the objects you are going to use both visually and on the chart.
b. Write the names of the objects into your notebook.
c. Take the bearings of the objects in quick succession. If the boat is moving fast and the objects are close, take the bearing of
0 r'' I -f C4a2er . /5o C
/000 I G~.1a~cd ~ust`t~, . ~C' ed ,~. 3kts,
}--- means abeam to Port • —I means abeam to Starboard --~ means right-hand edge
138 SMALL BOAT NAVIGATION PRACTICAL NAVIGATION IN A BOAT 139
the object nearest the beam last; this is the bearing that will be changing most rapidly.
d. Do not write the bearing of each object down before taking the bearing of the next. This wastes time. Instead remember the last two digits of each bearing in your head, e.g. og, 24, 67 for bearings of 309, 024 and o67. Then go to the notebook and write the two figures against the appropriate objects. The first digit in each case can easily be added by inspection of the chart. It is considerably easier to retain six figures in your head than nine whilst taking the bearings.
e. If you are in a boat that is only fitted with a steering compass with no horizon view from it, and there is no hand bearing compass available, the boat must be pointed at each object in succession and the ship's head read off the compass when the bow is pointed directly at each object.
Speed through the Water
If you do not possess a log, judgement of speed or distance run through the water is largely a matter of experience with that particular craft. The amount of wake produced at varying boat speeds is normally a good clue. Sailing craft, too, have a fairly set maximum speed which is a function of their waterline length. In powered craft, the maximum speed is also well defined in calm water but more difficult to assess when heading into a lop. Intermediate speeds can also be pretty accurately estimated in power boats from the engine revolutions and a speed/revolution table devised (a measured mile in your locality?).
Particularly when the boat is moving fairly slowly, quite an accurate judgement of speed can be made by using the `Dutchman's Log'. If the time taken for a piece of wood or other marker to travel from the bow to the stern is measured, the boat's speed can be calculated. If, therefore, you can persuade your wife to throw her handbag in the water (latest hat will not do—too much windage) abreast the bow and clear of the hull and, with a stop-watch, time the period elapsed until you fish it out of the water exactly abreast the stern, you will obtain an estimate of the boat's speed. The shorter the boat, of course, the more critical the timing becomes from all points of view.
In a boat of 30 foot length, suppose the marker thrown over parallel with the bow takes 5 seconds to reach the stern. Then:
Speed (knots) = Distance (miles) Time (hours)
or in this case Speed = 3o x 6o x 6o = 3.6 knots. 6000 c
Surface Drift
When the wind has been blowing steadily over the sea for some time, the water surface responds to the frictional drag of the air by moving in the general direction towards which the wind is blowing. This drift current, or surface drift as it is called, is deflected by the Earth's rotation to the right of the wind direction in the Northern Hemisphere and to the left in the Southern. The amount of deflection can be as much as 45°. The strength of these currents varies greatly depending on how long the wind has been blowing, its strength, and the amount of fetch (distance over the sea) it has had. An average figure for the current's rate is usually taken to be somewhere about I/5oth of the wind speed. Therefore a 20 knot wind blowing in a steady direction for a few hours could be expected to produce about I a knot of surface drift.
The major currents in the open oceans are, of course, caused by winds such as the Trade Winds blowing consistently over vast areas of ocean for many months of the year. The surface drift mentioned here is a much more local effect.
Leeway
In any boat, but particularly sailing craft, leeway (movement bodily downwind) has to be taken into account when estimating the course made good. This is over and above any current or tidal stream that may be present. The amount of leeway a boat makes is governed by its hull form, the relative wind direction and the boat's speed. Boats with deep keels or large centreboards will make less leeway than shallower, less stiff craft. Again, judging the amount of leeway a boat will make under given conditions is largely a matter of experience with that boat —and the helmsman! The angle the wake makes with the boat's heading is a good indicator; generally, the faster the boat is moving in a forward direction, the less the leeway made.
I40 SMALL BOAT NAVIGATION PRACTICAL NAVIGATION IN A BOAT I4I
.Navigating out of Sight of Land
It is is not the intention in this book to include anything about the taking and working of astronomical sights, frequently the only method of obtaining a reliable fix out of sight of land. A knowledge of astro-navigation is essential if you are contemplating any really long voyages in the open sea.
A point that might be made here is that the accuracy of your D.R. or Estimated position is bound to degrade with time—unless you are very lucky and all the drift and current errors conveniently cancel each other out; sadly, this does not often happen! The staleness of the D.R. or E.P. (since the last fix) is thus a big factor in any estimation of how closely you know your true position. The longer time away from a fix, the bigger the guess.
When the shore and known fixing marks are visible, it is not always necessary to keep a record of every tack or alteration of course as a fix can be taken when necessary to re-establish the boat's position. When out of sight of land, however, there is a real need to keep as accurate a D.R. or E.P. as possible. This can be done by plotting on the chart every course and every distance run on that course. If, when sailing, frequent tacking is necessary to make ground to windward, the plotting of each tack may be a tedious business. The alternative is to use a Traverse Table.
These are contained in most sets of nautical tables. Basically, all they do is to give a tabular solution for a right-angled triangle, the hypoteneuse being the Distance Run, the other sides being the distance resolved North–South (D' Lat.) and the distance resolved East–West (Departure). The course steered is the angle (or go° minus the angle) between these two sides (Figure 25).
Suppose a boat is steering o600 and runs on this course for 3 miles. Solving the triangle by plane trigonometry:
Sine 300 = D' Lat. Therefore D' Lat. = 3 Sine 30 3
= 3 X 0.5
= I.5 miles or minutes of latitude.
Cos 30° Departure
= Therefore Dep. = 3 Cos 30
3
= 3 x o.866
= 2.598 miles East (or West) .
Note. The Departure can be plotted directly on the chart—using the Latitude scale for its measurement—or converted into a difference of Longitude (D' Long.) by the formula:
D' Long. = Departure X Secant Middle Latitude.
departure
Figure 25
A Traverse Table, such as that given in Inman's Nautical Tables, may be arranged as on page 142.
The arrangement of this table is such that if the course angle is less than 450 the table is entered from the left-hand edge and the column headings at the top used for D' Lat. and Dep. If the course is greater than 450, the table is entered from the right-hand edge and the column headings at the bottom used. For example:
D' Lat. Dep.
Course 031°—Distance 2.98 miles 2.55 N 1.53 E
Course o6o°—Distance 3.00 miles 1.5 N 2.598 E
For courses between 0900_I 8o°, the table is used as if the course was South ... ° East; a course of 176° is the equivalent of
Course Dep. D'Lat. Dep. D'Lat. Dep. D'Lat. Dep. D'Lat. Course
South 40 East, therefore the D' Lat. and Dep. for a distance of 3.o miles are 2.933 S and 0.209 E respectively. Note that the D' Lat. and Dep. must be South and East in this case as the course has a Southerly and Easterly component. Obviously the D'Lat. is going to be much the larger as a course of 176° is within a few degrees of due South. Similarly, for courses between I8o°270°, the course is considered as being South ... ° West, and between courses 2700-3600 North ... ° West.
The following example illustrates the use of the Traverse Table in practice. A boat starts in position 50° 10.0' N 4° o6.o' W. It then steers these courses:
D' Lat. Dep.
N S E W
29.93 - 2.09 ‑
- 25.44 15.90 ‑
- 29.98 - 1.05
20.07 - - 22.29
50.00 55.42 17.99 23.34
PRACTICAL NAVIGATION IN A BOAT 143
As a result of these manoeuvres the boat has travelled:
50.00 N 17.99 E
55.42 S 23.34W
5.42 S 5.35 W
In other words, it finishes up 5.42' or minutes of Latitude or miles South of the starting position and 5.35 miles West. The simplest way of re-plotting the boat's position on the chart is to set these 'Northings' and 'Eastings' successively on the dividers-Departure is in miles East or West and therefore the Latitude scale can be used for both quantities-and plot the appropriate distances South and West from the initial position. Alternatively, the Longitude of the final position can be found by converting Departure into D' Long.-by a table or:
D' Long. = Departure Sec. Lat. Therefore D' Long. = 5.35 Sec. 500 05' = 5.35 X 1.55
= 8.3' West.
Starting Position 500 10.0' N 40 o6.o' W
Difference o° 05.4' S o° 08.3' W
Final Position 50° 04.6' N 4° 14.3' W
In practice, there would be no point in working to two places of decimals of a minute; that was done for illustration. The final position can then be plotted on the chart quite normally using the Latitude and Longitude scales. To simplify matters, a table for converting Departure into D' Long. is usually placed next to the Traverse Table in the book.
Having worked the courses and distances run in one of these ways to give the D.R. position, allowance must then be made for leeway, tidal stream and current to arrive at the Estimated Position. As for leeway, it is probably easier to allow for it by judging the difference it will have made on each of the courses steered and then working the adjusted courses in the Traverse Table instead of the straight courses steered. The effect of tidal stream or current over the whole period can be estimated and applied as one correction on the chart.
Course 004° for 30 miles Course 148° for 30 miles Course 182° for 30 miles Course 312° for 30 miles
I44 SMALL BOAT NAVIGATION
Making a Landfall
The acid test! But there is nothing more satisfying than to arrive at the right place at the right time in spite of all the quirks of wind, tide and weather.
The careful work required in keeping an accurate E.P. in the open sea, aided by soundings and radio aids if you have any to use, will now begin to pay off. On the other hand, a bit of careless work out in the middle may result in a good deal of nail-biting and the aspect of a selection of objects which bear no resemblance to the chart around your destination.
To take the worst case first; suppose that you have been at sea for a couple of days or more without a proper fix—it has been blowing hard and the visibility is not good. Your destination is sited on a low-lying, pretty featureless coast with no conspicuous off-shore marks; there are rocks well to seaward and the current is strong. Under these conditions, the wisest course of action would be to wait or anchor until the visibility improved. However, there may be urgent reasons for an early arrival in harbour. As the D.R. position may be well out, it is
ilk
too optimistic to expect a neat arrival straight off the harbour entrance. The first objective must be to get a positive identification of some land or sea mark as early as possible. Consequently, although the coastline may appear to be absolutely featureless from the chart, an exhaustive study of the Sailing Directions (Admiralty Pilot) may have revealed some clues which would help, i.e. the colour and appearance of the coastline along particular stretches, any small hills or dunes that are slightly more easily seen than than the rest. Having then amassed as much local information as possible, aim to close the coast in an area where the offshore dangers are least and where there is a good chance of identifying some feature on the shore from a good range. An alternative course of action is to deliberately aim to miss your destination on one side or other by an amount at least equal to your probable reckoning errors. Then, on finally making the coast, there can be no doubt in which direction to turn parallel with the coastline in order to reach harbour.
At other times, you may be making a landfall on a coastline which has plenty of artificial or natural features, but with tricky approach channels through shallows before the harbour can be reached. A bold headland is easy to identify, but a
PRACTICAL NAVIGATION IN A BOAT 145
coastline backed by broken country may prove more difficult to sort out. If there is a major lighthouse in the area, it may be preferable to approach the coast in the dark to gain a positive identification of the light by its characteristics. It is often more difficult to pick out a lighthouse by day, even if the visibility is good, if it does not stand out well against the background. Conversely, off some major rivers and ports, the host of navigational marks and shore lights may tend to confuse by their sheer weight of numbers. At night, it may not be easy to pick up the smaller navigational lights against a background of street lighting, traffic moving on shore, etc. Such considerations as these should be taken into account when approaching a strange harbour from seaward. Glean as much information as you can beforehand from the chart and Sailing Directions and then use common sense in planning the approach taking into account the possible inaccuracies in your Estimated Position. A lot of landfalls are quite straightforward but there may be the odd one which may cause some anxiety unless the appropriate precautions have been taken.
Approaching the Coast in bad visibility
As has already been said, the golden rule if visibility is bad and you are uncertain of your position is to anchor as soon as the water becomes sufficiently shallow—preferably clear of shipping lanes. It is most dangerous to press on in the blind faith that the mark which should have appeared some time back will sooner or later appear. Another `must', if it is possible, is to get a good fix before the visibility completely closes down; this applies when making a passage along the coast as well.
Anchoring in a chosen spot
There are many factors to be taken into account when choosing an anchor berth. The principal ones are:
(i) What is the direction of the prevailing wind? (Avoid anchoring if you can possibly do so on a Lee shore—if the anchor drags you will end up on the rocks, or at best have a most uncomfortable time.)
(ii) Is there sufficient room to swing safely round the anchor without danger to yourself or other craft?
146 SMALL BOAT NAVIGATION PRACTICAL NAVIGATION IN A BOAT 147
(iii) Is it reasonably easy to get out of your selected berth if the wind or current changes direction or strengthens?
(iv) Is the holding ground good?
(v) Is the water too deep for comfort?
There are a lot of supplementary considerations (or perhaps not so supplementary?) such as nearness to landing places, bright lights, etc., but the first requirement for an easy mind is to ensure the boat's safety. A swinging time ashore is slightly spoilt if on your return you find that the boat has had ideas of its own.
No anchor, however good its design, will hold properly if an insufficient length of cable or anchor warp is veered. The holding power of an anchor depends to a large extent on the angle the chain makes with the stock of the anchor whilst it is lying on the bottom. If the length of chain near the stock is lying flat, as the strain comes on, the initial pull on the anchor is applied to it almost horizontally. The anchor then provides the maximum resistance to being dragged along the bottom, and the additional cable acts as a spring (due to its weight) preventing any sudden jerk on the anchor. On the other hand, if only just enough cable is let out to allow the anchor to reach the bottom, the pull coming onto it is applied almost vertically and as a sudden jerk; thus the anchor is easily pulled out of its ground and has little holding power.
The amount of cable that should be veered depends on a number of factors such as the depth of water, strength of wind, duration of stay in that berth, etc. As a good general rule, if you intend to remain at anchor for an appreciable length of time, it is advisable to let out a length of cable equal to Eight Times the Depth of Water, i.e. in a depth of 5 fathoms veer 40 fathoms of anchor chain. This is a generously safe allowance under normal conditions. Some people might say that this is far too much, but if there is otherwise plenty of swinging room, it is much better to err on the safe side. Should a gale blow up and you intend to remain at anchor, it would be advisable to let out all the cable you have got—the more out, the better the chance of the anchor holding.
The quality of the various holding grounds has been discussed in a previous chapter. Sand and mud are the best, rock andweed the worst. Apart from the danger of an anchor slipping and dragging quickly across a rocky bottom, there is also the risk of it jamming in a crevice when you least want it to do so. If you have to anchor on a rocky bottom, it is a wise precaution to buoy the anchor before letting it go—this means attaching a rope (of greater length than the depth of water!) to its crown. Before the anchor is let go, the anchor buoy is thrown clear over the side. If the anchor subsequently jams on being weighed, the buoy can be recovered and a pull put on the anchor from a different direction to try and pull it out. If this in turn fails, the next recourse is to try and sail it out, i.e. heave in until the cable is taut, secure it and then gather headway until the cable draws aft. A strain from a different direction is thus applied, and this can be tried successively on different headings. If this also fails, and you are in a hurry to get clear, the final solution is to slip the cable, buoying it from the inboard end before you do so. This end of the cable can then be recovered on return to the berth.
The amount of swinging room to be allowed when coming to a single anchor depends on the length of cable out, the depth of water, and the length of the boat. The proximity of other craft and dangers must be taken into account. For example, a 27 foot boat anchoring in 4 fathoms might veer 32 fathoms of cable. The theoretical radius to be allowed for the swinging circle is therefore:
27 feet +6 (32 —4) feet = 27 +6 X 28 = I95 feet or 65 yds. However a safety margin should be allowed (over and above this) of at least two boat's length from the nearest underwater hazard. When anchoring near other boats, it may not be possible to allow this comfortable combined margin, but it is as well to remember that although normally they will all swing to the current or wind at about the same time, sometimes things do not work out so neatly. It is not unknown for two boats anchored close to each other to swing to a shift of wind in the opposite directions. Complications can result!
Tidal stream or current has a greater effect than wind on the direction at which a boat will lie at anchor, particularly when the boat is deep in the water. A I knot of current may have a greater effect than a 15-20 knot wind if they are in opposition.
148 SMALL BOAT NAVIGATION
The approach to the Anchor Berth
There is sometimes a bit more to the actual business of anchoring than just dropping the anchor over the bow, securing the cable and then reaching for the brandy and ginger ale. Such methods may suffice in the summer in the Mediterranean, but are not liable to lead to security of tenure under English weather conditions. The aim on anchoring must be to lay the cable out across the bottom in a straight line, then any strain applied to the anchor and cable at a later time is taken up steadily. If, on the other hand, the cable is piled up in an untidy heap, coil upon coil, any subsequent boat movement will rapidly unwind the coil until a sudden jerk is imparted to the anchor which may break it out. The same disadvantage lies, to a lesser extent, in laying the cable out in a bight (loop) across the bottom.
There are basically two ways of laying the cable out in a straight line on anchoring—by means of a dropping or running moor. The former is principally used when it does not matter a great deal whether the boat is precisely in its correct berth; the latter when it is important to anchor bang in the right spot. Whichever method is used, it is always preferable to anchor heading into the wind or tidal stream. In a dropping moor, the anchor is let go as soon as the chosen berth is reached; the boat is then immediately allowed to drop back downwind or current laying out the cable as it goes. The cable will then be laid out straight and the boat drops gently back, its speed being controlled by the rate of veering the anchor cable. No jerk need be imparted to the cable at any stage.
With a running moor, the boat must have a certain amount of headway as the berth is reached so that control of the boat's heading can be maintained right up to the last moment. This allows a more precise positioning of the anchor on the bottom. For example the intention is to anchor precisely in the position shown in Figure 26.
The wind is from the NNE and there is no tidal stream. The approach course is from the SW on a heading of 030° True. First, a look at the chart may show a conveniently placed object which can be used as a headmark during the final approach; here the church is well placed to give a good lead into the anchorage and one which will clear the rocks on both sides of the entrance. (Very rarely does it happen that absolutely no
PRACTICAL NAVIGATION IN]A BOAT 149
No mast
'(A) __ —+ Church
2c ~I~I i Jct r LCI
3c oeo"T ~c~
oso~r r , , , °°° ;; ~e i r
! 7~ ighthouse. °°°°°~~\ ; i ~/
Bn
Figure 26
(The anchoring position is marked by the symbol under (A))
suitable mark is available—in this case, if the church had not been there, the point just to the South of it on the shore would have served just as well on a slightly different bearing; though it might have been more difficult to identify.)
With a parallel ruler set between the church and anchor berth, lay off the bearing on the chart and measure it from the compass rose (o8o° T). If the church is kept bearing o8o° True during the run in, the boat must be on the right line of approach to the anchoring position.
However, the boat has to get onto this line from its approach heading of 030° and this leads to a word about `turning bearings'. When using a bearing to decide the moment to turn onto a new course, always use the bearing of an object as near as possible parallel to the new course. Doing this much reduces the chance of ending up off the intended track due to compass error or some other reason. In the above diagram, it would be much better to use a bearing of the church for the final turn rather than a bearing of the lighthouse.
Next, distance to go, or distance along the track is required. This is obtained by selecting charted objects on the beam during the final approach and measuring the bearings these should subtend at given distances from the berth. In the sketch, the 3 cable, 2 cable, I cable, and I cable (ioo yards) distances to go have been marked, with finally the L.G. ('Let go') position shown. To be clinically accurate, the distance from the bow to
150 SMALL BOAT NAVIGATION PRACTICAL NAVIGATION IN A BOAT 151
the compass should be laid back from the berth in marking the L.G. position (to allow for the length of the boat between anchor and compass) but this is peanuts in short boats—in the Queen Elizabeth II it would make quite a difference.
Using the parallel ruler on the chart, the individual beam bearings from each object at each distance to go are then measured and tabulated. Before writing the bearings into the table, they should be corrected for Variation and Deviation (for the final approach course) :
Total correction I o°W.
Run o800 True—ogo° Compass on Puddlecome Church
Distance Lighthouse Ro. Mast Beacon
3 C. 195° Compass — 125° C.
2 c. — 070° C. 140° C.
i C. — 055° C. 165° C.
I C. — 038° C. 182° C.
L.G. — 007° C. 196° C.
This preparatory work does not take long and with the facts written into a notebook, a perfectly safe and accurate anchoring can be achieved without any need for fixing.
If the church is kept bearing ogo° by compass by adjustment of the course to allow for leeway, the boat can be brought in at the right speed for anchoring by checking the beam bearings. It is wise to have more than one set of beam bearings up your sleeve, as one can then be checked against another; also it may be found that one mark is obscured, at an important time, by other craft or be difficult to identify against the background.
With a running moor, if there is no wind or current, headway can be maintained until the right amount of cable is out. If there is a strong headwind or current, however, it is better to take the way off the boat as soon as the anchor is down and then drop slowly back on the chain. Whichever way the approach is made, if it is impossible to anchor into wind or tide, the anchor should be dropped on the weather side (i.e. up wind or current). When the way comes off the boat, the anchor chain will then lead out clear as the boat drops to leeward. If the anchor is let go on the lee side, the chain may foul the keel or scrape paint from the bow.
Entering Harbour
Shiphandling is a subject in its own right, and it is not the intention to discourse on it here. However, one or two small navigational hints may be of some help. On entering a harbour, always keep an eye on other craft at anchor and buoys to see in which direction the current is setting. Then identify the berth for which you are making as early as possible and approach it, if you can, against the current. The same applies when aiming to pick up a mooring. In sailing boats, the direction of your final approach to a berth is inevitably determined by the wind direction. In power boats, if the wind is blowing off a jetty, the approach should be made at a wider angle than normal and the boat turned to parallel up with the jetty at the last moment. If the wind is blowing onto ajetty, a shallower approach aiming a short distance off the jetty should be made.
In any type of boat bigger than dinghy size, always have an anchor ready for letting go when entering (or leaving) harbour. You never know when this may come in useful and it is always a good precaution even when you intend to berth alongside or at buoys. In a strange place, you may not know what the berthing arrangements are in that port on first entering, and therefore keep a good look out for groups of other yachts, customs posts and harbour offices where you may be able to obtain quick enlightenment (or a rude answer).
Finally to emphasise once again the importance of looking up all the information you can on the tides, currents, depths, conspicuous objects before entering a strange place. A frantic thumbing-through the Pilot with one hand, whilst steering with the other, and holding the sheets with your big toe may not breed confidence amongst your fellow men and girls.
ii iI ifl[II' Iii
Any boat owner must have a working knowledge of the International Regulations for the Prevention of Collision at Sea. Otherwise, he may become something of a menace to others on the sea.
The Rule of the Road at Sea is published, in this country, under an Order in Council entitled `The Collision Regulations (Ships and Seaplanes on the Water) and Signals of Distress (Ships) Order 1965'. This is obtainable from H.M. Stationery Office for the princely sum of 71 new pence.
By international agreement, the rules were slightly changed in 1960, principally to take into account the fitting of radar into a large number of ships since the war. There are altogether some 3I Rules covering—the lights and shapes to be carried by day and night—the sound signals to be made in poor visibility—and the Steering and Sailing rules. Some of them have been re-grouped for convenience and paraphrased here, without altering their sense; and some of the more important have been quoted verbatim—where this is the case, they are shown within inverted commas. Some commentary is also given on specific rules. These alterations and additions are not `official' but merely aimed to help interpretation; needless to say, I disclaim any responsibility for accidents that might be deemed to have arisen as a result of reading these pages!
The Rules should not be confused with the `Racing Rules' which are only in force when boats are actually competing, or about to compete, in a race. The Racing Rules are themselves based on the Rule of the Road but have major additions and modifications to the basic rules.
Rules r and 3o—Preliminary Statements and Definitions
(i) The International Regulations do not interfere with LocalTHE RULE OF THE ROAD AT SEA 153
Authority rules made for particular harbours, rivers, and inland waterways. (Rule 30.)
(ii) The rules concerning lights must be complied with from sunset to sunrise in all weather conditions. Additionally, these lights may be shown in daylight hours in restricted visibility, or in any other circumstances when it is considered prudent to do so. No other lights should be exhibited which can be confused with the prescribed lights. (Rule x.)
(iii) A power-driven vessel is a vessel that is using an engine at the time, whether or not this is her principal means of motive power, whether or not she is also sailing, and whether or not it is an inboard or outboard motor. (Rule I.)
(iv) If a boat is using an engine with sails also hoisted, it should, by day, carry a black cone, point downwards, in the fore part of the boat where it can best be seen. The cone should be at least two feet in diameter at its base! (Rule I4.) (This, you may imagine, is not always complied with, but in the event of an accident...)
(v) A vessel is considered to be underway as soon as it is no longer attached by any means to the shore or bottom; i.e. it is underway as soon as the last line is slipped from a jetty or a buoy, and as soon as the anchor is clear of the bottom when weighing. A boat aground is obviously not underway! (Rule I.)
(vi) Vessels are considered to be in sight of one another only when one can be observed visually from the other. (This is put in principally to disqualify radar contact as a `sighting'.) (Rule I.)
(vii) Regarding lights, `Visible' means visible on a dark night in a clear atmosphere. (Rule i.)
(viii) The term `engaged in fishing' means fishing with nets, lines or trawls but does not include fishing with trolling lines (streamed when underway over the stern or from outriggers). (Rule i.)
Rules 2 to I4—Lights and Shapes
POWER DRIVEN VESSELS UNDERWAY
I. A power driven vessel underway and over 150 feet in length must carry:
(a) Two White Masthead Lights. These must be sited on different mastheads or parts of the ship's superstructure and
154 SMALL BOAT NAVIGATION
the forward one must be at least 15 feet lower than the after. Both must be visible from 5 miles over an arc of visibility stretching from two points (221°) abaft the beam on one side, through the bow, to two points abaft the other beam, i.e. from the relative bearings of Red I I2 to Green 1124°. (Rule 2.)
(b) Bow Lights. On the port bow, a RED light showing from right ahead (but not across the bow) to two points abaft the port beam. On the starboard bow, a Green light showing from right ahead to two points abaft the starboard beam. The visibility distance of both lights must be 2 miles. (Rule 2.)
(c) Overtaking Light. This is a white light, carried on the stern, visible over 12 points (135°) through the stern, i.e. from 67 on one quarter—through the stern—to 671° on the other quarter. Visibility distance 2 miles. (Rule I o.)
Summary
A ship underway over 150 feet in length seen from dead ahead will show both masthead lights, one above the other, and both bow lights. As you cross its bow from port to starboard (at a goodly range, I trust), the red light will go out, the green light remain visible, and the two white lights separate. At long range, the bow lights may not indeed be visible and the masthead lights, by their aspect, will give the first indication of the ship's course.
If your relative position is more than 224° abaft the beam of the other ship, its white overtaking light only can be seen.
2. A Power Driven Vessel less than z5ofeet in length must carry:
One White Masthead Light, Bow Lights, and an overtaking light. It can carry a second white masthead light (as above) but is not obliged to do so. All these lights have the same characteristics as those defined in paragraph (I) a, b,
and c, for ships over 150 feet long. (Rule 2.) f'.
3. A Power Driven Vessel less than 65 feet long must carry:
(a) A White Masthead Light not less than 9 feet above the gunwale and visible 3 miles. (Rule 7.)
(b) Bow Lights showing RED and GREEN over the same arcs as those of a ship but need only be visible at I mile. (Rule 7.) or (c) The bow lights instead of being individually sited one
THE RULE Off' THE ROAD AT SEA 155
on each side may be combined into the same lampholder (Combined Lantern) but they must show over the same arcs as before (Red—Port: Green—Starboard) and the combined lantern should be not less than 3 feet beneath the white mast-headlight. (Rule 7.)
(d) Overtaking Light. In small vessels, it may not be possible on account of bad weather or other sufficient cause for the overtaking light to be a permanent fixture. However, if a permanent overtaking light is not carried, a white portable light or electric torch should be kept at hand ready for use and be shown in sufficient time to prevent collision. (Rule Io.)
4. Power Driven Vessels less than 4ofeet long may carry:
(a) The White Masthead Light at less than 9 feet above the gunwale but not less than 3 feet above the sidelights or combined lantern. (Rule 7.)
(b) Separate Sidelights or a Combined Lantern. (Rule 7).
(c) A fixed or temporary overtaking light under the same conditions as for a boat of 65 foot length. (Rule I o.)
SAILING VESSELS UNDERWAY
These follow the same lights, depending on their size, as the equivalent length large power driven vessels with the important exception that they never carry the white masthead lights of the power vessel. They may carry instead on their foremast a RED light over a GREEN light both visible from Red 1121° through the bow to Green 112-°.
These foremast lights should be visible at 2 miles. (Rule 5.)
A large sailing vessel underway may therefore be showing a Red over a green at the foremast, its bow lights, and an overtaking light. The latter lights are obligatory, in accordance with the rules for craft of their size, even though the foremast lights are not.
Sailing Vessels and Boats under oars less than 4o feet long must carry:
(a) Bow lights. If they are not fitted with the normal side‑
lights, they must carry, where it can best be seen, a combined
lantern Red to port and Green to starboard, visible I mile. If
it is not possible to fix this light permanently, it should be
kept ready for immediate use and exhibited in sufficient time
156 SMALL BOAT NAVIGATION THE RULE OF THE ROAD AT SEA 157
to prevent collision. (You will get no marks under these rules if Red is allowed to show to starboard or Green to port.) (Rule 6.)
(b) Overtaking Light. The normal overtaking light. Again, if it is not possible for this light to be fixed, a torch or lantern showing a white light should be kept at hand ready for immediate use. (Rule I o.)
Small Rowing Boats under Oars or Sail:
The full concessions are allowed here. They need only have an electric torch or lantern showing a white light available which must be exhibited in time to prevent collision. (Rule 7.)
Notes on Sailing vessels and Rowing Boats
As can be seen, there is no stipulated maximum length, under the 40 foot limit, above which the use of the single white light is forbidden. This is left to the discretion of the owner, noting that it is in his own interest to carry the proper lights if it is practicable so to do.
VESSELS TOWING OR BEING TOWED
(a) The Towing Vessel. A Power-driven vessel if towing (or pushing) other craft, and if the length of tow exceeds 600 feet, must carry:
Three White Masthead Lights in a vertical line on the same
mast and visible from two points abaft the beam, through the bow, to two points abaft the other beam. These, therefore, show over the same arc as the normal masthead lights but are carried instead of them. (Rule 3.) (By day, the towing vessel shows a black diamond shape.)
Bow Lights. As normal.
Overtaking Light as normal, or a small white light abaft the funnel or superstructure for the tow to steer by, but this light must not show forward of the beam. (Rule 3.)
(b) Power Vessel Towing—with a length of tow of less than boo feet must carry:
Two White Masthead Lights again in a vertical line and
having the same characteristics as the 3 Masthead Lights mentioned above. (Rule 3.)
Bow Lights.
Overtaking Light or a small light for the tow to steer by.
(c) Power Driven Vessels of less than 65 feet in length Towing. When towing or pushing other vessels, they must carry:
The Two White Masthead Lights not less than 4 feet apart,
and one of them must be in the same position as the ordinary white masthead light, i.e. an extra masthead light is needed. (Rule 7.)
Bow Lights or a Combined Lantern.
Overtaking Light or a small white light for the tow to steer by. (Rules 7 and Io.)
The Vessels being Towed
A vessel being towed must carry bow lights (or a Combined Lantern) and if it is the last vessel in the tow, an Overtaking Light. Craft in the middle of the tow must carry Bow Lights, but need not carry a proper overtaking light, but may show a small white light aft. (Rules 5 and Io.)
Vessels being Pushed by a Tug or another Boat
(a) A single vessel being pushed carries the normal sidelights or a Combined Lantern.
(b) A group of boats being pushed must be lit as one vessel, i.e. the port bow light on the port outer craft and the starboard bow light on the starboard outer craft. (Rule 5.)
LIGHTS AND SHAPES FOR SPECIAL CIRCUMSTANCES
A Vessel Not Under Command
A vessel is said to be `Not under Command' when she has had some sort of failure such as a steering or engine breakdown which precludes her from manoeuvring; she cannot therefore keep out of the way of other ships or craft. At night she shows, where they can best be seen, TWO RED LIGHTS vertically one above the other, visible all round the horizon at a distance of at least 2 miles. A power vessel will switch off her masthead lights, but the sidelights should be kept on in both power and sailing craft, under these circumstances, if the vessel is making way through the water. By day, a vessel not under command hoists
L
158 SMALL BOAT NAVIGATION
two black balls in lieu of the red lights (colloquially known as N.U.C. Balls). (Rule 4.)
Vessels Carrying out Special Operations
Vessels engaged in special tasks such as laying Telegraph Cables or navigational marks, surveying, carrying out underwater operations, warships replenishing at sea or operating aircraft, cannot easily get out of the way of other vessels approaching them. Instead of the normal masthead lights, at night they carry 3 lights in a vertical line—RED WHITE RED—visible all round the horizon for at least 2 miles.
By day, these lights are replaced by shapes in a vertical
line—RED BALL WHITE DIAMOND RED BALL. (Rule 4.)
Minesweepers
When engaged in minesweeping operations or exercises, a minesweeper carries at night a GREEN all round light at her fore truck, and a GREEN light at the yard on the side on which danger exists. If the minesweeper has sweeps out on both sides, therefore, a TRIANGLE OF GREEN LIGHTS will be visible; these lights show all round the horizon; and are in addition to her ordinary lights.
By day, black balls are shown in lieu of the lights. (Rule 4.)
The showing of these lights or shapes by sweepers indicates that it is dangerous for other craft to approach closer than 3000 feet astern or 1500 feet on the beam on the side (or sides) from which the sweeps are streamed. (Rule 4.)
Note: When any of these vessels (Not under Command, conducting special operations or minesweeping) are actually making way through the water they continue to show their bow lights and overtaking light.
Pilot Vessels
Steam Pilot Vessel. When on her station and underway at night will show her bow lights, overtaking light, and a special signal of an all round WHITE LIGHT over an all round RED LIGHT at the masthead. She should also show an intermittent all round white flare or light every ten minutes or so. When not under way, she switches off her bow and overtaking lights. If she is not engaged on pilotage duty at the time, she merely shows the ordinary lights for a similar vessel of her length. (Rule 8.)
Ii
THE RULE OF THE ROAD AT SEA 159
Sailing Pilot Vessel. When on her station on pilotage duty under way, she carries her bow and overtaking lights plus an all round WHITE LIGHT at the masthead, and burns one or more white flares or lights at intervals not exceeding Io minutes. If not engaged on pilotage duty, she shows the lights appropriate to a vessel of her size only. (Rule 8.)
Fishing Vessels. Rule 9
When they are not actually engaged in fishing, they should show the normal lights appropriate to a vessel of their length. When they are fishing, they carry the following lights:
Trawlers. These craft drag a dredge net or other gear along or near the sea bed; when trawling they carry:
GREEN LIGHT over a WHITE LIGHT visible all round the horizon.
Masthead Light. The ordinary masthead light may also be carried, but it must be lower and abaft the green and white lights.
Bow Lights and Overtaking Light. These are shown, as normally, if the trawler is making way through the water; if she is stopped, they are switched off.
Fishing Vessels Other than Trawlers. They carry:
RED LIGHT over a WHITE LIGHT visible all round the horizon.
Bow Lights and Overtaking Light if making way through the water.
Fishing Vessels with gear extending more than 500 feet horizontally into the Seaway carry:
RED LIGHT over a WHITE LIGHT.
An additional all round WHITE LIGHT in the direction in which the gear is extended. This extra light should show above the sidelights but not at a greater height than the other all round white.
Bow and Overtaking Lights if they are making way through the water.
A drift net vessel may well be showing lights of this nature.
Note. Fishing vessels may use a flare or searchlight to attract the attention of approaching vessels endangered by or en‑
16o SMALL BOAT NAVIGATION
dangering their gear. They also sometimes use clusters of bright working lights.
By day fishing vessels over 65 feet long when engaged in fishing carry two black cones point to point where they can best be seen. If they are less than 65 feet long, they may carry a basket instead. If their outlying gear extends more than 500 feet horizontally, they should carry an additional cone—point up— in the direction of the outlying gear.
Vessels at Anchor. Rule I I
A Vessel more than zso feet long when at anchor must show, at night, an all round white light forward and an all round white light aft; the latter being lower than the forward light. Both should be visible at 3 miles.
A Vessel less than i jo feet long need not carry the after light, and the required visibility range of the forward anchor light is 2 miles.
By day, the anchor lights of vessels of any size are replaced by a black ball carried in the forepart of the craft.
A Vessel Aground. Rule I I
Must show at night:
Anchor Lights appropriate to her size.
TWO RED LIGHTS one above the other, visible all round the horizon.
By day, she should show three black balls in a vertical line.
Rules 15 and i6—Sound Signals and Conduct in Restricted Visibility
A power-driven vessel 40 or more feet in length must have a whistle or siren, a mechanical foghorn and a bell.
A Sailing vessel 40 or more feet in length must have a mechanical foghorn and a bell.
A vessel of less than 40 feet in length, a rowing boat (or a seaplane!) shall not be obliged to make any of the following signals in poor visibility, but must make some other efficient sound signals at intervals of less than one minute.
This is the gist of the slightly ambiguous statement in Rule 15 (ix) but it means that, in bad visibility, as in good, the onus for avoiding collision is on all seafarers and not only on those in large ships. In any sort of cruising boat, 40 feet long or not, there should be some form of foghorn or claxon. The investment
it
THE RULE OF THE ROAD AT SEA 161
of a few shillings in one of these will seem justified if you find yourself in the middle of a busy shipping lane in dense fog.
Means
The signals described below for vessels underway should be given by:
(i) power driven vessels on a whistle or siren.
(ii) sailing vessels on the foghorn.
(iii) vessels being towed on a siren, whistle or foghorn.
Duration of Blasts. Rule I
A `short blast' means a blast of about I second's duration. A `prolonged blast' (or `long' blast) means a blast of between 4-6 seconds' duration.
Sound signals in Bad Visibility
In fog, mist, falling snow, heavy rainstorms or any other condition similarly restricting visibility, whether by day or night, the following sound signals must be made:
(The signals are listed here in tabular form for ease of reference, although in the Rules they are written out in full.)
Made by Made on Interval Signal
Vessel more than Bell forward 350 feet in length Gong aft
AT ANCHOR
Vessel less than Bell 350 feet long
AT ANCHOR
Every Rapid ringing of
minute bell for 5 seconds, followed by gong aft
Every Rapid ringing of
minute bell for 5 seconds
Siren, whistle As requisite or foghorn
Bell forward Every (Gong aft) minute
One short—one prolonged—one short blast ('R' in Morse)
3 distinct strokes on the bell, followed by 5 seconds' rapid ringing, followed by 3 strokes. (Gong as before for large ship)
Any vessel AT ANCHOR sensing danger from an approaching vessel
A vessel AGROUND
162 SMALL BOAT NAVIGATION
Made by Made on Interval Signal
Conduct of Vessels in Fog or Poor Visibility. Rule 16
A great number of the collisions at sea occur in fog or in conditions of poor visibility, and Rule 16 is one of the most important of the `Rules of the Road'. It is therefore quoted here in full:
(a) `Every vessel, or seaplane when taxi-ing on the water, shall, in fog, mist, falling snow, heavy rainstorms or any other
THE RULE OF THE ROAD AT SEA 163
condition similarly restricting visibility, go at a moderate speed, having careful regard to the existing circumstances and conditions.
(b) A power-driven vessel hearing, apparently forward of her beam, the fog signal of a vessel the position of which is not ascertained, shall, so far as the circumstances of the case admit, stop her engines, and then navigate with caution until the danger of collision is over.
(c) A power driven vessel which detects the presence of another vessel forward of her beam before hearing her fog signal or sighting her visually may take early and substantial action to avoid a close quarter situation but, if this cannot be avoided, she shall, so far as the circumstances of the case admit, stop her engines in proper time to avoid collision and then navigate with caution until danger of collision is over.'
Many court cases have been won and lost on the careful (or otherwise) observance of this rule. Paragraph (c) was inserted in the 1960 version of the Regulations to take into account the detection of other vessels by radar, but it does not absolve the radar fitted vessel from stopping engines if a close quarter situation develops.
From the yachtsman's point of view, it must be borne in mind that the very conditions which produce bad visibility, heavy rain for instance, can also degrade radar performance due to the attenuation of radar signals by the water droplets in the atmosphere. The degree of attenuation suffered depends in part on the frequency of the individual radar set. At worst, small craft may escape detection at all by a ship's radar, particularly if the sea is rough and the small echo from a yacht or boat becomes lost amongst the signals reflected by the waves (this is usually called `Sea Clutter'). At best, a boat which only presents a small reflecting area to the radar pulse cannot be detected at long range. It is, therefore, foolish to assume that a small boat or yacht is bound to be detected by a ship's radar and to take any unnecessary risks based on this assumption; for example trying to get across the bows of a ship in fog.
Sound Signals made by Vessels in Sight of one another. Rule 28
Certain other sound signals are restricted for use when vessels are in sight of one another by day or by night. It is important that,
Power vessel
Siren or
Every two
One prolonged
MAKING WAY
whistle
minutes
blast
through the water
Power vessel
Siren or
Every two
Two prolonged
under way but
whistle
minutes
blasts with an
STOPPED
interval of one
second between
Sailing vessel on
Foghorn
Every
One blast
the STARBOARD
minute
TACK
Sailing vessel on
Foghorn
Every
Two blasts
the PORT TACK
minute
Sailing vessel
Foghorn
Every
Three blasts
with the wind
minute
164 SMALL BOAT NAVIGATION
when taking avoiding action in close quarter situations, one
boat's intentions should be made clear to the other. This can
and must be done by the use of sound signals which are tabu-
Y g
lated below:
Made by Made on Signal
A power driven vessel about to ALTER Siren or One short
P
COURSE TO STARBOARD whistle blast
A power driven vessel about to ALTER Siren or Two short
COURSE TO PORT whistle blasts
A power driven vessel about to PUT Siren or Three short
ITS ENGINE(S) ASTERN whistle blasts
A power driven vessel having the Siren or At least
right of way over another vessel, but whistle 5 short and
in doubt whether the other vessel is rapid blasts
taking sufficient action to avert a
collision. (This signal is optional)
Note. Strictly speaking the last signal only applies to one power
vessel having the right of way over another. However, there are
other Rules (zo and 25) which state that a power driven vessel,
of less than 65 feet in length, or a sailing vessel must not hamper,
in a narrow channel, the safe passage of a ship which can only
navigate inside the channel. A liner or heavy tanker entering or
leaving harbour may well be unable to alter course to avoid
small craft in the channel and may use this signal to indicate
`Get
that she is concerned, i.e. out of my way—I cannot get out
of Yours'.
SOUND SIGNALS—INTERNATIONAL CODE
These are not taken from the Regulations for the prevention of
Collisions at sea, but from the International Code of Signals.
They are included here for easy reference.
Signal Morse Letter Meaning
S.O.S. ...---... Distress Signal.
F • . -. I am disabled. Communicate k
with me.
K –•– You should stop your ship
instantly.
L –•• You should stop. I have something
important to communicate. THE RULE OF THE ROAD AT SEA 165
0 – – – Man overboard.
P - -. Your lights are out or burning
badly.
R – • The way is off my ship. You
may feel your way past me.
– (Could be used in fog.)
U You are standing into danger.
V • • • - I require assistance.
W • - – I require medical assistance.
These signals may be made by flags, morse, semaphore, or on
a siren, whistle or foghorn in any weather.
THE STEERING AND SAILING RULES (I-3o)
Green to Green and Red to Red
Perfect safety, go Ahead,
If to starboard Red appear
'Tis your duty to keep clear.
A well known doggerel verse which summarises some of the
main rules regarding power vessels. This section of the Regula‑
tions is, of course, the vital section. Those rules already covered
about Lights, Shapes, and Sound Signals are designed' to assist
r comply with main Steering and Sailing
the mariner to com w t the
~ Y g an
Rules which follow here.
They cover the actions to be taken by individual vessels
approaching one another where a risk of collision may exist.
They tell the seaman which vessel must give way under various
circumstances and when the other vessel may be expected to
take the appropriate avoiding action.
Some of the basic principles are:
(i) A vessel must establish first that, if she holds her present
course and speed, a risk of collision with another vessel exists.
(ii) Power-Driven (Steam) vessels generally give way to Sail
or boats under oars.
(iii) When two Sailing Vessels approach each other, the
vessel which, owing to the wind direction, would lose more
ground by an alteration of course has the right of way.
(iv) In all circumstances where a risk of collision exists, the
vessel that has the right of way (sometimes called the `privileged'
vessel) should hold her course and speed whilst the other vessel
ABAFT the beam
A vessel TOWING,
Siren, whistle
Every
One prolonged
picking up
or foghorn
minute
blast followed by
CABLES, or NOT
two short blasts
UNDER COMMAND
('D' in Morse)
A vessel being
Siren, whistle
Every
One prolonged
TOWED, or if
or foghorn
minute
blast followed by
more than one
three short blasts
vessel is being
('B' in Morse)
towed, the last
vessel in the tow
A vessel ENGAGED
Siren, whistle
Every
One prolonged
IN FISHING, when
or foghorn
minute
blast followed by
under way or at
two short blasts
anchor. (Does not
('D' in Morse)
apply to vessels
trolling, which give
normal signals)
A power driven
Siren or
As requisite
The normal fog
PILOT VESSEL
whistle
signal followed
when engaged in
by four short blasts
PILOTAGE
(`H' in Morse)
166 SMALL BOAT NAVIGATION I THE RULE OF THE ROAD AT SEA 167
(sometimes called the `Giving Way' or `Burdened' vessel) must alter course and/or speed. However, if at the last moment collision appears to be inevitable, both vessels must do their best, by altering course or speed, to avoid it if at all possible or at least lessen the impact.
(v) When obeying these rules, the action taken by the burdened vessel must be positive and taken in ample time.
(vi) An Overtaking vessel, whether power or sail driven, must always avoid the vessel being overtaken. This overrides all the other rules.
(vii) All vessels not engaged in fishing (except those `Not under Command', engaged in special operations, etc.) must keep out of the way of vessels actually engaged in fishing. This does not, however, give fishing vessels the right to obstruct a fairway.
Sailing Vessels. Rule I
~ 17
This Rule is quoted here in toto:
`(a) When two sailing vessels are approaching one another, so as to involve risk of collision, one of them shall keep out of the way of the other as follows:
(i) When each has the wind on a different side, the vessel which has the wind on the Port side shall keep out of the way of the other.
(ii) When both have the wind on the same side, the vessel which is to Windward shall keep out of the way of the vessel which is to Leeward.
(b) For the purposes of this Rule, the windward side shall be deemed to be the side opposite to that on which the mainsail is carried or, in the case of a square-rigged vessel, the side opposite to that on which the largest fore-and-aft sail is carried.'
Interpretation
This rule was reworded in the 1960 Revision, and is now stated in much simpler form than it used to be. It is quite straightforward—Port Tack gives way to Starboard Tack when the boats have the wind on the opposite sides. The Windward boat gives way to the Leeward boat when they are both on the same
tack.
Some examples of these rules, applied to particular situations,°~ w
i
N D
Figure 27
are given in Figures 27A. In all the diagrams, the `burdened' or `giving way' vessel is shown shaded.
The Overtaking Rule (Rule 24) is over-riding; in Example (d), the Leeward boat is overtaking the other from a relative position more than two points abaft the Windward boat's beam, and in this situation the Leeward boat has to give way. (The obligations of vessels overtaking others are further expanded in the section under Rule 24.)
Power-Driven Vessels. Rules 18 and I
The basic rule is quite straightforward and is quoted here in full: `When two power-driven vessels are crossing, so as to involve risk of collision, the vessel which has the other on her own starboard side shall keep out of the way of the other.'
L
A
168 SMALL BOAT NAVIGATION i THE RULE OF THE ROAD AT SEA 169
In the normal course of events, the `burdened' vessel, which has the other to starboard of her, will alter course to starboard to avoid crossing ahead of the other ship.
The end-on situation is usually more difficult; this is covered in Rule 18 and this is quoted in full here (apart from the paragraph covering seaplanes) :
`When two power-driven vessels are meeting end on, or nearly end on, so as to involve risk of collision, each shall alter her course to starboard, so that each may pass on the port side of the other. This Rule only applies to cases where vessels are meeting end on, or nearly end on, in such a manner as to involve risk of collision, and does not apply to two vessels which must, if both keep on their respective courses, pass clear of each other. The only cases to which it does apply are when each of the two vessels is end on, or nearly end on, to the other; in other words to cases in which, by day, each vessel sees the masts of the other in a line, or nearly in a line, with her own; and by night, to cases in which each vessel is in such a position to see both the sidelights of the other. It does not apply to cases in which a vessel sees another ahead crossing her own course; or by night, to cases where the red light of one vessel is opposed to the red light of the other or where the green light of one vessel is opposed to the green light of the other or where a red light without a green light or a green light without a red light is seen ahead, or where both green and red lights are seen anywhere but ahead.'
Interpretation
The italics in the above text are mine, but see to what great (and almost exhausting lengths) the Rule goes in defining the `end on' situation. This is because it is one of the trickiest for assessment of the correct action to be taken. There is also always the doubt as to what action the other ship is going to take in these circumstances.
Suppose, for instance, two ships or boats sight each other at comparatively long range very fine on each other's starboard bow. Each knows that if a true end on situation exists, each should alter course to starboard, viz. Figure 28A.
But if the other is not exactly dead ahead, the two vessels may pass clear starboard to starboard, viz. Figure 28B.0~ ~ i 0
30
0* 0*
3 Y
Figure 28
Dangerous doubts can arise if the situation is allowed to develop into a close range situation when neither is sure whether the other will hold her course with the hope of passing clear to starboard or alternatively is about to alter course thinking it to be too risky to hold her present heading (Figure 28c).
In ship `X', the skipper is doubtful whether by continuing on his present course he will risk a head-on collision with `Y'. The same thoughts may well be in `Y's mind. For both of them at this late stage there are four possible alternatives:
(a) To hold their present course and speed in the hope that they will pass clear.
(b) To alter course to starboard.
(c) To alter course to port.
(d) To take the way off the ship by going astern.
A
C
I70 SMALL BOAT NAVIGATION THE RULE OF THE ROAD AT SEA 171
Of these alternatives:
(a) May well be a vain hope and is probably too risky.
(b) Is a viable action if there is room enough for `X' to cross `Y's track before they get too close to each other and similarly for `Y' to cross `X's track.
(c) An early alteration to port on both ships' parts at long range would have prevented this situation from developing, but a late alteration to port on either ship's part would be extremely dangerous. Suppose `X' at the last moment decided he was not going to clear on his present course and made an alteration to port. Simultaneously `Y', worried by the closeness `X' is likely to pass on his present heading, decides to alter course to starboard. The result is very probably a nasty collision. (See Figure
28D.)
(d) Taking the way off by going astern may be a way of preventing collision (or minimising its results) if it is done in time.
Avoiding the Close Quarter Situation
The main point to remember in any interpretation of these rules is that the close quarter situation should never be allowed to develop. An early, bold alteration by either ship in a doubtful situation will clarify it from the start and remove the dangerous elements of doubt inherent in late, hasty avoiding action. This applies to all combinations of circumstances and not just to the particular case quoted above.
A good indication whether a close quarter situation is likely to develop later is given by carefully watching the compass bearing of every approaching vessel. If the compass bearing does not change, and both vessels maintain their present courses and speeds, the two will collide. If the bearing is only changing slowly, the two craft may approach each other later at too close a range for comfort (the rate of change of bearing is, of course, a function of the range between the two ships). In the long range nearly head-on situation, the watching of the other ship's bearing can be the only reliable way of judging the relative motion of the two ships, particularly if the other ship is yawing in a seaway and it is difficult to determine its mean course.
At long range, the compass bearing is normally taken of the other ship's bridge or mast. At short ranges, it may be necessary to take the bearing of the right section of the other vessel; forexample, if you are trying to cut under a super-tanker's stern, the bearing of its stern, and not its bridge, must be watched (cutting across such a ship's bows is in no way recommended—bearings or no) .
Power Driven Vessel's Meeting a Sailing Vessel. Rule 20
`When a power driven vessel and a Sailing vessel are proceeding in such directions as to involve risk of collision, except as provided for in Rules 24 (the Overtaking Rule) and 26 (Vessels engaged in fishing) the power driven vessel shall keep out of the way of the Sailing Vessel.
This rule shall not give the Sailing vessel the right to hamper, in a narrow channel, the safe passage of a power driven vessel which can navigate only inside such channel.'
Interpretation
This rule is quite simple. Normally, sailing vessels have the right of way over power-driven vessels—except when they are overtaking, meet power driven vessels engaged in fishing, in a narrow channel, not under command, etc.
Holding Course and Speed. Rule 2I
`Where by any of these rules one of two vessels is to keep out of the way, the other shall keep her course and speed. When, from any cause, the latter vessel finds herself so close that collision cannot be avoided by the action of the giving-way vessel alone, she also shall take such action as will best aid to avert collision (see Rules 27 and 29).'
Interpretation
It is important that the `privileged' vessel holds her course and speed, otherwise early avoiding action by the `burdened' ship may be nullified and the situation confused.
Avoiding Crossing Ahead. Rule 22
`Every vessel which is directed by these rules to keep out of the way of another vessel shall, so far as possible, take positive early action to comply with this obligation, and shall, if the circumstances of the case admit, avoid crossing ahead of the other.'
I72 SMALL BOAT NAVIGATION THE RULE OF THE ROAD AT SEA 173
Slackening Speed. Rule 23
`Every power driven vessel which is directed by these rules to keep out of the way of another vessel shall, on approaching her,
if necessary, slacken her speed, or stop, or reverse.' X 1121 A
The Overtaking Rule. Rule 24
'NOTWITHSTANDING ANYTHING CONTAINED IN THESE RULES, every
vessel OVERTAKING any other shall keep out of the way of the overtaken vessel.
Every vessel coming up with another vessel from any direc‑
tion more than 222° (2 points) abaft her beam, i.e. in such a t
°2Z 0
position with reference to the vessel she is overtaking that at D B
night she would be unable to see either of that vessel's sidelights, X
shall be deemed to be an overtaking vessel; and no subsequent alteration of the bearing between the two vessels shall make the overtaking vessel a crossing vessel within the meaning of these Rules, or relieve her of the duty of keeping clear of the overtaken
vessel until she is finally past and clear. Y
If the overtaking vessel cannot determine with certainty y
whether she is forward or abaft this direction from the other
vessel, she shall assume that she is an overtaking vessel and keep Figure 29
out of the way.'
her beam; she must also be going faster than `X', for otherwise her bearing as seen from `X' would be drawing aft.
Interpretation `Y' is now an overtaking vessel, being more than 2 points
The capitals in this extract from the rules have been added for abaft `X's' beam, and is obliged to alter course to avoid `X' and
emphasis, as this is one of the most important regulations. Its keep out of `X's' way until she is completely clear ahead. If `Y'
application hinges on the relative direction of approach is doubtful when first seeing `X' whether or not she is more than
between the two vessels, two points abaft `X's' beam, she must assume that she is an
In Figure 29A, two power driven vessels are on converging overtaking vessel.
courses but `X' has just sighted `Y', and `Y's' bearing appears to
be steady. A risk of collision must therefore exist. .Narrow Channels. Rule 25
`X' must keep clear of `Y' in this case, as `X' has the other `In a narrow channel every power-driven vessel when proceed‑
}I
on, or just before her beam on the starboard side. `X's' best ing along the course of the channel shall, when it is safe and
action would be to alter course to starboard to pass under `Y's' } practicable, keep to that side of the fairway or mid-channel
stern, whilst `Y' must hold her course and speed. Alternatively which lies on the starboard side of such vessel.
`X' could reduce speed to allow `Y' to pass unhindered ahead. Whenever a power-driven vessel is nearing a bend in a
Suppose, however, a slightly different situation exists (Figure channel where a vessel approaching from the other direction
29B) when `X' is first aware of `Y's' presence. The bearing is' cannot be seen, such power-driven vessel, when she shall have
again steady, but `Y' is now approaching `X' from well abaft arrived within one half mile (? mile) of the bend, shall give a
M
174 SMALL BOAT NAVIGATION
signal by one prolonged blast on her whistle which signal shall
g y p
g g
be answered by a similar blast given by any approaching power-driven vessel that may be within hearing around the bend. Regardless of whether an approaching vessel on the farther side of the bend is heard, such bend shall be rounded with alertness and caution.
In a narrow channel a power-driven vessel of less than 65 feet in length shall not hamper the safe passage of a vessel which can navigate only inside such channel.'
Interpretation
(a) Keep to the starboard side of a channel. ~.
(b) Sound one long blast when going round the bend! (only applies to power boats—and the mad presumably).
(c) Self evident.
g
Fishin Vessels. Rule 26
`All vessels not engaged in fishing, except vessels to which the provisions of Rule 4 (vessels not under command, etc.) apply, shall, when under way, keep out of the way of vessels engaged in fishing. This rule shall not give to any vessel engaged in fishing the right of obstructing a fairway used by vessels other than fishing vessels.'
Limitations of Craft. Rule 27
`In obeying and construing these rules due regard shall be had to all dangers of navigation and collision, and to any special circumstances, including the limitations of the craft involved, which may render a departure from the above rules necessary in order to avoid immediate danger.'
Neglect of Seamanlike Precautions. Rule 29
`Noting in these Rules shall exonerate any vessel, or the owner, master or crew thereof, from the consequences of any neglect to carry lights or signals, or of any neglect to keep a proper lookout, or of the neglect of any precaution which may be required by the ordinary practice of seamen, or by the special circumstances of the case.'
Interpretation
So Watch Out!
THE RULE OF THE ROAD AT SEA I75
Local Harbour Regulations. Rule 30
`Nothing in these rules shall interfere with the operation of a special rule duly made by local authority relative to the navigation of any harbour, river, lake, or inland water, including a reserved seaplane area.'
Distress Signals. Rule 31
`When any vessel ... is in distress and requires assistance from other vessels or from the shore, the following shall be the signals to be used or displayed by her, either together or separately, namely:
(i) A gun or other explosive signal fired at intervals of about a minute. (Not normally a thing to hand!)
(ii) A continuous sounding with any fog-signalling apparatus.
(iii) Rockets or shells, throwing red stars fired one at a time at short intervals.
(iv) A signal made by radiotelegraphy or by any other signalling method consisting of the group • • •---• - • (S.O.S.) in the Morse Code.
(v) The International Code signal of distress indicated by
N.C.
(vi) A signal consisting of a square flag having above or below it a ball or anything resembling a ball.
(vii) A signal sent by radiotelephony consisting of the spoken word "Mayday".
(viii) Flames on the vessel (as from a burning tar barrel, oil barrel, etc.
(ix) A rocket parachute flare or a hand flare showing a red light.
(x) A smoke signal giving off a volume of orange-coloured smoke.
(xi) Slowly and repeatedly raising and lowering arms outstretched to each side.
The use of any of the foregoing signals, except for the purpose of indicating that a vessel or seaplane is in distress, and the use of any signals which may be confused with any of the above signals, is prohibited.'
Direction on the Earth's
Surface
A good compass is an essential piece of equipment. However, it is not much use having a dependable compass but no idea of the proper course to steer to reach a particular destination or how long it will take to get there.
We are fortunate these days in that practically the whole of the Earth's surface has been either charted or mapped, and therefore the business of finding the course between two places is a simple matter. It was the British Admiralty that gave the major impetus to the science of cartography (map-making) in the 18th century by commissioning such men as Cook to carry out explorations and meticulous surveys and to record the observations they made. Nowadays, many nations produce charts from their own or other nation's hydrographic information, but the Hydrographic Department of the British Admiralty alone produces over 6000 different charts and diagrams in common use by seamen all over the world.
Distance on the Earth's Surface
The basic unit for the measurement of distance at sea is the nautical mile. This is a distance of 6o8o feet. Why this awkward figure ? It is in fact the distance subtended at the Equator by an angle of i minute of arc at the Earth's centre. The significance of this chosen distance is that I minute of Latitude = I Nautical Mile and therefore, as will be seen later, distance can simply be measured from the latitude scale on a chart without conversion or a separate scale being necessary.
For general purposes, the mile is considered to be 2000 yardsDISTANCE, COURSE, SPEED AND DIRECTION 47
long, divided into io cables of zoo yards apiece. The old fashioned term `league' was a distance of just under three miles but this is seldom used at sea today; nor are the statute mile of 1760 yards, furlongs etc.
Speed
A knot is defined as a speed of one nautical mile per hour. Therefore, do not fall into the old landsman's trap of saying in airy fashion `Oh! I was doing 20 knots per hour' as any bloke around might presume you were serving your time. Twenty knots means 20 nautical miles per hour.
If you have difficulty converting time and distance into speed and vice versa, remember this simple formula:
Distance (miles) = Speed (knots) x Time (hours) D=ST
Therefore, if you have covered I o miles in 40 minutes your speed must be:
I o= S x 60 or S= 6 0= 15 knots. 4
Another very useful dodge to remember is the `six minute' rule. Six minutes is, of course, one tenth of an hour. Therefore, if you know or can estimate the distance you have covered in six minutes, a straight multiplication of this distance by ten will give your speed direct.
e.g.
You travel 0.2 miles ( 2 cables) in six minutes. Speed 2 knots You travel 0.5 miles ( 5 cables) in six minutes. Speed 5 knots You travel I.2 miles (12 cables) in six minutes. Speed 12 knots You travel 2.5 miles (optimist!) in six minutes. Speed 25 knots
Obviously you can adapt this rule in your head to cover different times, e.g. for a three minute run multiply by 20, and for twelve minutes' run multiply by 5, etc.
Direction on the Earth's Surface
This is not quite as simple as it may seem at first sight as the Earth's is not a plane surface; nor is it a sphere but a spheroid
this figure the curve is exaggerated for illustrative purposes, but the courses are correct for an uninterrupted Great circle track.
Thirdly, the fact that the Earth's spherical surface has to be translated onto a flat sheet of paper sets a problem for cartographers and some distortions are inevitable.
Before discussing the practical solution of these problems it is necessary to explain the geographical reference framework that is superimposed over the Earth's surface to define position —the Latitude and Longitude grid that has been in use for centuries.
Latitude
As the Earth is almost spherical, distances between a point on its surface and a reference axis must be measured in angular units. The reference used for Latitude is the Equator, the great circle midway between the poles. Latitude is measured North and South from o° at the Equator to go° N at the North Pole and go° S at the South Pole. The Latitude of intervening places is defined by the angle that it subtends at the Earth's centre from the Equator. e.g. in Figure 7B places X, Y, and Z all subtend the same angle to the Equator and are therefore at equal Latitude. The small circle (not passing through the centre of the Earth) joining these places is called a Parallel of Latitude, in this case that of 50° N.
The Latitude of a particular point is given in degrees, minutes and seconds; for instance 500 00' oo" North (60 minutes in a degree-6o seconds in a minute). Remember also that I minute of Latitude = I Nautical Mile. Therefore, the parallel of 50° N must be 5o X 6o =3000 miles North of the Equator.
Difference of Latitude D'Lat.
The difference of latitude between two places both North of the Equator or two places both South of the Equator is the straight arithmetic difference between them, e.g.
First point 56° 29' 35" N Second point 31° 53 2711 N
D'Lat. = 24° 36' o8"
Longitude
This is also measured in angular units. As the Earth is circular, it could have been defined from an arbitrary place on the surface from o0-3600 in one direction. However, the terms East and West had been in use for centuries, and as England was in the forefront of the maritime powers in the 18th century, the position of the Greenwich Observatory was chosen as the datum line at Longitude o° or zero. From this place, Longitude is measured i8o° East and 1800 West. The `join' at i 8o° Longitude occurs in a line that runs through the Pacific (the Date Line). Lines joining places of equal Longitude are called MERIDIANS—these circles (Great circles) start from one pole, cut the Equator at right angles and pass down to the other pole. Figures 8A and 8B illustrate the Longitude grid superimposed on the Earth's surface.
It can easily be seen that a person standing at the Pole can travel from the Greenwich Meridian through I8o° East and
back through West merely by turning on his heel, but he would
Direction and Bearing
iIf have to walk around the whole Earth's circumference to change
Direction is basically measured from the meridian, i.e. the
longitude by the same amount at the Equator. In other words,
•
direction of True North if referring to the geographical
the meridians converge at the poles, and the distance between
meridian. Both True Course—and the True Bearings of
one meridian and another on the Earth's surface varies with
objects—are measured clockwise from the meridian and
flatitude. Also, if you travel up or down one meridian, you must
expressed as an angle between o° and 3600.
North
be travelling Nor or South.
III
Course
THE TRUE COURSE
Diference of Longitude
The angle between the boat's fore & aft line and the meridian
In much the same way as D'Lat., difference of longitude is
'
is her course. So if the boat is steering 260° True she will be
measured in degrees and minutes (and seconds if necessary) as
heading through the water in a direction I o° South of West.
an algebraic difference, e.g.:
8c.
(Figure)
First place 123° 00.2 E
I
If there is no tidal stream, current, wind, or compass error,
Second place 25° 43.6 E
she will also be making good this course over the ground.
D'Long. = 970 16.6
COURSE MADE GOOD
Life, however, is seldom so simple that one or more of these
or
disturbing effects does not exist in practice. Therefore though
First place 22° 10.4 E
the boat may be heading 260° through the water, a current
Second place 02° o8.6 W
setting to the South may result in the boat making a `course
made good' of 250°. This `course made good' can only be
D'Long. = 240 I9.0
precisely established by fixing, i.e. finding the geographical
position of the boat at a number of successive times, or it can be
Aslight trap arises when the two places in question are
estimated from knowledge of the currents and the amount of
l 8 i l i d the
neary 1o° apart in longitude, maybe across te Date ne.
Line.
`::
leeway the boat makes in a wind of a given strength and
The shortest way round must be taken—thus the D'Long
direction.
between
The course steered and the `course made good' are, therefore,
I O° o5.o E T
more often than not different directions, the first related to the
water and the second to the ground.
and
$ 70° 00.0 W
Compass Course and Magnetic Course
As explained in the last chapter, the Magnetic course is the
is NOT 1800 05.o but 1790 55.0' (18o° —I Io° 05.0 +180°70°)
angle between the Magnetic Meridian and the boat's fore &
aft line. Similarly, the Magnetic Compass course is the angle
Departure
between the direction of Compass North and the boat's fore
I: This is the distance in miles travelled in an East or West direction
& aft line. To arrive at a True Course, both variation and the
over the Earth's surface.
compass deviation (on that particular heading) must be
applied.
Departure = D'Long. x Cosine Mean Latitude.
The following example may help to summarise this:--
Bearings
True Bearings and Compass Bearings are defined in very much the same way. The True Bearing of an object is its angle measured clockwise from the meridian.
In Figure 8F, the lighthouse bears Ioo° True from the observer. The True Course to steer for the light is also I oo°.
As a reminder—when using a magnetic compass for taking
4i the bearing of an object, the compass bearing must be corrected for deviation and variation to obtain True Bearing (again remember to use the deviation for the boat's heading and NOT the direction of the object). Example:
Compass Course 300°—(Deviation on this course 30 E) Compass Bearing of object 125°
Deviation (30 E) + 30
Variation (12° W) —12°
TRUE BEARING = i16°
Relative Bearing
Relative bearings are commonly used for indicating the position of another object or craft relative to one's own vessel.
A relative bearing is measured in degrees from one's own ship's head, 18o° in each direction from the bow. The Starboard (right) side is designated GREEN and the Port (left) side RED.
In Figure 8G, the relative bearing of the other boat is Green 30°, i.e. 30° on the Starboard bow. As the boat's course is 025°, the True Bearing of the other boat would be 055°
DISTANCE, COURSE, SPEED AND DIRECTION 55
(025+30). Were the other vessel 30° on the Port bow, the Relative Bearing of it would be Red 300, and the True Bearing 355° (360 +25-30).
The terms Bow, Beam, and Quarter are also commonly used. Strictly speaking, an object said to be `on the Port Bow' without any number of degrees specified should be bearing Red 45°, i.e. 450 to port of right ahead—'on the Port beam' Red go°—and `on the Port Quarter' Red 135°. However, these terms are often more loosely used to indicate a general area, `,Port Quarter' meaning anywhere on the port side from right astern to Red 135°, `Port Beam' meaning anywhere from Red 135° to Red 45°, and `Port Bow' anywhere from Red 45° to right ahead.
7lie Great Circle and the Rhumb Line
Earlier in this chapter it was stated that the Great Circle route is the shortest distance between two places on the Earth's surface, and that steering a Great Circle course means a constantly changing heading. In practice, on a leg of less than about 200 miles, the distance lost in steering a steady course said not a Great Circle is not significant. Even when following a Great Circle route, it is not a practical proposition to alter the boat's heading continuously and gradually to follow the curve, as the rate of change of heading is so small even at high speed.
The Rhumb Line
I u order to steer a steady course over the Earth's curved surface, the boat must follow a track that cuts successive meridians at the same angle (otherwise it would be altering course). Such a track is called a RHUMB LINE—by definition a linc that cuts all the meridians at the same angle. On the Earth's surface it is, in fact, a spiral (Figure 8x).
In this example, a Rhumb Line course of 070° is shown. In I,ractice, the navigator always steers a succession of Rhumb Line courses, even when following a great circle track. This is explained in Appendix II.
Charts
The chart is to the seaman what a map is to the landsman—a pictorial representation of the Earth's surface. The chart, however, shows features both above and below the water line.
Different projections, or methods of transferring the Earth's curved surface onto a flat piece of paper can be and are used in chart construction. But by far the most common projection used for charts is the Mercator's projection. This is used for practically all charts of a natural scale smaller than i I inches = i mile.
Mercator's Projection
The simplest way to explain the Mercator's projection (which came into use at sea in about 1630) is to imagine a paper cylinder wrapped around the world. Imagine also that the Earth is fairly transparent and that a lamp is burning at the Earth's centre. The various land and sea features would then be shown up as shaded images on the surrounding paper.
Figure 9A is a representation of a standard Mercator projection where the cylinder touches the Earth at the Equator.
A Transverse Mercator projection is similarly constructed but in this case the paper cylinder is turned through ninety degrees and touches the Earth at the poles.
In fact, the simple explanation so far given is not entirely accurate, as the projection is mathematically adjusted to give the chart certain properties. The principal ones are:
i. Rhumb lines are straight lines on the chart. Therefore courses can be laid off on the chart or read off without
difficulty.
2. The Equator, which is both a Rhumb line and a Great circle on the Earth, is a straight line on the chart.
Longitude ' 111°'°"
of Greenwich
Long —
Longitude sc le C
IS C
10 ~.
0 5 io is
Latitude stale (S'to lin.) Long.
180°
Figure 9
3. The Meridians (great circles converging at the poles on the Earth) are straight, equidistant parallel lines on the chart, cutting the Equator at right angles.
4. The Parallels of Latitude (small circles on the Earth) are straight, but not equidistant, lines parallel to the Equator on the chart.
5. Great circles, on the chart, are curves whose apexes lie towards the poles.
Thus the Latitude and Longitude grid (or graticule as it is sometimes called) appears on the chart as a lattice of straight lines, with the meridians equally spaced, but with the parallels of latitude increasingly far apart the further they are from the Equator.
You will see in Figure 9B that the length of the line BC is greater than AB on the chart although both correspond to a difference of io° in latitude. The latitude scale on a Mercator chart is thus variable, steadily expanding the further North or
South the area covered, until at the Poles it is infinitely large. As this scale is also used for the measurement of distance, it is not possible to measure distances from anywhere on the scale.
Distance MUST BE MEASURED FROM THAT PART OF THE LATITUDE SCALE IN THE MARGIN OPPOSITE THE POINTS CONCERNED. For
example, the distance AD would be measured with a pair of dividers at EF on the scale.
The Longitude scale (across the top and bottom of the chart) must never be used for the measurement of distance.
Disadvantages of the Mercator's Projection
On the Earth, the parallels of latitude remain a constant distance apart and the meridians converge at the Poles. On the Mercator's chart, the Longitude scale is constant whilst the Latitude scale is made to expand with Latitude (it is actually related to the Secant of the Latitude). The result is that the land masses on the Mercator chart become increasingly `blown up' in size (although they retain their correct shape) the further North or South they are, and the ordinary Mercator cannot be used at the Poles where the latitude scale would be infinite. Except in the Polar regions this distortion does not matter practically. For instance, although Iceland and an Equatorial island of the same actual size on the same Mercator chart would appear totally out of proportion in size (Iceland seeming to be three times bigger than the other), the Latitude scale opposite to Iceland would also be three times bigger. Thus shape, distance, and bearing are preserved, but areas are distorted.
Constructing a Latitude and Longitude grid on a plain
sheet of paper
In ocean areas well away from land, it quite often happens that the scale of the only chart available is very small—a pencil point may be the equivalent of one mile. In such situations it is impossible to plot an astronomical fix accurately. It is therefore necessary to draw a Latitude/Longitude grid on a plain plotting sheet using a much larger scale, for instance 5 miles equals one inch.
This is easily done (as illustrated in Figure ga) by following this procedure:—Draw your Latitude scale horizontally across the paper.
(i) From the zero of the Latitude scale, draw a line at an :angle equal to the Latitude of your position—in this case 600.
(ii) Drop perpendiculars to this line from the Latitude graduations. Your Longitude scale is then given along the inclined line.
It is fairly easy to see from this that at Latitude go°, the longitude scale would be zero, whilst at the Equator (Latitude o°) the two scales would be the same This is the `mirror Picture' of the Mercator chart graduations (Longitude scale constant and Latitude scale infinite at the Poles). The main thing to remember is that, except at the Equator, the latitude graduations will always be further apart than the Longitude uses. (Note: This construction is only reasonably accurate over a comparatively small area—about 200 miles each way.)
[he Gnomonic Projection
The main property of this projection is that Great Circles appear on a gnomonic chart as straight lines.
The chart is drawn by projecting the Earth's surface onto a plane touching the Earth's surface at a chosen tangent point. II' the tangent point selected is a Pole, the Latitude and Longitude grid produced is quite simple, as shown in Figure n and E.
The Parallels of Latitude are concentric circles round the Pole, and the Meridians straight lines radiating from it.
Unfortunately, Rhumb lines are not straight lines on a Great Circle chart, and therefore course and direction cannot I determined directly from a small scale gnomonic projection.
On a much larger scale they are, however, used for harbour plans, where the small area covered can safely be assumed to be `flat'. Courses can be laid off on these charts, and a separate distance scale is normally given on gnomonic charts of this t yl>c, together with the Latitude and Longitude of a given `sl4Ot' on the chart.
The other uses of Great Circle charts and diagrams are for I Iar charts (where the Mercator projection becomes useless) and for Great Circle sailing.
British Admiralty Charts
The ordinary navigational charts published by the British Admiralty cover all the areas of the world. They are grouped in Folios, each folio covering a particular geographical area and containing charts of both large scale (harbour plans) and small scale (passage and oceanic charts). Each chart is given an individual distinguishing number (e.g. Chart No. i824a shows the East Coast of Ireland together with the Irish Sea and St. George's Channel).
The numbers of all the charts published by the Admiralty, their folio groupings, and their prices are given in the Catalogue of Admiralty Charts, which is itself re-published annually.
These charts are fairly expensive, averaging about 83 pence for an ordinary chart to CI .2o for one that is overlaid with a radio aid grid—such as a Decca lattice. The average yachtsman will not want to be too lavish when buying charts from a chart agent. The best way, therefore, to see what charts are available and to obtain the necessary area coverage without excessive cost, is to consult the diagrams in the chart catalogues showing the individual limits of each chart and then to choose accordingly. A reputable chart agent will hold copies of the catalogues and help with the selection, so a personal purchase of a catalogue is not essential. References to charts (and their limits) are also contained in the Admiralty Sailing Directions (or Pilots). However, the catalogue is the better reference, being republished every year whereas the Sailing Directions are not.
Yachtsman's Charts
Until recently, the Admiralty also published a series of yachtsman's charts and these are now produced commercially by firms such as Stanford and Imray. They have the big advantage for small boat use of being much smaller in size, which allows them to be used on small chart or saloon tables—or even on a portable board. Their current price is about go pence per chart.
Sizes of Charts
The ordinary Admiralty charts are designed to fold into a uniform size for convenient stowage in a folio, but they are not all identical in overall size; for instance some charts (mainly harbour plans) are printed on a single unfolded sheet; others (passage charts for example) are folded double and must be opened for use. If the space is available, an important point in a boat is to have a chart table of large enough size to allow a comfortable working surface—the minimum area needed to take an ordinary Admiralty chart unfolded is about 38 inches X 25 inches. Some Yachtsman's charts are 13 inches long X 8 inches deep, other about 21 inches X 15 inches.
Selection of Charts
To sum up—there are these two basic types of chart: the Admiralty charts in use by seamen, in ships large and small, all over the world and the commercial adaptations of these charts specifically designed for use in small craft in limited areas. Charts are not cheap and you will not want to spend more than is necessary. However, when planning a voyage, it is essential to have sufficient charts to cover all eventualities—for example, if going on passage down the Spanish coast it would be unwise to rely merely on a small scale passage chart; in the event of a blow, you might have to run for cover and it might prove difficult to trip your way through the rocks on the back of a small postage stamp. Therefore, you would need the coastal charts and harbour plans on that coast. Combine prudence with thrift!
Chart Titles and Numbers
Each chart has its own individual title and number. These chart numbers are geographically somewhat haphazardly distributed, so in ships carrying complete folios it is customary to give the charts another number—the consecutive number. The top chart of the folio would be given consecutive number I, and so on. Most boats will not be carrying complete folios, but it is not a bad idea to adopt a similar numbering system for the charts carried. A folio list on the front of the chart cover showing these consecutive numbers against each title much
simplifies the task of finding or stowing away a particular chart.
Chart Scale and Type of Projection
The natural scale of the chart and the type of projection on which it is drawn is shown just underneath the title. The natural scale is a straightforward comparison of distance on the chart against distance on the Earth's surface. With a scale of I/150,000, an inch on the chart therefore represents 150,000 inches on the Earth's surface, or about 2.08 miles.
This is of general interest, but of more practical importance is the scale of distance. Remember that on Mercator projections one minute of LATITUDE equals one mile, and that when using dividers to measure distance on the chart, that part of the scale (at the side of the chart) opposite the distance being measured must be used. NEVER use the Longitude scale, running across the top and bottom of the chart, for distance measurement.
On harbour plans—quite often drawn on the Gnomonic projection—a special distance scale is provided.
INFORMATION ON CHARTS
It is impossible to list here all the conventional symbols that are used on charts and getting to know them is largely a matter of experience. However, a special chart is published by the Admiralty (No. 5011) which gives all the symbols used on British charts and this is an easy means of reference.
Land Features
The amount of information given on land features is limited to that which is of practical use to the navigator at sea. Thus a considerable amount of detail is shown of the coastal areas, whilst inland areas are largely left blank—except where, for instance, a hill or mountain can be seen from seaward. In recent years, with the increased use of radar for navigation, contour lines and spot heights (e.g. ggo—a hill 990 feet in elevation) have become of more direct interest and greater detail is shown of these.
A B C D E
,.ry
O Chy o9 O Orr
Bn Bn
CHURCH LIGHTHOUSE CHIMNEY BEACON TOWER F
Figure r o
.just a few of the more common land symbols are shown in 'Figure io (A–E).
An object that stands out boldly when viewed from seaward may have the notation (conspic.) written on the chart against its symbol—a conspicuous object.
Lighthouses
These are a minor subject in theriselves. They are shown by the conventional symbol for a lighthouse surmounted in the case of a major light with a magenta coloured flash (Figure I OF) so that they readily catch the eye. Obviously on small scale ocean charts it would be impossible to include all the navigational lights on a seaboard, so only the biggest and best are shown.
The British Admiralty publishes a book called the Admiralty list of Lights (in a number of different volumes covering different parts of the world) which gives complete details on all the navigational lights. Their characteristics, heights, power etc. are all listed and also whether they are attended (lighthouse keeper) or unattended (nobody around!). In some places, a light may only be switched on at particular times or for a particular event. Do not therefore expect a light that is marked (Occas.)—occasionally—on the chart or in the Light Lists to lbc always switched on when you desperately need it—it may he that the local luminary has gone home, and one is not
rmally handily placed to ring him up and ask that it be lit.
' I'he lighthouses along a particular stretch of coastline all have different characteristics, so that one may not easily be confused with another. The main varieties of lights are Fixed, l- lashing or Occulting. A fixed light, self-evidently, is one that is on all the time; a flashing light is one whose characteristics
64 SMALL BOAT NAVIGATION CHARTS 65
are such that the period of darkness exceeds the period of light; and an Occulting light is one that is ON more than it is off.
If no colour is specifically mentioned, then it is white. The letters (W) (R) (G) are used on charts to indicate colour whenever necessary; white, red and green respectively. There is a complication in the existance of Alternating Lights (marked Alt. on the chart) . These successively flash the colours indicated.
obscured I white
1
green red
Figure ii
Thus Alt. W.G. means that a light will successively show first a white flash followed by a green flash throughout its visibility range.
These alternating lights must not be confused with Sectored lights, which show different colours continuously in different sectors. These sectors are normally marked on the chart by dotted lines, and the Red sector of a light is usually chosen to cover some particular hazard in that area. One other thing to remember about sectors—in the Light Lists, the sectors covered by a particular colour are always given in BEARINGS FROM SEAWARD. For example, a light showing White from 18o° to 29o0, Red from 29o0 to 000°, Green from 000° to 030°, and
Obscured from 030° to 18o0 would be shown on the chart as shown in Figure i I.
Some other points on lights that need to be mentioned. The `Period' of a light is the time it takes to complete one whole cycle of its operation, measured from the start of its first flash. Some examples again—a light that is shown on the chart as:
(/.Fl. (2) is a light that gives two white flashes in quick
I o sec. 29M succession—followed by an interval of darkness —every I o seconds. The 29M means that it is visible on a clear night 29 miles away from an observer whose height of eye is 15 feet above sea level.
0cc. W.R. This is a sectored light (white shown in one
I o sec. sector, red in another) which goes out for a
brief period once every io seconds.
Gfi.Fl. (3) This lighthouse shows two lights. The major
Io sec. 125 ft. light flashes three times in quick succession
17 M F.R. every io seconds. It is at an elevation of 125
I 12 ft. IoM feet above sea level, and visible 17 miles.
The second is a Fixed Red light at a slightly
lower elevation and only visible io miles.
(p.Occ. (q.) This light goes out four times in quick suc‑
W.R. 16 sec. cession every 16 seconds. The white light is
I 7,14M visible for 17 miles and the red for 14. Again
it is a sectored light.
All. W.R. This gives a white flash followed by a red
20 sec. 15M flash every 20 seconds all round the horizon
and is visible from 15 miles.
These are but a few of the characteristics of lights that may )c met with. The majority of the markings against lights on the charts are fairly self evident. If further information on a I).trticular light is required, a glance at the Light List will give the full details, for example how long each successive flash is in a Group Flashing light. In practice it is not often necessary to consult the Light List unless you do not possess a good chart of the area concerned.
Providing the visibility is good, the range at which a light is
66 SMALL BOAT NAVIGATION CHARTS 67
U
sighted can give a good indication of your distance from it. The loom of very powerful lights can normally be seen long before the light itself is sighted, but it should be remembered that lights at very high elevation may be obscured by cloud; and weak lights may be obscured by haze.
Depths on a Chart
At the present time, the depths on existing British Admiralty charts are normally shown in fathoms and feet (a fathom =6 ft.) On some large scale charts such as harbour plans, the depths may be shown in feet alone. It is normally self evident which depth scale is being used, as with the fathom scale, the odd number of feet between whole fathoms are shown as a small figure beside the fathom digit. A depth of 52 on a chart indicates that the depth in that particular spot is 32 feet i.e.
5 x 6+2=32). However, if there is any doubt, check the notation under the chart Title-it is always stated whether the depths are given in fathoms, feet or soon (perish the thought) metres.
For at the present time British charts are being converted to the metric system already in use on many foreign charts. The soundings will then be shown in metres and points of a metre. A conversion table for putting fathoms and feet into metric quantities is given below.
CONVERSION TABLE-FATHOMS AND FEET INTO METRES
Feet 6 12 18 24 30 36 42 48 54 6o
Fathoms I 2 3 4 5 6 7 8 9 10
Feet - i.8 3.6 5.5 7.3 9.1 10.9 12.8 14.6 16.4 18.3
1 0.3 2.1 3.9 5.8 7.6 9.4 11.3 13.1 14.9 16.7 18.6
2 o.6 2.4 4.2 6.1 7.9 9.7 11.6 13.4 15.2 17.0 18.9
3 0.9 2.7 4.5 6.4 8.2 10.0 11.9 13.7 15.5 17.3 19.2
4 1.2 3.0 4.9 6.7 8.5 10.3 12.2 14.0 15.8 17.7 19.5
5 I.5 3.3 5.2 7.0 8.8 io.6 12.5 14.3 16.1 18.o 19.8
Chart Datum
The depths shown on a chart cannot be taken as absolute depths, i.e. ones that never change, because of the rise and fall of the tide; indeed, meteorological conditions in some parts of the world can alter the level of a whole sea area at different seasons of the year.
When a survey is carried out, therefore, the depths found at all the different points on the sea bottom have to be corrected for the tidal conditions appertaining at the time and referred to a common datum before they are plotted and the chart drawn. This datum is known as Chart Datum, and for safety reasons is so chosen that the sea level seldom falls below it (it approximates to the lowest predictable tide level under average Incteorological conditions).
height‑
charted elevation
of lighthouse
__m_eanH~hWaterSprings____~
sea surface
mean sea level
heightoftide- i5ft. mean lnw Water Springs _ , _ _ _ _ , I _ _ _ _
actual depth beneath
charted depth 32+ 15 .- 47ft.
51I.31ft 35.23ft
bed " depth m 32ft.
Figure 12
From a practical point of view, it is not necessary to know the actual datum level chosen-i.e. 52.9 feet below an ordnance survey bench mark set into the Vicarage wall at Little Belchillglon; it is, though necessary to appreciate that the depth of water over a patch marked 21 (two fathoms one foot) will seldom be exactly 13 feet. It is likely for the majority of the iiine to be more than that, but there may be a few occasions when it is less. To find the actual depth over that patch at a particular time, the height of the tide must be calculated and added to it. Again, to err on the side of caution, the elevation of land features on a chart are referred to a different plane-Mean High Water Springs. (An explanation of Spring and
68 SMALL BOAT NAVIGATION CHARTS 6g
Neap Tides is given in the next chapter.) This reference is chosen so that their charted elevation will rarely be less than the figure shown. This point is perhaps of less importance these days in large ships when ships' navigators use radar instead of vertical sextant angles for taking the range of an object—but a vertical sextant angle can still be useful to the yachtsman not possessing any other ranging device.
Figure I2 may help summarise the difference between the various levels and datums:
The yacht is shown over a spot marked 52 on the chart; however the actual depth of water under her keel at that particular moment in time is 47 feet, the height of the tide being 15 feet.
Mean sea level is also shown; this can be taken as the average level of the sea surface over the whole year.
Fathom Lines
These lines, joining points of equal depth, are drawn on the chart with each different line having a conventional symbol
B yY'
-b-. 61 6L /
61
62 , 6ij
54
44 S 53 55 55` 54 ~oa +
D
F G H E
~ K `~ 1 ' ~7rcul
,~
PD Eb .~
Figure 13
C Rocky patch which does D Individual dangerous rock
not uncover—dotted line with less than 6 feet of water
encloses danger area over it
E Rock awash at the level of
Chart Datum
F Position of a wreck that is NOT G Wreck that is considered
dangerous to surface navigation dangerous to surface navigation
H Dangerous wreck—Position J Wreck—Existence Doubtful
Doubtful
K Foul
composed of dots or dashes (sometimes combined with colour shading) to indicate its value. However, it is not necessary to remember the individual symbol for each fathom lines as the soundings on either side of the line indicate its numerical value. For instance, the 6 fathom line might appear as shown in Figure 13A.
Drying Heights
Certain features are covered and uncovered by the sea, depending on the state of the tide; reefs or rocks near the shore, [r example, or sandbanks out to sea. A patch that dries 2 feet above Chart Datum is shown by the figure 2 with a line drawn underneath it, e.g. 2. Sometimes features are marked with the words `Dries ... feet' written alongside. The latter method is normally used with individual features—a rock for example, and the former to indicate a small area or patch.
An above water rock that does not cover at all would be marked as in Figure 13B. The numeral alongside gives its elevation above M.H.W.S.
1)angers
Some of the most important chart symbols for the dangers to navigation that lie beneath or on the surface are given in Figures 13c J.
1%oul Ground
The remains of a wreck or other debris on the bottom which is not dangerous to surface navigation, but which should be avoided by vessels anchoring. (Figure 13K.)
Type of bottom
The nature of the bottom is indicated by various letters written alongside the soundings. These are primarily of interest from the point of view of anchoring, as the nature of the sea bed will affect the holding power of the anchor. The best holding ground is thick mud or sand and the worst, rock and weed. The types of holding ground are (with the chart symbols) in approximately the correct descending order of preference:
70 SMALL BOAT NAVIGATION CHARTS 71
S......Sand
M ...... Mud Good Holding Ground Cy...... Clay
G...... Gravel Sh ...... Shingle
P ...... Pebbles Moderate
St...... Stones Sh ...... Shells
R ...... Rock Poor
Wd...... Weed
Buoyage
Navigational buoys are used where it is not possible or economic to build permanent marks such as lighthouses or beacons. Some dangers, such as sandbanks, shift their position quickly and therefore the marks showing the limit of these hazards have to be moved quite frequently. The major hazards in busy shipping lanes out to sea are sometimes marked by Light Vessels instead of Buoys.
The charted position of a buoy is the position of the small circle in its base. However, it must always be remembered that buoys, which are moored to the bottom by a length of chain, may be shifted by heavy weather, and therefore total reliance should not be placed in their position.
Two systems of buoyage are used—called the LATERAL and CARDINAL systems. The former is used in United Kingdom waters (and in the tidal waters of many other countries) and is the one described here. The other, CARDINAL system, is more commonly used in areas where there exist a large number of offshore islands, rocks or shoals, and the shape of a particular buoy is governed by its true bearing from the danger.
The Lateral System of Buoyage
With the Lateral system, the character of the buoys used is governed by the direction of approach, i.e. a port hand buoy (of a particular shape) must be left to Port WHEN ENTERING
A CHANNEL OR GOING with THE FLOOD STREAM.
The Flood Stream, as its name implies, is the Tidal Stream associated with a rising tide (the EBB stream with a fallingtide). Around the British Isles, the Flood Stream flows up the Channel as far as Dover. The Flood also flows up the Irish Sea around the North of Scotland and down the East coast of England. There is thus a meeting point near Dover where the water coming from two directions—up Channel and down the North Sea—meet.
The offshore buoyage round the British Isles is governed by this pattern of water movement. Coming up Channel from the West, a buoy marking the limit of a shoal extending from the Sussex coast would therefore have Port Hand Characteristics. A buoy in a similar position off Newcastle would have a Starboard Hand characteristic, and be left to Starboard.
The most important buoys used in the Lateral system are shown in Figure 14.
MIDDLE GROUND BUOYS
(Used to mark a shoal in the middle of a channel)
The topmarks on the buoys at the inner end of the channel are different from the outer marks. The lights shown by these buoys are distinctive whenever possible but follow the normal rules, e.g. red flashing or an even number of white flashes for the `Main channel to the Right or Channels of Equal Importance buoys' and white flashing with an odd number in the group for `Main channel to the Left' buoys.
Mid Channel Marks
These buoys are sometimes used to mark the centre of a channel —particularly at the entrance to some harbours where the position of the channel cannot otherwise be marked effectively. They can be of any shape, but must be easily distinguishable from the Port, Starboard, and Middle Ground buoys. The characteristics of their lights must also be distinctly different from neighbouring lights. See Figure 14.
Spoil Ground and Quarantine Anchorage Marks
Spoil grounds may either be the result of the dumping of refuse in an area or may be made unhealthy by the outflow of sewers, etc. Quarantine anchorages are used by ships with cases of infectious disease on board. In either event, the area is not
74 SMALL BOAT NAVIGATION CHARTS 75
altogether satisfactory as no indication is given of the varying rates at which the tidal stream increases in strength or slackens. This depends on the interval in time from the local High or Low water and whether it is a Spring or Neap tide.
The modern way of showing tidal information on a chart is by the use of Tidal Diamonds. Inscribed on various places on a chart are capital letters enclosed in a diamond, for example
~. Elsewhere on the chart, normally near the edge, will be found a block of tables, each with its alphabetical indicator above it. A typical example is:
490 52.2 N
5° 10.9 W Dim. Rate
Sp. Np.
Hours
6 256 r.8 o.9
0
5 254 1.2 o.6
A 4 234 0.4 0.2
3 045 0.4 0.2
0 2 054 1.0 0.5
V
1 059 i.8 0.4
H.W. 067 2.3 I.!
1 075 i.8 0.9
0
p 2 082 o.8 0.4
3 203 0.4 0.2
4 233 1.3 0.7
5 247 2.3 1.1
6 257 1.9 0.9
All that is necessary for the use of this table is to look up in Tide Tables the time of High Water on that particular day.
Let us assume that H.W. Devonport was at 1315 (GMT) and it was Spring Tide period. The tidal stream at 0715 would be setting in the direction 256° at I.8 knts and at 15io–o82° at o.8 knts.
Chart Corrections
When originally bought, the charts should normally have been corrected up-to-date for any details that have changed since the chart was published, e.g. a buoy moved, or newly discovered shoal inserted etc.
The date of publication of a chart is shown outside the bottom margin. However, if a lot of detail has changed in the course of time, a new edition of the chart may be printed—again the state of this new edition is shown near the date of publication. Then, if important information comes to light which is too complex to insert by hand on the chart, a Large Correction may be made. This, once again, involves the re-printing of the chart by the Admiralty—the date of the Large Correction being shown under the date of the new edition.
Outside these reasons for reprinting the chart in toto and discarding the old copies, a small correction system is used for keeping charts fully up to date. Every week, Admiralty Notices to Mariners are issued which give all the important corrections that need to be made to all charts. The corrections are suitably indexed so that each individual chart affected by a correction can be seen at la glance. These permanent corrections are normally made in pen and waterproof violet ink on the chart copy held by the individual owner. Sometimes, if the detail is sufficiently complicated to merit it, `blocks' are enclosed in cash copy of the Notice to Mariners; these can be cut out and pasted on the chart. It is a good tip to align these blocks into the correct position on the chart before applying any glue, and tick the corners with a pencil mark—then apply the glue and stick them on.
Some corrections are classed as Temporary or Preliminary corrections when a (T) or (P) is shown against the particular notice affected. These corrections should only be applied in pencil for, as their names imply, they concern impermanent changes—a light vessel being removed temporarily from its station for maintenance would warrant a temporary notice to
76 SMALL BOAT NAVIGATION
Mariners, Preliminary notices may later need to be inked in. The numbers of these small corrections are listed at the bottom of the chart concerned outside the bottom margin on the left, against the year concerned, e.g. 1968-22. 156. 273. means that in 1968, Notices to Mariners 22, 156 and 273 affected that particular chart and the necessary corrections to it have been made.
Chart correcting can be a tedious business, particularly in a small boat with limited space, but for general navigational safety in strange waters, it is a very necessary chore.
Admiralty Notices to Mariners can be obtained via any chart agent.s
Tides and Tidal Streams
or practical purposes, it is not necessary to have a detailed knowledge of the complex astronomical factors that give rise to the tide generating forces. The Moon and Sun, in that order, have a differential attractive or repellent effect on the water covering the Earth. It is largely the relative position of these bodies to points on the Earth's surface which determines the amount of attraction they impose on the water surface at a particular place and time.
As you know, the Earth rotates anti-clockwise about its Polar axis once every 24 hours. The Moon orbits around the Earth—in the same direction as the Earth's rotation—once every 291 days or so. Thus the lunar day—from an earthworm's point of view—is longer than the ordinary day by a period of about 50 minutes. If a spot on the Earth was directly underneath the moon at 1400 on the 17th July, it would not be directly underneath the Moon again at 1400 on the i 8th July, because the Moon would have moved on in its orbit and the transit will occur at a later time.
The Earth is also in orbit round the sun, completing a full revolution once every 365.3 days (hence the Leap Year every four years). To an observer on the Earth, the Sun appears to be • rotating around the Earth once every 24 hours; this is due to the Earth's rotation. But it is obvious that the Sun does not rise or set at the same times or on the same bearings every day of the year. This is principally due to the fact that the plane of the Earth's orbit round the Sun is not aligned to our Equator, but makes an angle of about 231 degrees with it. The apparent orbital path of the Sun around the Earth (the reverse way of looking at it) is called the ECLIPTIC. (See Figure 16A).
Twice during the year, on about March 21st and September 21St, the sun is directly over the Equator and the period of light
F
78 SMALL BOAT NAVIGATION
it
equals the period of darkness—I2 hours each. These are called the EQUINOXES (equal day and night). At other times of the year, the Sun's Declination—its angle above or below the Equator—varies until it reaches a maximum of 23 Z South on December 21st (the Winter Solstice) and a maximum declination North on about June 21st (the Summer Solstice). This is speaking as a Northern Hemispheric man! Down under, no doubt, they would want to arrange things differently.
Similarly, the Moon's Declination, or its position above or below the plane of the Equator, is not constant but varies between 28+ North to 281 South approximately every 13 days.
More important still, as far as the tides are concerned, is the phase of the Moon, its position relative to the Sun as seen from the Earth. A new moon—only a thin crescent visible—occurs when the Moon approaches a position between the Earth and the Sun. (When the Moon and Sun have almost the same declination, an Eclipse of the Sun may occur as the Moon gets between Sun and Earth.)
About 7 days later, the Moon will be at go° to the Sun—as seen from the Earth—and half of it will be illuminated. The Moon is then said to be in its First Quarter and it will be waxing —the illuminated portion increasing each night with the horns of its crescent pointing to the left.
Another 7 days and it will be a Full Moon on the opposite side of the Earth from the Sun. A full Moon will always, therefore, start rising over the eastern horizon near sunset.
For the next seven days, the Moon wanes until it reaches its Last Quarter—the other half illuminated—and in a further seven days it will be a New Moon once more.
When. the Moon is in its New or Full phases, the Sun's and Moon's gravitational forces combine to exert the maximum pull on the Earth's water surfaces.
Supposing Figure 16B shows the situation at a particular instant in time. The Moon and Sun—drawn wildly out of scale—are in `conjunction' and on the Earth's Equator, i.e. both have zero declination. In accordance with the normal gravitational laws, both the Moon and the Sun will exert an attractive force on the Earth. This force is considered as being directed from the centre of one body to the centre of the other and is inversely proportional to the square of the distance
80 SMALL BOAT NAVIGATION TIDES AND TIDAL STREAMS 81
between them. Therefore, although the Sun has a far greater mass than the Moon, the Moon's attractive force in this context is the greater.
Obviously, these attractive forces must be balanced by other forces due to the bodies' orbital speed around one another or a collision would result, but these other forces need not be considered here.
The Moon and Sun, then, exert an attractive gravitational force of a certain value at the centre of the Earth (C). At Point (E) in the Diagram they will exert the equivalent force PLUS a certain quantity, as that point is nearer to them. At Point (W), they will exert the equivalent force MINUS a certain quantity.
These differential forces have their greatest value on the Earth's surfaces nearest the body and farthest away—at points N and S they would be zero as those points lie at the same relative distance as the centre of the Earth. The effect of the forces on the solid land surface is negligible, and as far as the Earth's atmosphere is concerned they merely cause a slight change in atmospheric pressure. But the water on the Earth's surface, like the air, is free to respond, and being fairly dense, reacts to them.
As a result, water is drawn towards E and away from the heavenly bodies at W. At other points, e.g. between N and E and N and W, the attractive force is not directly towards or away from the local vertical.
At these intermediate points, the total force can be resolved into vertical and horizontal components, and the horizontal components will move the water towards E and away from W. The result of all these forces is to cause a `bulging' of the water surfaces at E and W which progressively reduces towards the points go° away—N and S as shown in Figure 16c.
The Earth's Rotational Effect
Clearly, if the Earth did not rotate, nor the Moon and Sun change their declination, there would be a permanent high water at E and W, and a permanent low water along the Meridian of N to S.
But the Earth does rotate and the Moon likewise around the Earth. In one Lunar Day (of about 24 hours 50 minutes) any particular point on the Earth's surface will therefore feel (Moonand Sun having zero declination) two High Waters and two I ow Waters. This is called a SEMI-DIURNAL Tidal Pattern, i.e. in ordinary time, a High Water followed by a Low Water about 6 hours 12 minutes later, another High Water at 12 hours 25 minutes, a Low Water at i8 hours 37 minutes, and the cycle completed by the following day's High Water at zq. hours 50 minutes.
,Spring Tides
The earlier diagram illustrated the situation with the Sun and Moon working in conjunction on the same side of the Earth—at the period of a New Moon. The tide raising forces are pulling together and are at a maximum. These are occasions when SPRING TIDES, those that rise highest and fall lowest, will occur. '1 The example also illustrated the particular time of the year when the Sun had zero Declination, at the Equinox, when the highest and lowest tides of a semi-diurnal nature are likely to occur in the whole year—hence the expression EQUINOCTIAL
SPRING TIDES.
Spring tides will also occur when the Sun and Moon are in `opposition' at the opposite sides of the Earth at the time of a full moon, as shown in Figure 16D.
In this context, the term `opposition' is misleading as, in fact, the tide raising forces of the two individual bodies are still additive. The Moon's gravitational pull exerts Force M at the Earth's centre, M +m at W and M +m at E. The Sun exerts Force S at the centre, S +S at E and S—s at W. They both exert therefore a differential force towards E and W as before. SPRING tides result, and when a FULL Moon occurs at the Equinox, EQUINOCTIAL SPRING TIDES will occur.
.Neap Tides
'These are tides which have the smallest rise and fall in height. They occur when the Moon is in its Quarters, i.e. Half-Moon. The differential forces of the Sun and Moon are now applied at c)o" to each other.
As a result, the Moon is trying to raise High Water at N and S, whilst the Sun is trying to raise High Waters at E and W as shown (diagrammatically only) in Figure I6E. The Moon, being closer, will win, but smaller amplitude tides will follow.
82 SMALL BOAT NAVIGATION TIDES AND TIDAL STREAMS 83
The Elect of Declination
So far, the Moon and Sun have been shown at zero Declination. However, in the case of the Sun this occurs but twice a year, and for the Moon every 6-' days. To consider the Moon alone for a moment, and the diagram in Figure 16F.
Having a certain Northerly Declination, the Moon will raise the maximum `humps' in the water at points A and B. A point `D' as it rotates with the Earth's rotation will experience no Tide Raising force at D, but a maximum at D1. Thus only one High Water and one Low Water per day will occur. These are called DIURNAL TIDES.
The Sun's Declination will have a similar, but smaller effect. The Semi-Diurnal Tide Raising effect is therefore greatest when the Sun/Moon's Declination is zero, and the Diurnal Tide Raising effect greatest when both have maximum declination.
Each ocean area has a natural period of oscillation—the `slop' in a basin. The Atlantic responds readily to semi-diurnal tide generating forces, whereas the Pacific is most affected by those of a diurnal nature. The tides round the British Isles, therefore, are mainly semi-diurnal in character.
Summary
To summarise these effects: At New and Full Moon, the attractive forces of the two bodies combine to produce SPRING tides, i.e. tides with the greatest difference in height between High and Low Water. When the Moon is in its Quarters, NEAP tides result, tides that have the smallest difference in height between High and Low water. (The Spring tides actually occur usually about I day after the Moon is New or Full as there is a time lag in the force taking effect.)
At certain times of the year, all the factors attracting the tide generating power of the bodies—such as Sun's declination, Moon's declination, phases of the Moon, combine to produce the highest tides of the year. This occurs about the time of the Equinoxes—hence the expression Equinoctial Spring Tides.
In European waters, the tides are almost invariably semi-diurnal in character; two high and two low waters a day, with the time of the first high water becoming about fifty minutes later each successive day. However, although the astronomicalforces are the basic cause of tides, their effect in particular areas is much affected by the shape of the sea or ocean, the gradient of the sea bed, etc. If a particular ocean has a natural oscillatory frequency near to the period of the tidal forces, then high tides will result. The North Atlantic is subject to some of the highest tides in the world; on the other hand, there is very little tidal movement in the smaller enclosed basin of the Mediterranean.
In some other areas, notably in the Pacific, the tide is diurnal in character, i.e. only one high water and low water a day.
In other places, local anomalies may cause a stand of the tide; in Southampton water, for example, there may be as many as four high waters a day, the shape of the Solent and Spithead causing double high tides.
Over the years, a great deal of information has been gathered to enable the tides, particularly in European waters, to be predicted with considerable accuracy. However, these predictions only apply when the normal meteorological conditions for the time of year pertain. Large falls or rises in barometric pressure can alter the height of the sea to quite a marked extent, and strong, persistent winds can build up the sea level against a coast line (remember the floods on the East Coast). Any tidal prediction is subject to unforeseen factors such as this, and cannot therefore be infallibly correct.
Tide Tables
The British Admiralty publishes annually three volumes of Tide Tables, and based on much the same data, a good number of other tide tables are produced, either for local use, or for inclusion in Almanacs, such as Reed's.
Part I of the Admiralty Tide Tables is published in three volumes:
Vol. i—covers European Waters (including the Mediterranean)
Vol. 2—covers the Atlantic and Indian Oceans
Vol. 3—covers the Pacific Ocean
These tables give the times and heights of High and Low waters for a large number of the major ports—called Standard Ports. They also provide methods of working out the heights
84 SMALL BOAT NAVIGATIO `.
and times of tides at an even greater number of smaller place (Secondary Ports) based on a Standard Port in their vicinity.
Tables are also provided for working out the height of the'. tide at times between high and low water. The method of doing these calculations is not identical in each volume of the Tide Tables. It is not intended to go into these calculations here, as a full explanation of the calculations and examples of their practical solution are given in each volume.
Apart from the Admiralty Tide Tables, there exist a large number of other tables produced by publishers and local authorities. These are normally perfectly satisfactory for finding out the heights and times of high and low water, but do not always give a facility for calculating the height of a tide at intervening times. There is not much doubt that the Admiralty Tide Tables will give the widest range of ports and the most accurate predictions, but there are others, notably those contained in Reed's Nautical Almanack, which may suffice for the average yachtsman's needs.
Whichever Tide Tables you use, always be careful to check the Zone Time of the predictions. For example, a lot of the U.K. tables are published in G.M.T. and therefore in England, when Summer Time (Zone —r) is being kept, one hour needs to be added to the predicted times, e.g.:
10.00 G.M.T. = I I.00 B.S.T. (Zone-I or A) (I IooA)
DEFINITIONS
A few simple definitions may be of help in de-cyphering Tide Tables.
Height of a Tide—is its height above or below Chart Datum. If the tide falls below chart datum, the height is shown in the tables as a minus quantity, e.g.—x.2 (feet).
Range of a Tide—is the difference in feet between the heights of successive High and Low waters, e.g.:
High Water Height 16.5 feet 19.5 feet
Low Water Height o.2 feet —o.6 feet
Range of Tide = 16.3 feet 20.1 feet
TIDES AND TIDAL STREAMS 85
Duration of a Tide—is the time interval between successive High and Low waters.
The Interval—is the time difference between High (or Low) water and the time at which it is desired to predict the height of tide. Spring Tides—Occur roughly once a fortnight—normally in the day succeeding a New or Full Moon. They are tides with the greatest range, and consequently produce the strongest Tidal Streams.
.Neap Tides—occur at the intervening weekly intervals. Smallest range, and therefore weakest Tidal Streams.
Mean High Water Springs and Mean High Water .Neaps—are the
average heights of high water at springs and neaps, taken over a long period. Mean Low Water Springs and Mean Low Water Neaps are calculated on the same basis.
Other Methods of Tidal Prediction
The Tide Tables are produced by the careful analysis of a large number of harmonic constituents related principally to the movements of the Moon and Sun, but this sort of work needs the services of a tidal machine or computer, and only the results are of interest to the seaman. Next down the ladder is a method of prediction using a few of these harmonic constituents to produce a graph of a particular tide at a particular place. This gives a very accurate result in working out the height of a tide at a secondary port, but it is fairly complex and slow and would not normally be used by small boat sailors. (Those interested will find it in Part III (not Volume III, Part I) of the Admiralty Tide Tables.) Next in accuracy come the predictions for the Secondary ports given in the Tide Tables.
There are, however, a couple of rough and ready, and not very accurate, methods of predicting tides where these are semi-diurnal in character. These are by using the non-harmonic constants shown on some charts. The constants are:
Mean High Water Lunitidal Interval (terrible mouthful—so
M.H.W.I.)—This is the interval between the Moon's transit at Greenwich and the next following High Water. To find the time of the Moon's Meridian passage you will need to look in a Nautical Almanac; to this time you must add M.H.W.I. to find the time of High Water. To find the time of Low Water, add or subtract 6 hours I2 minutes from the High Water time.
86 SMALL BOAT NAVIGATION ' TIDES AND TIDAL STREAMS 87
High Water Full and Change (H. W.F. & C.)—This term is sometimes given on the older charts. It means that when the moon is full or changing, the next high tide in the area concerned will take place at the stated interval after the Moon's meridian passage. It is therefore similar to M.H.W.I. above.
Both these constants can be made use of, without reference to the time of the Moon's meridian passage, if the chart you are using states the constant and you know the time of High Water at another place along the coast—and the constant there. The difference between the two constants, added or subtracted from the time of High Water at the other place, will give a reasonable approximation for the time of High Water in your own area.
There is one other useful approximation concerning the likely height of a tide at intermediate times between High and Low Water, assuming that there is a regular semi-diurnal tidal pattern. The level is likely to change in the following way:
During the First hour (after H.W. or L.W.) By 7%
During the Second hour (after H.W. or L.W.) By 18%
During the Third hour (after H.W. or L.W.) By 25%
During the Fourth hour (after H.W. or L.W.) By 25%
During the Fifth hour (after H.W. or L.W.) By 18%
During the Sixth hour (after H.W. or L.W.) By 7%
So, assuming that the Range of a particular tide is zo feet, and the height at High Water was 24. feet, the expected heights of tide would be:
At the end of the First hour after H.W. 22.6 feet
At the end of the Second hour after H.W. Ig.o feet
At the end of the Third hour after H.W. 14.0 feet
At the end of the Fourth hour after H.W. 9.o feet
At the end of the Fifth hour after H.W. 5.4 feet
At the end of the Sixth hour after H.W. 4.0 feet
Note: This method is only very approximate. The decimal points
are entirely unrealistic in practice.
Tidal Stream Tables
Apart from the Tidal Stream and Current information shown on charts by means of tidal diamonds and arrows, other books exist which give additional information. For Tidal Streams,
some of the most useful are the Pocket Tidal Atlases covering the waters around the British Isles. There is one, for example, for the Channel, another for the Solent, and another for the Irish Sea. They display, by means of arrows, tht* expected set and rate of the tidal stream for each hour before or after High Water at the port on which they are based (in most cases—Dover). These are particularly useful books for yachtsmen as they are small, comparatively cheap, easy to use and contain a lot of information in condensed form.
Unfortunately their equivalents do not exist, to my knowledge, outside U.K. waters. However, The Admiralty Tide Tables for foreign waters (Vols. II and III) include some Tidal Stream tables amongst the other tidal predictions.
i rr-‑
This is the nub of the business of navigation and pilotage; messy and unmethodical chartwork can quite quickly lead to that `indefinable feeling of impending doom' and the hire of a prayer mat. So decide that you are not going to wear out the latter from the start by being accurate and neat in your workings on a chart. Speed is sometimes also necessary, but should not normally be sacrificed for accuracy.
The simple instruments required for use on a chart are:
I. A Soft pencil-2B or softer. Hard pencil marks are difficult to erase and the chart will not remain usable for long if it is deeply scored with hard pencil marks. A clutch pencil, with a box of spare leads, is ideal as it can be sharpened easily without spraying the chart table, boat, and the soup with chippings.
2. A good soft rubber.
3. A pair of dividers—these can be bought in various shapes (bow or straight legged) and sizes; a pair with 6" legs is convenient for general use, although you may occasionally need a longer pair. Personally, I prefer the straight type as they seem more easily handled with one hand. They should be workable with one hand, but not so slack that they will not stay fixed at a set spread.
4. A pair of compasses—any pair will do providing they work satisfactorily and will hold pencil or lead.
5. A Station Pointer or Douglas Protractor. The former may be
quite expensive unless you can pick up a second-hand one cheaply but is extremely useful for plotting horizontal sextant angle fixes. It consists of a central circular scale, engraved in degrees, to which is attached three long legs; the central leg is
CIIARTWORK, FIXING AND PILOTAGE HINTS 89
fixed to the scale but the outer legs can be set and clamped at any angle. Good instruments of this type are normally made of brass or gunmetal and have a vernier arrangement to allow the angles to be set accurately to within a few minutes of arc.
You will probably be familiar with the Douglas Protractor—a large square plastic protractor engraved with parallel lines across its face and in degrees around the edge. This is cheap, and also useful for plotting fixes; its limitation lies in its size for when plotting fixes on a large scale chart you may `run out of protractor' whereas the Station Pointer will still cope. Also, of course, angles cannot be marked very accurately on a Douglas Protractor.
6. A Parallel Ruler. This is essential. It is used for transferring courses or bearings from one part of the chart to the compass rose, so that they can be read off, and for the plotting of position lines. Parallel rulers are available in two types, roller or hinged. The roller type is much the quickest and easiest to use if the chart table is sufficiently large to allow the chart to be laid out flat upon it. A sketch of this type of ruler is shown in Figure 17A.
It simply consists of a rectangular strip of brass, wood, or plastic. At each end is a roller projecting slightly beneath the under surface, the two rollers being joined to a common axle (in some newer types with nylon rollers there is no axle but a double roller arrangement which serves the same purpose). These rollers only allow the ruler to move in one direction unless they are lifted off the chart surface, and therefore parallel lines can be transferred from one area of a chart to another. This type of ruler does not, however, work satisfactorily on uneven surfaces.
The second type is the hinged variety (Figure 17B) wherein two rectangular strips are moved by two pivoted metal arms. The ruler is moved across the chart by successively keeping one strip held to the paper whilst pivoting the other. It is much slower and more fiddly to use than the roller on a flat surface, but it is lighter and easier to operate when the chart is folded or laid on a bumpy surface.
For laying of a course or bearing with a parallel ruler, see Figure 17c.
With either type in hand, suppose that you want to lay off a course of o30° from Point X on the diagram above. The parallel
eIIARTWORK, FIXING AND PILOTAGE HINTS 91
ruler is laid on the nearest `compass rose' on the chart and one edge aligned through the centre of the rose to the 0300 graduatlurr. The ruler is then slid across until an edge passes through I'4)irnt X. A pencil line is then drawn from this edge and can be extended as necessary.
Additionally, it will be necessary to find the bearing between WI) points on a chart, or the course between them. The ruler in t lien first aligned with one edge through both points and su I I )sequently moved across to the nearest compass rose. The edge joining the centre of the rose to its circumference will give (lie course or bearing required. (Point Y bears 2100 from Point X i r i the diagram above.)
7. A Notebook for `keeping the reckoning'. A lay-out for this
r rt ~ I (,book is discussed at the beginning of Chapter 9.
The easiest way of getting oneself lost at sea is to keep no II u ord of the boat's position on the chart, and no written details If course alterations, fixes, etc., in a notebook. The starting
I r~ psi lion is always known (presumably!) but a plot must be kept oil t lie chart of the boat's subsequent movements.
'l lie Dead Reckoning Position
The Dead Reckoning or D.R. position is arrived at by allow‑
J u ge only for the boat's true course and speed through the water nce leaving a known point. Looking at Figure 17D, let us
s u y a boat left Point `S' at iioo. It steered ogo° for I hour, :r l i ered course to 1300 at 1200, and altered course again to ogo° it i400; then its D.R. position at 1700 allowing for a boat's nIuc•ed of four knots is Point `T':
There are certain conventions in plotting the D.R. The 4u„rses steered are sometimes written against the track as rullurwn, and the D.R. position is always shown as a small `tick' o r ross the line with a time against it. The distances (bracketed l're) would not normally be written on the chart, but merely measured off with a pair of dividers.
It is also conventional to use a single arrow to indicate the u Irse being steered through the water.
1 he Estimated Position (E.P.)
'Tile Estimated position allows for the best estimate of tidal rit ream and leeway, etc., being applied to the D.R. position
92 SMALL BOAT NAVIGATII
In other words, it is the best estimate of the boat's position o~ the ground allowing for course, speed, current or tidal stream as leeway, since the last known position of the boat.
The symbol for Estimated position is a triangle with a dot it, viz. p 1300. (The time is also written alongside—as with D.R. position.)
Figure I 7E shows much the same example as before; ti D.R. position at 1700 is Point `T'—but the E.P. at 1700 Point `T1' allowing for an estimated set to the North West i
over the six hours since i ioo. Note that the direction of set is normally indicated by three arrows along the line.
A Fix
A fix is the positive establishment of the boat's position at given time (although some fixes may be more positive an honest than others). The conventional sign for a fix is a circl with a dot in it—and a time against it, viz. 0 1400.
A fix may be obtained by a variety of means and its accurac' will depend on many factors. The methods of obtaining a fi: given below are those mostly commonly used in a small boa without radio aids or radar.
I. THE TERRESTRIAL FIX
(Visual bearing of charted objects)
This is the simplest form of fix and one of the most accurat( under normal circumstances with a good compass.
If a bearing is taken from the boat of a single charted objet on shore and this bearing (corrected for compass errors) i plotted on the chart, it has been established that the boat mus lie somewhere on a line reciprocal to that bearing. For example in Figure 18A, the compass bearing of a lighthouse at i000 wa: 085°. The Variation was io° W and the Deviation (for th( boat's heading) was 2°E.
Compass Bearing 085°
Variation —Io
Deviation +2
True Bearing 077°
94 SMALL BOAT NAVIGATION CHARTWORK, FIXING AND PILOTAGE HINTS 95
This bearing (077° True) can now be laid off on the chart (using a parallel ruler from the nearest compass rose) as a single position line. The boat must lie somewhere along that line, but how near or far from the lighthouse is not known. Note also that the lighthouse bears 077° from the boat, but the line, with its arrowhead, is drawn away from that bearing, and the time is written along the line.
A single position line is not, in itself, a fix as the distance off the object has not been established. However if TWO or more almost simultaneous bearings can be taken of two or more different charted objects, a fix is obtained (Figure 18B).
The lighthouse again bore 085° compass, but a bearing of the beacon was taken a few seconds later and turned out to be 350° compass. The True bearing of the two objects (Variation Io° W and Deviation z° E) was therefore 077° and 342° respectively. Using a parallel ruler, the two lines are laid off on the chart from the objects and the boat must lie at their point of intersection. A circle is drawn round this point and the time written alongside the fix.
A word of warning about a two bearing fix. The boat need only be at the point of intersection of the two lines if:
a. The compass is accurate—its errors known and properly allowed for.
b. The charted objects have been correctly identified and the bearings taken accurately.
c. The plotting is also correct.
Factors (b) and (c) are largely a matter of practice and experience and (c) is not the most common cause of error. The accuracy of the compass or object identification are more often the doubtful factors. For these reasons alone, it is unwise to put unwarranted faith in a single two point fix, particularly where the angle of cut of the two position lines is less than 30°. In such a situation, any compass errors (or any other errors for that matter) have a greatly magnified effect. Suppose that bearings were taken of two different objects—the bearings were o85° and 105°. These were then plotted on the chart—assuming for simplicity that both the variation and deviation were nil (Figure 18c).
By all appearances the boat should be at Point Y. But there isonly zo° difference between the two bearings, and suppose that the compass was a few degrees in error ... say 5° when the bearings were taken. The plotted bearings should have been ogo° and I Io°. The boat is, in reality, at Point X, a considerable distance away from its supposed position.
On the other hand, a go° cut between the two bearings taken is ideal; whatever the compass error, its effect is minimised. Assuming the same error ... 5° and bearings of 355° and o85° (Figure I8D) .
Point S is the supposed position and Point T the true position. T is much closer to S than in the previous example.
Therefore, when taking a two-point fix:
I. Try and obtain bearings of objects about go° apart.
2. Do not use objects less than 30° apart if this can possibly be avoided—if you must, treat the resulting fix with some suspicion.
It is usually far better, if sufficient objects are visible, to use a 3 or 4 point fix. Again, with a three-point fix the optimum choice is three objects each 6o° apart, and for a four-point 45°, viz. Figures 18E and F.
However, these separations are not critical; the aim must be to keep a difference of at least 30° between position lines.
The `Cocked Hat'
The great advantage of using three or more position lines in a fix is that the quality of the fix is immediately apparent when it is plotted on the chart. The three lines should pass through a point, and if they do, you can be pretty sure that the boat was in the position shown at the time of the fix. (There is one exceptional circumstance when this does not hold true and this is mentioned below.)
Perhaps though, the three lines do not pass through a single point when they are plotted. In this event, the area of uncertainty produced is called a `Cocked Hat' (see Figure i8G).
All that can be assumed initially in this case is that the boat is somewhere within the area enclosed by the three lines. In open water it is normal to assume that you are in the centre of the area of doubt. But close inshore or in the proximity of
g8 SMALL BOAT NAVIGATION CHARTWORK, FIXING AND PILOTAGE HINTS 99
III. FIXING BY RANGE AND BEARING
This is a common method of fixing in use at sea today; a compass bearing and simultaneous range is taken of a charted object. In ships, the range is normally obtained by radar— outside the capabilities of the normal small boat. Some yachtsmen may possess a small optical range finder but these are comparatively expensive instruments. The majority of the larger cruising boats will, however, have a sextant on board. The principal use of this instrument is in finding the altitude of the sun, moon, stars and planets above the visible horizon but it can also be used for measuring the vertical and horizontal angles subtended by charted objects. Vertical sextant angles of objects of a known height, with the appropriate corrections applied, can be looked up in tables listing angles against range. There are several books which include tables of this nature, but one of the most compact and useful, containing tables specifically for this purpose, is Lecky's "The Danger Angle and Offshore Distance Table'.
The procedure is quite simple. For instance, a lighthouse is shown on the chart to have an elevation of 130 feet. The sextant, held vertically, is used to measure the angle between the centre of the lantern of the lighthouse (the charted elevation) and the sea surface beneath it. Suppose the angle measured was o° Ig.8'. The first correction that must be applied is the Index Error of the instrument itself (see Chapter 8).
Sextant Altitude of lighthouse o° I9.8'a
Index Error +01.2
Observed Altitude = o° 21.0'
Consulting Part I of Lecky's tables—Heights of objects from 50 to I I oo feet and distances from o. I to 7.5 miles—the range of the lighthouse can be read straight out as being 3.5 miles. The only correction to be applied when using this part of the tables is that for Index Error, as shown. A range is therefore obtained very simply.
Part II of these tables covers heights of objects from 200 to
I 8,000 feet and distances from 5 to I 10 miles. This section is
normally used when trying to find the range of the land whilst you are still a considerable distance away from it. The peak of the island of Fogo in the Cape Verde Islands has an elevation of 9281 feet and may be seen at great distances. Approaching from the South, from your D.R. position you estimate that you are 48 miles off it but wish to find the range accurately for a fix. Using Part II of Lecky's tables, there are two additional+corrections which must be applied to the Sextant Altitude before entering the tables to obtain the range. These are the Angle of Dip (for the observer's height of eye above the water) and the other (I/12th of the estimated distance) for refraction—the bending of the light rays in the atmosphere. The Sextant Angle is therefore corrected for:
a. Index Error.
b. Dip (from another small table in Lecky's)
c. Refraction—I/12th of estimated distance (correction in minutes of arc) .
FOGO, elevation 9281 feet. Your height of eye in the boat is io feet, estimated distance off, 48 miles. Having taken the Sextant Altitude between the peak and the horizon between you and the island, this turns out to be 10 20.0'.
Sextant Altitude I ° 20.0'
Index Error +01.2 (same as before)
Observed Altitude 10 21.2
Dip —03.4 (always minus—this for height of eye of I o feet)
10 17.8
Refraction —04.0 (always minus—this for
(1/12th of estimated 48
distance) 12 =4 minutes of arc) Angle = I0 13.8'
Looking up I ° 13.8' in Part II of the tables against an altitude of 9200 feet gives a range of 52.1 miles for Fogo's peak. Note
I00 . SMALL BOAT NAVIGATION CHARTWORK, FIXING AND PILOTAGE HINTS I01
that Part II of Lecky's tables are intended for use with objects lying beyond the sea horizon.
The plotting of a visual range and bearing fix on the chart is very straightforward. The bearing must, of course, first be corrected for compass errors to obtain the true bearing before it is plotted. Theoretically also, when using bearings of objects at very long range there is a discrepancy between observed bearings and those taken from, or plotted on, the chart, due to distortions involved in the construction of the Mercator projection. However, for practical purposes, this is of no significance except in high latitudes. It is important, though, to plot `long range' bearings very carefully.
Therefore, plot your true bearing first, draw the position line from the object, and then with a pair of compasses or dividers set to the range obtained, draw the range circle from the object on the chart. The point of intersection of the bearing position line and the range circle is the fix position. Then write the time against it. This fix is shown in Figure I8M.
IV. FIXING BY TWO OR MORE RANGES
If the ranges of two or more objects are taken almost simultangeously, using vertical sextant angles or any other means,the position circles from these objects intersecting on the chart will give a fix. This is a good method of fixing if suitably defined objects of known height are available. (Figure I 8N.)
V. FIXING BY USING HORIZONTAL
SEXTANT ANGLES
This is a highly accurate method of fixing which can be used when precision is vital, e.g. for laying a buoy or mooring in the correct spot or in charting the position of a navigational mark. It is aethod that has been used by surveyors for many years in the normal course of their business. Its main disadvantage is that it is a comparatively slow method of fixing and the plotting of horizontal sextant angles is a somewhat pernickety job.
Horizontal sextant angles are taken by holding the sextant on its side, handle underneath; one object is viewed directly through the clear glass section of the horizon glass, and the reflected image of the other object is superimposed on it by moving the scale micrometer or vernier. The angle between the two can then be read off.
A minimum of three charted objects must be used to obtain a fix, as the angles on each side of the centre object are needed. If there is a large height difference between any of the objects as viewed from the observer's position, it may be difficult to superimpose the objects correctly; therefore objects of more or less equal height are preferred. The objects chosen should also be more than 300 apart to give a good cut. Ambiguity can again exist if the objects and the boat all lie on the circumference of the same circle (as commented upon earlier when discussing `cocked hats'). It is therefore safer to choose a centre object nearer to you than the other two—or objects in a straight line—so that such a situation cannot arise.
Once the angles have been observed, they can be plotted onto the chart with the aid of a `Station Pointer' or `Douglas Protractor'. The Station Pointer is the more accurate of the two.
To use the Station Pointer, set the two angles on the outer arms. Supposing that the central object was a church, the left-hand object a lighthouse, and the right-hand object the right-hand edge of a cliff (Figure I9A) .
The Station Pointer, with its arms set to the appropriate angles, is laid on the chart and moved around until the bevelled edges of each leg pass exactly through the charted position of the objects observed. The boat's position is then the centre of the instrument, normally marked by a small `V' notch through which a pencil mark can be made on the chart.
The result is a good, accurate fix achieved independently of any compass bearing.
The Douglas Protractor is designed for use on exactly the same principles, but it merely consists of a transparent plastic square, with an engraved, matt upper surface graduated round the edge in degrees, and with a small hole at its centre. ,.
The left and right angles are marked with a pencil on the matt surface, and lines drawn from the edge to the centre of the protractor. It is then placed on the chart and moved until the
F.
I04 SMALL BOAT NAVIGATION
cannot choose, so a less accurate method of fixing has to be
4 used. The Running Fix is that most commonly used in these circumstances. A bearing is taken of the only object visible, the time noted and the position line from this bearing plotted on the chart. Later when the bearing of the object has altered sufficiently—due to the boat's movement—a second bearing is taken and plotted. The first position line is then `transferred' up to the second line, allowing for the boat's course and speed over the period between the two bearings. This should be the boat's course and speed `over the ground' if it can be estimated from past history. If not, the boat's course and speed through the water must be used. It is this `distance run'—or the distance travelled in the time between the two bearings—which is the weakness of this type of fix. Any inaccuracy in the estimation of the distance run will show up as fix inaccuracy, without, of
course, the navigator being aware of it.
An example again. Variation 8° E, Deviation 3° W for a course of 2700 by compass, speed 6 knots. A wireless mast bears 3400 at 1030 and 0500 at 1245. What is the boat's position at this time? (See Figure IgD.)
Compass bearing 3400 Compass bearing 0500
Total correction +5 +5
True bearing 345 True bearing 055
True Course 275°
As you do not know precisely where you are at 1030, it is normal to take the point on the position line nearest the D.R. position—thus Point X. From Point X, allow for a run of 2'J hours at 6 knots = 13.5 miles. Plot this, Point Y, on your presumed track since 1030. Then draw a parallel to the 1030 position line through this point. The intersection of the transferred position line (conventionally shown with two arrows) and the 055° (1245) bearing is the fix position at 1245•
The more accurate the run between fixes, the better the fix itself. If you had been moving along a coastline prior to obtaining this running fix and had been able to take two three-point fixes in the previous few hours, you might estimate your distance run for the running fix differently (Figure I 9E).
Between o600 and o800, although steering a course of 275°CHARTWORK, FIXING AND PILOTAGE HINTS I05
True, the course made good over the ground (measured with a parallel ruler between Points X and Y) is found to be 2710. Also, by measuring XY with a pair of dividers, the distance made good in the 2 hours was 14 miles=a Speed Made Good of 7 knots. From Fix Y, now lay off a course of 2710 and allow 21 hours at 7 knots = 17.5 miles to bring you to a 1030 Estimated Position—Point Z.
The beacon has now disappeared, but the transferred position line from the Radio Mast at o800, combined with the 1030 bearing of the same object, gives you a fix at 1030 to the North of your Estimated position but South of the course steered.
Doubling the Angle on the Bow
An old dodge, but this in fact is really a form of running fix. If the time is taken when an object is at a certain angle on the bow—say 30° on the port bow—and it is taken again when the object is twice this angle on the bow (6o°), the distance run between the two times will be equal to the boat's distance off the object at the later time. For example, in Figure i 9F, a lighthouse was 30° on the Port bow at i000, course ogo° speed 6 knots, and was found to be 600 on the bow at I03o. The boat should then be 3 miles off the object at 1030.
This check is not much use if any tidal stream or current is present, i.e. the course and speed made good is different from that steered. If a set is expected, it is better to use a Running Fix allowing for an unknown tidal stream. Finally, in the case of 45° (doubling to go°) on the bow—called a `four point bearing' —the distance run will be equal to the distance off the object when it is abeam.
Running Fix with unknown Tidal Stream (Figure I 9G)
Suppose that a fix was taken previously at Point W at ogoo. The lighthouse bore 308° at i000 and 346° at 1030. Presuming that you were at Point X at 1000, the speed made good is
WX - =4 knots. Then insert Point Y allowing for half an Time I
hour's run at the speed over WX, i.e. 4=2 miles for the distance 2
XY. Your fix position at 1030 is then point Z, the course made good WZ, and the current YZ, setting towards Z.
Io6 SMALL BOAT NAVIGATION CHARTWORK, FIXING AND PILOTAGE HINTS I07
Other Dodges
Some less positive methods of obtaining position are given below. Strictly speaking, these are not `fixing' methods in the accurate sense, but may nevertheless be very useful in giving a good indication of the boat's probable position.
Using Soundings
A sharply defined fathom line can, on many occasions, be a good navigational aid, particularly when approaching land. Perhaps you might be approaching Cape St. Vincent from the West in thick weather and be uncertain of your exact position. The bottom shelves steeply in this area and the Ioo fathom line is sharply defined about 5 miles off the Cape. If your boat is luxurious enough to be fitted with an echo sounder that will reach that far down, well and good. Providing you keep outside this depth, the boat cannot come to any harm from grounding—the hazard is more likely to come from large ships rounding the Cape. But not all boats have echo sounders nor a lead line much more than 20 fathoms in length, so it is a question of getting the best out of the equipment held on board. A sharply defined 20 fathom line is an obvious target in these circumstances for the gaining of a position line at the least risk, and a glance at the chart will soon show whether this is a practical proposition.
In many places, the seabed slopes too gently or is too undulating for the crossing of a particular fathom line to immediately ring up three red cherries. However, unless the bottom is absolutely flat, there is a technique which can assist in the use of soundings to establish a position. This is to take soundings regularly, noting the time and depth, and adjusting the latter for the likely height of tide. At the same time, keep a careful record of the course and speed. After a long enough time to allow a reasonable picture to be built up, place a piece of tracing paper on the chart, marking on it a line to indicate North. Then estimate the boat's probable course and speed made good over the period, laying off this course on the tracing paper. Finally, plot and mark the depth of each sounding along the track, using the chart scale to measure the distances between soundings. The resulting sounding pattern on the tracing paper is then moved around on the chart, keeping the North/South line roughly parallel with the chart meridians,until the pattern fits the sounding marks on the chart in the vicinity of the probable position.
Horizon Range
At night, the approximate range of a lighthouse can be estimated from the moment when it just appears above, or dips below, the horizon. Visibility must be good, of course, and the light of sufficient power or intensity to reach the horizon and some distance beyond it. The intensity of all lights is given in the Admiralty Light Lists, but a glance at the chart usually gives sufficient clue on this point as the major lights will show long visibility ranges. It is the actual raising of the light itself above the horizon or disappearance below it that must be taken. The Loom—the beam only visible sweeping around the horizon— may be seen long before or after the actual light is seen.
A table in Lecky's, mentioned previously under vertical sextant angles, can be used for calculating the range; there are similar tables in each List of Lights, and in other marine navigational tables, such as Inman's Nautical Tables and Reed's Almanac/c.
Taking Cape St. Vincent as an example; the light there is described as Fl. 5 sec 264 ft. 23 M—i.e. it flashes white once every 5 seconds, has an elevation of 264 feet, and is visible to an observer, whose height of eye is 15 feet, at 23 miles. If your height of eye is I o feet, the light should appear above the horizon at the following range:
Range of light for its 264 feet elevation 18.7 miles
Range of light for your I o feet elevation 3.63 miles
Light appears or dips at 22.33 miles
Range from Sound Reflection
Only of practical use really when one is confronted with the situation of being stuck in the middle of a Scottish Loch or Norwegian Fjord with fog coming down, depth too great for anchoring, and the beer running out.
The echo of any noise you care to make (which might be considerable) will return to you, so that:
I08 SMALL BOAT NAVIGATION
Distance in feet of boat from cliff = Time in seconds X 1130
2
or if this sum is too difficult to work out in your head, the rough distance is:
Distance in Cables = Time interval in seconds x o.9
SOME PILOTAGE HINTS
`Shooting Up' Objects
In an unfamiliar area, it is important to be able to identify objects quickly and accurately; otherwise a lot of time will be wasted trying to plot position lines that do not meet and close cocked hats that refuse to shut. Or, there may be a very conspicuous object in the area which is not charted but would otherwise be useful for fixing.
There are several ways of `shooting up' an object. The quickest method is to get the unknown object in transit with a charted object, and at the same time take a bearing of the two. The bearing, laid off with a parallel ruler on the chart from the known object, will often identify the doubtful one.
This system can also be used in another way. For example, going up a narrow channel, it may be necessary to identify buoys quickly and there is not much time for plotting fixes (see Figure 2oA) :
The target is the mooring buoy at X, passing between the Port hand buoy at Y, and the Starboard hand buoy at Z. From the chart, it is obvious that there are a lot of buoys in this channel and it may be difficult, on rounding the corner, to identify one from the other. To save the embarrassment of either going aground or pinching someone else's mooring, before reaching this point find out from the chart the true bearings of these three buoys when they are in transit with the pier-head on the South shore. Convert these bearings to compass bearings by applying the variation, and the deviation for a heading of o65° Then, quick bearings of o8o°, I io° and 115° on the pier-head on the approach will help identify these buoys.
Getting an uncharted object marked on the chart; if the
a
Tidal Stream and Current
Problems
Tidal streams and currents vary greatly in strength not only from one ocean or sea to another but also between adjacent local waters. The coasts of the North Atlantic are uncomfortably placed in this respect, the tidal range being generally large, and therefore strong tidal streams abound. Tidal streams of over 5 knots are not unknown round the British Isles.
Due respect must therefore be paid to these streams and currents, particularly in small craft where the boat's speed in poor sailing conditions may well be less than the rate of the stream.
With respect to chartwork, there are several individual problems—where tidal streams are concerned—that need to be solved:
I . To shape a particular course to a given point allowing for a known tidal stream; and as a refinement to this, to arrive at a particular place at a particular time allowing for the set.
2. To estimate the boat's position after a given time allowing for a known tidal stream.
3. To find the rate and direction in which an unknown tidal stream is setting.
4. To use this information to arrive at a destination at a given time.
i. To shape a course to reach a given point allowing for a known tidal stream (Figure 21 A)
In this case, you know your starting and end positions, the direction and rate of the tidal stream, and the boat's speed through the water. Wanted—the course.
You are at Point X and wish to reach Point Z, i.e. the line XZTIDAL STREAM AND CURRENT PROBLEMS I13
A x
3 w
B X
--. 3 u
Y
2'S 1300
y 1800
D M
L Z x 1930
2400 K 6'
C Y
z x
1200 0 _ --- K
x E
Y
Figure 21
must be the course made good (C.M.G.). The tidal stream is setting (towards) 2500 at 3 knots, and your boat's speed is 5 knots. The construction is quite simple. Draw XZ and XY, the set and rate of the tidal stream-25o° length 3 miles (if using a I hour triangle on the chart). Set your boat's speed (5 miles) on a pair of dividers or compasses and with one point on Y, cut off along the line XZ. Then:
YW = The course to steer
XZ = The course made good
XW = The speed made good along XW/WZ
Measuring XW off the same scale as that used previously for XY and YW, you can find its value in knots (about 2.3 knots) .
XZ (distance to go) Then XW (speed made good)
allows the calculation of the time of arrival at Point Z.
I14 SMALL BOAT NAVIGATION TIDAL STREAM AND CURRENT PROBLEMS I15
The example shown above has assumed a fixed boat's speed‑
5 knots. However, you may wish to arrive at Point Z at a particular time, and must therefore calculate not only the course to steer but also the boat's speed through the water to achieve this. The basic construction is the same, but in this case you must first work out what the speed made good has to be over the distance XZ. For example: You are at Point X at 1200, and would like to arrive at Point Z at 1600 (Mother-in-law's waiting).
The distance between the two, from the chart, is I2 miles—I2 miles to be covered in four hours. Therefore the speed made good has to be 3 knots. (Figure 2IB.)
XW must therefore be 3 (miles/inches/units of any sort). XY, the tidal stream is also 3; the triangle can only be closed by joining YW, and this will give the course to steer and the boat's speed required. About 087° at just under 6 knots. (This is the True course—not compass.)
2. To estimate the boat's position after a given time allowing for a known tidal stream (Figure 21 c)
As has been explained in Chapter 5, the characteristics of the streams around the British Isles are now fairly well known and information about them for particular times and places is given in the Tidal Diamonds on the charts. Let us say that the Diamond off the Isle of Wight at 1200 on October 2nd states that the stream is setting o65° at 2.5 knots. Your position is south of St. Catherine's Point Light and making for the Needles, steering 305° True at 6 knots. Wanted—your E.P. at 1300.
Lay off XY your course and speed through the water-305°
6 knots. Then lay off YZ, the direction and rate of the tidal stream, which was 065° 2.5 knots. Your E.P. at 1300 is then Point Z, your course made good the direction XZ, and the speed made good is the distance XZ.
With any of these problems, you can use any units you like in the speed triangle, i.e. I knot= I inch, mile, centimetre or what have you. But as you are working on the chart, it is more convenient to- use the chart scale and make, when you can, I knot = i mile. On some charts of small scale, this would not be satisfactory and then you could use I knot =5/10 miles. Nor is it necessary solely to use one hour's run for each vector of thespeed triangle. Providing the same time is allowed for each vector, any period can be used—j- hour, 2 hours' worth, etc. These comments only apply to the speed triangle. Distances must, of course, be measured on the scale of the chart and it is easy to forget this if using different units for speed.
In the example above, it is obviously simpler to use an hour triangle at i knot = I mile. Note also the conventional arrow symbols on the sides of the triangle:
Course and speed through the water One arrow
Course and speed made good Two arrows
Tidal stream direction and rate Three arrows
3. To find the set and rate of an unknown Tidal Stream, and
4. to use this information to reach a destination at a given time (Figure 2ID).
In this example, a fix is obtained at Position J at I 800. The second fix is obtained at Position K at 1930, the course and speed steered in the interval being 24o°-3 knots. You wish to reach Position L by midnight if possible, and the distance between K and L on the chart is 15 miles. What course and speed should be steered assuming that the tidal stream or current remains the same?
JK must be the course made good between 1800 and 1930. The course and speed steered during this time are then laid off from J-240° at 3 knots=24o° for 4.5 miles over II hours. This gives Point M. MK must be the direction and distance by which the boat has been set to the South-East over the II hours, i.e. 120° for 2 miles. Therefore the current must be setting
towards 12o° at a rate of 2 X 2 = 1.3 knots.
3
You wish to make good a course between K and L and have qj hours in which to cover the 15 miles. That means a speed
made good of 15 X 2=33 knots.
9
Anywhere along the line KL, lay off the tidal stream vector XY-12o° at 1.3 knots. Then from X along XL, lay off the speed to be made good—XZ—which is 3.3 knots. Then YZ is the course to steer, and the length YZ the boat's speed required —about 4.5 knots.
116 SMALL BOAT NAVIGATION
Alternatively, if insufficient wind or power is available to make this speed, you might want to find out at what time you will reach Position L at your present speed through the water.
Lay off the current vector as before—then with the dividers set to your speed, cut off along XL at Z. XZ is then the speed made good (Figure 2IE).
XZ turns out to be about i.8 knots. Therefore it will take
I$ = 8 hours 20 minutes to reach Position L at your present speed, and you will not be there until 0350. Bad luck!
._ 1 'l ,
Magnetic Compasses and chart table instruments have been described in earlier chapters, but there are other instruments and navigational aids in common use which are of interest to the small boat owner.
Perhaps one ought to start with the Gyro Compass. This is not generally used in the smaller boats, mainly no doubt because of its cost, but also because it needs a continuous power supply of one sort or another and a certain amount of maintenance. Small North-seeking gyro compasses are fitted in some fairly small craft, and it is fair to say that they are generally more accurate than a magnetic compass—and, of course, no irritating corrections for variation and deviation to worry about. They do suffer from errors, but unless something is radically wrong, these are normally of a fixed variety. Incidentally, gyro error is normally classified as `High' or `Low'. A gyro error of 10 High means that the compass is over-reading by i°, and therefore one degree must be subtracted from the compass reading to obtain True course or bearing. They have another advantage in that a number of repeaters can normally be driven off the master compass, and therefore a good all-round view of the horizon can be gained for the taking of bearings, and the steering repeater sited in an ideal position.
However, all these advantages must be set against the price. Before leaving the subject, though, it might be worth mentioning an interesting (and cheap) adaptation of a gyro compass which I came across a few years ago. The owner of this boat had on board a small portable air-blown free gyroscope, which had probably been originally fitted in an aircraft. This was connected by a rubber pipe, through a reducing valve, to a
II8 SMALL BOAT NAVIGATION NAVIGATIONAL INSTRUMENTS AND AIDS I19
compressed air bottle. The procedure was then quite simple—when a bearing was required, the gyro was fished out of its stowage (it only weighed about a pound) the card aligned to the True boat's heading, and the air switched on. It ran up to full speed in a few seconds and could then be used anywhere in the cockpit area for the taking of bearings. Its advantages over a hand bearing magnetic compass were that the card was absolutely steady, however much the boat was moving about, and it was not, of course, affected by any local magnetism in the boat.
A free gyro, as was this instrument, is not North-seeking and suffers from the normal apparent drift due to the Earth's rotation. This is 15° per hour, so four minutes would have to elapse before the gyro drifted one degree. This is more than enough time to take the bearings for a three-point fix. If anybody wishes to copy this idea, then I should refer them to the owner, Wing Commander Crammond, but I regret that I do not know his whereabouts at the present time. Perhaps, also, similar instruments are now available commercially.
Sextants
There are two main types; those that use the horizon as a reference (marine) and those that use a bubble (aircraft). The marine type is the more accurate—naturally!—but to dispose of the opposition first. Bubble sextants are difficult to use at sea —bubble acceleration caused by the boat's or ship's movement makes it very difficult to hold the bubble steady. This is vital, of course, to the accuracy of the readings obtained as the bubble is the horizon. It is unfortunate that they cannot be used satisfactorily at sea, for they would give the facility for taking astronomical sights throughout the night, which is not possible with the normal marine sextant.
Marine sextants have remained largely unchanged in design for many years, although the method of taking readings from the sextant scale has been altered, the `micrometer' drum replacing the now obsolescent `vernier' scale. A standard marine sextant, capable of being read to an accuracy of about o. I of a minute of arc, now costs about £75 new. A second-hand vernier sextant can probably be picked up for about £50. Such an accuracy is desirable for astronomical sights, but is not reallyneeded for horizontal and vertical sextant angles. There are a few smaller marine sextants about, which may be picked up second-hand for about £Io to £2o, that only read to the nearest minute of arc but are good enough for sextant angle fixing.
A marine sextant has two mirrors, the top one (called the Index glass) being fully silvered and the bottom one (the
incoming Index I ss
for
from object '~--- ~ Screw V
A Horizon (adjustment perpendicularity)
glass
Screw W Frame
Eyepiece
of telescope
incoming from /
horizon or I
another Handle
r
Index bar
e es
r ~/
Screw X Scale
Scale window
IMicrometer ~'*—Spring clamp drum r'" 340 33° 32° 31° 30°
Main scale { B
on arc— I
Vernier- t 2' I'0-8 0.604 02 0'
Y Z
Figure 22
Horizon glass) half silvered and half clear glass. An observer can thus see the horizon (or any object) directly through the clear part of the Horizon glass. At the same time, he can see the image of another object reflected from the top mirror to the silvered portion of the bottom mirror. If the two mirrors are exactly parallel, the reflected and direct images of a single object should be coincident and the scale reading o° oo.o'.
Figure 22A gives a rough idea of what a sextant looks like when held vertically (as it would be when taking sights) and
I20 SMALL BOAT NAVIGATION NAVIGATIONAL INSTRUMENTS AND AIDS I2I
viewed from the side. This is only for those who are not familiar with the instrument, and it does not show various details, such as frame supports, shades, etc.
The Index Bar is movable over the whole arc of the scale, the spring clamp grips holding it locked at roughly the desired angle once they are released by the thumb and forefinger. Fine adjustments, also affecting both the reflected image and the scale reading, are made by turning the micrometer drum. The sextant can be set, therefore, to about the right angle by moving the Index Bar until the pointer in the scale window shows the correct number of degrees and the micrometer drum will click into place near this reading once the spring clamps are released. It is important to refrain from trying to move the Index Bar without, or only partially, releasing the spring clamps as this may cause damage to the worm on the micrometer drum or to the rack on the sextant scale.
Suppose that the instrument is to be used for the taking of a vertical sextant angle. First make sure that the smaller (star) telescope is fitted. (The sextant box normally contains two telescopes; the longer (sun) telescope gives greater magnification but produces an inverted image.) Then focus the telescope using the horizon or any other convenient object. Holding the sextant by its handle in the right hand, point it at the object keeping the horizon in view through the clear glass. Release the clamps with your left hand, and move the index bar until the top of the object heaves into view in the silvered part of the horizon mirror. Let the clamps go, and move the micrometer drum until the horizon and the top of the object are exactly level with each other. Now read ofd and on!
Reading off an angle is much easier with a micrometer than a vernier sextant, particularly when the light is not very good. Micrometer first; the degrees in the angle are read off opposite the pointer in the scale window—simple! The minutes are read off a scale inscribed around the drum opposite a pointer attached to the frame—also simple. The points of a minute are dealt with on the vernier principle; beside the frame pointer near the micrometer drum, you will find marked divisions—o.2, 0.4, o.6, o.8 of a minute. Whichever of these divisions is most nearly opposite one of the divisions on the micrometer drum, that division gives the points of a minute.
The vernier sextant is used in the same way, except that fine adjustments are made by a tangent screw attached to the bottom of the index bar, and the method of scale reading is different. In the first place, the main scale on the are is more finely graduated than on a micrometer sextant, so a movable magnifying glass is attached to the frame to assist. Secondly, the degrees, minutes, and points of a minute have to be deduced from the relative positions of the main scale on the are and the vernier scale outside it.
The vernier scales used on sextants can be read to 0.2 minutes of arc. In Figure 22B above, it is shown as a straight scale for ease of illustration although, in practice, it would be curved.
Each of the small divisions on the main scale is the equivalent of I o minutes of arc. Therefore the arrowhead indicating the zero of the Vernier scale is showing a main scale reading of between 300 Io' and 300 20' (Point Z). The starting point is, then, 300 io'. To find the additional minutes and points of a minute, look along both scales until a mark on each exactly coincides. This occurs at the point marked Y. There, the reading on the vernier scale is 2.2'. Thus the whole angle is 300 10' plus 02.2 =300 12.2'.
When taking horizontal sextant angles, the sextant is laid on its side, handle underneath, and the left-hand object viewed through the horizon glass. The actual image of one object is then brought into coincidence with the reflected image of the other on the dividing line between the clear/silvered parts of the horizon glass. This is easier to do than it sounds and should cause no problems.
Sextant Errors
Some errors, to which a sextant is susceptible, can be removed by its owner, and these are listed below in the order of treatment (potential doctors please note) :
a. Perpendicularity. The index glass must be perpendicular to the plane (frame) of the instrument.
Remedy: Put the index bar roughly in the centre of the arc. Hold the sextant horizontally and look into the index glass from outside—towards the arc of the scale. The reflected image of the arc should join (be in line with) the direct view of the arc. If it is
I22 SMALL BOAT NAVIGATION NAVIGATIONAL INSTRUMENTS AND AIDS I23
not, take the small cover off Screw V (see diagram) and turn this screw until all is as it should be.
b. Side Error. The horizon glass must be perpendicular to the plane of the instrument.
Remedy: Set the scale reading to about zero. Hold the sextant vertical and point it at a star. When the index bar is moved by the micrometer, the reflected image of the star should pass directly over the direct image. If it does not, but passes to one side or other, take the cover off Screw W (this is the one on top of the glass away from the sextant frame) and adjust it until the correct answer is achieved.
c. Index Error. Mirrors not exactly parallel when the scale reading is zero.
Remedy: This is the most important error, but the one which must be dealt with last as adjustments of the other two errors may well affect the index error.
By day, choose a section of the horizon which is clear and sharply defined, hold the sextant vertical, and adjust the micrometer until the reflected and direct images of the horizon are exactly in line horizontally. Then read off the scale. At night, select a bright star, point the sextant directly at it with the scale reading set near zero, and superimpose the two images. Read off the scale as before.
The Index Error readings thus obtained may either be `on' or 'off' the scale. Supposing that when the images or the horizon, or a star, were brought exactly into coincidence, the scale showed a reading of o° 02.3'. This is `on' the scale and indicates that the sextant was over-reading by that amount. The Index Error was then—o2.3' of arc. If, on the other hand, when image coincidence was obtained, the scale reading was 58.8', the sextant would be under-reading and the error `off the scale' by o1.2'. The Index Error was then +I.2 minutes.
Either way, this Index Error can be corrected by adjusting Screw X—at the side of the frame of the horizon glass. Watching the horizon or star through the sextant, and with the scale set precisely to o° oo.o', turn the screw until the images are coincident. Finally, re-check the Side Error, and have a last Index Error check.
Note: Particularly with vernier sextants, it may not be worthtrying to remove Index Error if it is less than about 3 minutes. It can always be allowed for by adding or subtracting its value to/from observed altitudes or bearings.
Care of a Sextant
Sextants are fairly expensive instruments and should there- fore be looked after properly. One or two points:
a. Always pick a sextant up by its frame, and carry it either by the frame or handle. Never carry it by the mirrors, or put it down on a surface upside down so that the mirrors are taking the instrument's weight. Obviously it should not be knocked about, and its replacement in its box after use is a good safety precaution.
b. Do not stow the sextant away for long periods encrusted with salt—wipe the mirrors and frame over carefully with a soft cloth and fresh water. Then oil the scale and drum.
INSTRUMENTS FOR MEASURING DEPTH
The Echo Sounder
Quite a few boats are now fitted with echo sounders, so perhaps a few remarks about these instruments would not come amiss. The basic principle on which they work is very simple—A sound pulse is transmitted from an oscillator attached to the boat's hull below the waterline, reaches the seabed, and is reflected back to a receiver in the boat. As the speed of sound in water is known (although it varies between waters of different densities—sea and fresh for example), if the time is measured between transmission and reception of the pulse, a depth can be obtained.
In shallow water, the time interval between transmission/ reception is very short and can only be measured by electronic means. Most echo sounders, too, work on a supersonic frequency so that one cannot actually hear the transmissions. The main difference between the various echo sounders on the market is in the way the information received is displayed.
Some of the more sophisticated echo sounders use a recorder-type display where the incoming bottom echoes are marked on
I24 SMALL BOAT NAVIGATION NAVIGATIONAL INSTRUMENTS AND AIDS I25
a moving trace of impregnated paper by an electric stylus. This has the advantage of enabling a permanent record to be kept of the seabed over which the boat has passed. This can be very useful in constructing a sounding trace—as explained in Chapter 6—and it is more or less essential for surveying work. However, most yachtsmen are only interested in knowing what their depth is at the time, and there are other types of display which show this more simply. Some take the form of a circular gas-filled tube, marked with an appropriate depth scale. When a transmission is made, a spot of light starts rotating around the tube, and this brightens when any echo is received from the seabed. This is quite a good, practical display. The choice of an echo sounder really depends, once again, on your pocket. There should not be a great deal that can go wrong with an echo sounder, as it is a comparatively simple piece of electronic equipment, but on the whole the more expensive—the more reliable.
One point before leaving this subject—be careful to check whether your echo sounder is set to read from the waterline or the keel. In certain circumstances, this might make quite a difference!
The Lead Line
For those working on a limited budget, the luxury of being able to take a depth at will merely by pressing a switch is probably unattainable. So, one must resort to the bracing life and time-honoured handraulic methods; the lead line, or possibly in very shallow water, a suitably marked stave or boathook.
A lead line should be made out of very pliant rope that does not kink or retain any loops or bights once it is uncurled. Ordinary stranded rope is no good for this purpose but special plaited rope is sold. The rope is spliced through a ring on the top of the lead weight. The lead itself has a hollowed-out section at the bottom which can be `armed' with tallow. When the lead is recovered, it should be therefore possible to identify the nature of the bottom from whatever is sticking to the tallow. If it comes up with a bit of shell attached—shells. If it comes up with a fly-button ... ?
There is a standard way of marking a boat's lead line, although this will presumably be changed when the charts all
become metric. There is no necessity to use this method if you
do not wish to do so—any method will do providing it is clear
to the user—but it has the advantage of being familiar to a lot
of people. The fathoms—starting from the lead end—arc
marked as follows:
One fathom One strip of leather (One Fathom
Two fathoms Two strips of leather =6 Feet)
Three fathoms Three strips of leather
Five fathoms Piece of White bunting (cloth)
Seven fathoms Piece of Red bunting
Ten fathoms Piece of leather with a hole in it.
Thirteen fathoms Piece of Blue bunting
Fifteen fathoms Piece of White bunting
Seventeen fathoms Piece of Red bunting
Twenty fathoms Two strips of leather with holes in them
These are known as `Marks'. So if the lead line came out of the water with a piece of blue bunting resting on the surface, the leadsman should, in his best Metro-Goldwyn-Mayer voice, call out `By the Mark—Thirteen'. The intervening fathoms, between the marks, are estimated and called `Deeps'. `Deep Four', `Deep Six' and so on.
When taking a sounding with the lead line, the rope is coiled up in the leadsman's inboard hand, leaving a small bight between it and where the other hand is grasping the lead line a few feet above the lead. The lead is then swung and released to land in the water a sufficient distance ahead to allow for the boat's speed—the object being to take the sounding when the lead line is up and down in the water right underneath the leadsman. In most boats, with a fairly low freeboard, this can be achieved with an under-arm swing, but in ships the man has to be more ambitious and whirl the lead overhead with consequent hazard to the rest of the community. However, if when first practising you instruct your friends to go below, there should be no need for the first-aid box.
Logs
These either measure speed directly or distance directly; no matter, for one can easily be deduced from the other. There are basically two types:
I
I26 SMALL BOAT NAVIGATION NAVIGATIONAL INSTRUMENTS AND AIDS I27
a. A bottom log that works on pressure differences between static pressure orifice and an impact pressure orifice—The Pitometer Log.
b. A bottom or towed log that is driven by a small propeller, or impeller, whose rotation is measured by electrical or mechanical means—The Chernikeef Log, Walker's Patent Log and others.
The Pitometer Log
This measures speed directly, and is generally used in large ships. It works on very much the same principle as an aircraft's pitot tube. A rodmeter projects below the hull, one shielded pipe in it measuring the static pressure at the depth of the rod-meter and the other pipe facing forward to sense the full pressure due to the ship's speed through the water. These two pressures are compared in a mercury (or other) differential, which also acts as a damping device, and a speed produced. This can be converted into distance for display on another dial.
Advantages. No moving parts outside the hull which can get clogged with sea-weed, etc. Very accurate, particularly at low/ medium speeds.
Disadvantages. Comparatively expensive and rather bulky for small craft. Needs an opening in the hull for the rodmeter.
The Chernikeef Log
This measures distance directly, and has been used in all sizes of craft. A small propeller is sited at the end of a shaft projecting through the hull (this can normally be raised and lowered through a gland—as can the rodmeter of a Pitometer Log). As the vessel moves through the water, the impeller rotates and this rotation is transmitted electrically to a distance recorder. This is converted to speed for display on another dial.
Advantages. Possible to measure very slow speeds as the impeller is quite sensitive. The log can also be adjusted (calibrated) to a certain degree by bending the impeller blades.
Disadvantages. Expense, and the impeller can become quite easily fouled, or perhaps damaged, by debris, seaweed, etc. Needs an opening in the hull for the impeller shaft.
The Walker's Patent Log
This measures distance directly, and is the log (or variety of log) most used in small craft—and in big ships too, sometimes as an emergency log. It is a towed log, where a small screw propeller, called the rotator, is towed at the end of a long piece of log line. The boat's passage through the water causes the rotator to turn, the rotation being transmitted via the line to a mechanical register fitted on a bracket in the stern of the boat. A metal wheel, called a governor, is sometimes used in the line just astern of the boat, clear of the water, to smooth out the rotation. The distance travelled can either be read off the mechanical register or, with electric logs, transmitted to a receiver sited elsewhere in the boat.
Streaming the log is quite simple. The rotator is paid out over the stern and the inboard end attached by its hook to the ring of the mechanical register. If the governor wheel is going to be used, this should be attached by its short length of line to the register—before the rotator is paid out—and kept inboard. The rotator is then paid out and the inner end of the log line clipped to the governor. This is then eased over the stern.
Recovering the log is a bit more difficult, particularly if the boat is making sternway—so haul in the log in good time! First recover the governor, unclip it, and if the boat is still making headway, pay out the inner end of the log line over the other side of the stern whilst hauling in the rotator. When the latter is inboard, the rest of the line can be brought in free of turns.
The log line is made specially for this purpose. Generally speaking, the longer the length used the better for the purposes of accuracy, but do not forget that if the boat's speed is very low, the rotator may trail across the bottom and possibly get snarled up in something down there. The recommended lengths* are:
Maximum speed 10 knots —40 fathoms 15 knots —50 fathoms 18 + knots — 65 fathoms
Advantages. Cheap and simple. No holes in the hull. Spares easily carried.
Disadvantages. The main disadvantage is that the log has to be
* A lot of boats only use about 75-100 feet of line with fairly good results.
1;2$ SMALL BOAT NAVIGATION I NAVIGATIONAL INSTRtUMENTS AND AIDS 129
hauled in whenever the boat goes very slowly, stops, or goes astern (unless in deep water—or in powered boats you want a rope round the screw). The rotator is liable to be fouled with seaweed; in foreign parts, also, it seems to have a strong fascination for sharks—with somewhat violent results. It is also inaccurate in a heavy following sea.
Other methods of estimating speeds, without the use of a log, are mentioned in the next chapter.
Radio Aids
There are quite a number of Radio Aids in constant use by both ships and aircraft, but not all of these, mainly for reasons of equipment expense or complexity, are available to the average yachtsman.
Those aids which require sophisticated equipment on board include the following:
a. Radar. Very widely used at sea by ships and an invaluable aid to navigation. Some large yachts and power boats have been fitted with radar, and there are, in fact, fairly small and portable radar sets now being produced commercially. However, the vast majority of small craft are without it.
b. Loran. This is an ocean aid, and Loran transmitting stations are sited in the North Atlantic, Pacific, and other areas. There are two systems, one of which is very accurate, but an expensive receiver is required.
c. Decca. A very well known, and widely used, coastal aid of high accuracy. Decca chains surround the coasts of the British Isles, the North Sea, the Channel and a growing number of localities abroad. The chains are generally usable within about 250 miles of the master station. However, again a special receiver is required. .
The Admiralty publishes special charts, overlaid with the Decca and Loran lattices. These can be obtained from any chart agent, together with small booklets showing the worldwide coverage of these aids.
Coastal Radio Direction Finding Stations
This is an aid which falls into an intermediate category as far as yachtsmen are concerned. RDF stations are spread around the
coasts and will, on request and on the booking of a small fee (free, I believe, in an emergency), take a bearing of the vessel asking for it and then transmit the bearing back to the boat. This, of course, means that the boat must be fitted with both a transmitter and receiver which can be tuned to the correct frequencies. Details of the services provided by these coastal RDF stations can be found in the Admiralty List of Radio Signals. These stations should not be confused with Radio Beacons (q.v. below). The station takes the bearing, not the boat. With morse (continuous wave) transmissions from the boat, bearings can be taken over very long ranges and this really is probably most useful as an ocean aid.
Consol
This is an ocean radio aid, freely usable by all who care to read the signals on an ordinary radio receiver. The range of Consol
Figure 23
is about 1500 miles and there are stations at Bushmills (Northern Ireland), Stavanger (Norway), Ploneis (France) and Seville (Spain). Consol charts are published, and details of frequencies, together with Consol position tables, are given each year in the Admiralty List of Radio Signals, and in other almanacks and publications.
The system works on the principle of phase-differences, and was developed from the radio aid the Luftwaffe used to find targets in Britain during the night bombing raids of the war. A very simple explanation of phase-difference; drawn diagrammatically a radio wave can be considered, not as a straight line, but as a Sine wave (see Figure 23).
The wave starts at phase o°, rises to a maximum value in one
Phase
130 SMALL BOAT NAVIGATION I NAVIGATIONAL INSTRUMENTS AND AIDS 131
direction go°, sinks again to zero 180°, and rises to a maximum in the other direction 27o0 before returning to zero again. During this time, depending on the wave length (frequency) of the transmitter, the wave will have covered a certain physical distance over the land or the water. If a boat was precisely half-way between two transmitters of the same wave length, the two signals would reach it at exactly the same phase, e.g. both at the go° point. If, on the other hand, the boat was further away from one transmitter than the other, the signals would arrive out of phase, each with its own phase value, e.g. one at go° the other at 270°.
The Decca receiver uses this principle by actually measuring the phase differences with considerable accuracy. In Consol, the transmissions are modulated in such a way that a different sound is produced in an ordinary radio receiver as the phase-difference changes. For example, a continuous note at zero phase-difference, 3o dots and 3o dashes at maximum phase-difference, and so on.
If on a chart or a map a line is drawn joining all the points equidistant from two fixed points, this line will turn out to be a hyperbola. A line joining places of equal phase-difference will therefore produce a hyperbolic position line. Thus the Consol lattice on a chart is a lattice of hyperbolic position lines identified by the number of dots and dashes each represents.
The fixing procedure is fairly simple—if you have a good ear! (I haven't, and have never yet achieved a really satisfactory Consol fix.) Tune into a Consol station. The frequency is given on the chart as well as in the books. Listen until you hear the call-sign and then start counting the dots and dashes. The total received until the start of the next cycle should add up to 6o. It is not always easy to discern when the dashes start or the dots end as they merge into a continuous note (called the equisignal) . If, for example, only 4 dots and 5o dashes are heard, i.e. 6 symbols are missing, the numbers should be adjusted to read 7 dots and 53 dashes. On the chart, pencil in the position line equivalent to this Consol reading nearest the D.R., and then tune to the next station and repeat the procedure. This should give a Consol fix. Remember that this is an ocean aid and that therefore great accuracy cannot be expected in the fix—as a rough estimate 5 to 10 miles might be taken as a guide, but alot depends on the radio conditions, distance from the transmitters, and the efficiency of the count.
Radio Direction Finding (D/F)
Quite a number of boats these days are fitted with a Radio Direction Finder—or D/F set as it is commonly called. These sets usually have some form of rotatable circular `loop' aerial. By tuning the receiver to a particular shore station and moving the aerial around, a maximum or minimum received signal is found. The signals are then amplified, and compared aurally or on a meter. When the minimum signal is reached, the plane of the boat's aerial must be at right-angles to the bearing of the transmitter. The aerial bearing at this time will, of course, be relative to the boat's heading, but if the aerial bearing is related to the compass, the compass bearing of the transmitter can be established.
D/F sets of this nature can give quite good results over comparatively short ranges—in the order of I oo miles. The accuracy of the bearing obtainable depends on a number of factors. The character of the set being used is one, range from the transmitter another. A principal cause of trouble is the interference between radio waves (from the same transmitter) that follow a ground-wave path and those following a sky-wave track. The latter pass out through the Earth's atmosphere but are reflected back by an ionospheric layer, to return to Earth at varying distances from the transmitter. A pure ground-wave signal will give the best bearings, but particularly at dusk and dawn, and also at night, sky-wave signals may intrude. With loop aerials, in sky-wave interference conditions, serious errors may result at ranges over 25 miles from the transmitter. As far as reception is concerned, there is no indication whether a sky- or ground-wave is being received.
Another possible cause of trouble in some ship or boat-borne D/F sets is that a receiprocal bearing may be obtained (i.e. a bearing that is 18o0 out). A simple loop aerial cannot differentiate between the signal strengths received when it is turned through exactly 18o°. Some sets are fitted with a sensing device which removes this difficulty, and the prospective purchaser of a D/F set would be wise to check this point.
Another cautionary note: depending on the position of the
132 SMALL BOAT NAVIGATION
x
Figure 24
aerial in the boat, other fittings such as masts and shrouds may deflect the incoming signals on certain relative bearings. A D/F set should, therefore, be calibrated `in situ' to discover whether any of these local errors are important enough to be added to or deducted from the received bearings.
Over ranges of less than I oo miles, the fact that radio waves travel on Great Circle paths can be ignored and little error will result if the bearing of the transmitter is plotted directly on a Mercator chart. But over long ranges—IOO-2000 miles or so—the difference between the Great Circle and Rhumb Line bearings must be taken into account.
In Figure 24, the boat at Point A takes the D/F bearing of a transmitter at Point B. The radio wave from the transmitter follows the Great Circle course BZA; the tangents to this curve from A and B meet at D. The bearing of B from A on the chart is the Rhumb Line bearing AB. A correction has therefore to be applied to the observed bearing equal to the angle DAB. This angle is called `Half convergency' as it is equal to half the convergency angle ADX.
The correction for this angle is always applied towards the Equator as the Great Circle curves towards the Pole. Its value can be found from the following formula:
D' Long Sine Mean Lat. Half-Convergency (in minutes) =X
(minutes) 2
(The D' Long. and Mean Lat. between boat and transmitter)
NAVIGATIONAL INSTRUMENTS AND AIDS 133
An easier way to find the value of Half-Convergency is to use the Traverse Table (the use of this table is described in Chapter 9).
Enter the D' Long. as the Distance and Mean Latitude as the Course.
Departure = Half-Convergency in minutes.
2
There are now many Radio Beacons, a lot of them fitted in lighthouses, sited around the coasts. They transmit at fixed intervals, normally sending their identification signal in Morse Code first, followed by a long dash to facilitate D/F bearings. The Radio Beacons (Ro. Bn. on the chart) and Coast Radio stations transmitting for D/F purposes are ringed with magenta circles. However, the charts give no details on callsigns, frequencies and times of transmission. Complete details of these are given in the Admiralty List of Radio Signals, from diagrams available at chart agents or from the makers of the D/F sets. (Coast Radio Stations may only transmit on request.)
A D/F set is therefore very useful to the yachtsman. Two plotted D/F bearings will give a fix, and many an owner must have been grateful for his D/F set on a foggy night in mid Channel. However, the limitations and possible causes of error in D/F bearings must always be remembered. It is an Aid—not God!
E
Practical Navigation in a Boat
The preceding chapters have described some of the tools and techniques used in the art of navigation; the time now approaches when theory has to be put into practice. The real work should start well before you slip from your moorings or leave that nice, comfortable jetty where the boat has been lying whilst you have been busy with other things.
Planning the Passage
The first thing to ensure is that all the charts, tables, books, instruments, are on board to cover the whole route and any possible diversions it may be necessary to make should the weather turn foul or one of the crew go sick; precautions first, fun later. Having done this check, start to plan your passage on the charts. Even though you may not be able to stick to them rigidly because of weather conditions, it is always a good plan to draw in the intended tracks on the chart; whilst doing this, mark back from your destination the distance to go so that, on whichever chart happens to be in use at the time, you know how far away the target lies. On long passages, it will probably be necessary to draw out the general track initially on a fairly small scale chart, and then transfer it to larger scale charts, amending, it as necessary to skirt any dangers that may show up more clearly on the latter. If in any doubt as to how wide a berth to give a particular point or headland, always consult the Pilot for the advice it may offer. This will also give pretty detailed information on the tidal streams and currents to be expected even if the chart is a bit vague on the subject. Look out for any unhealthy spots which it would be unwise to approach too closely at unfavourable moments, e.g. unlit rocks or sandbanks in the middle of a pitch-dark night. There are a host
PRACTICAL NAVIGATION IN A BOAT 135
of things to look for, and they cannot all be listed here, but it is largely a matter of common sense.
When transferring positions or courses from one chart to another, it is usually easiest to do this as a range and bearing from a particular point common to both charts. It can also be done by Lat. and Long., but this is usually slower and in a few areas still the Lats. and Longs. vary slightly between charts.
When you have got the general track marked on the chart, perused any doubtful places or corners, got a general idea of the tidal streams or currents likely to be met, the final considerations should be planning for leaving harbour and the weather.
To take the weather first. In U.K. waters, it is very simple to get a weather forecast. They are given frequently on the ordinary radio programmes, and the Met Office will give a forecast on request by telephone. Abroad it is not quite so easy, as the local radio (damn it) may be incomprehensible. However, in harbour it is usually possible to get a local forecast from the yacht club, port authorities, or even a chap stroking a piece of seaweed on the beach; it is a question of where English is spoke, if you do not know the country's mother tongue, rather than a lack of available forecasters—which are legion everywhere. At sea, details of area forecasts in English are given once again in the Admiralty List of Radio Signals. General weather information is also contained in the Pilots. Your final decision on when to leave harbour must rest, obviously, on the local and area weather forecast and the state of the tide. Even if the wind is fair, it may be prudent to wait until the worst of the flood tide has passed before making out an estuary. On the other hand, it may be worth beating out against the tide for a while if greater advantage can be gained later. It is impossible to generalise on this.
Finally, the plan for leaving harbour. If it is a familiar place, there should be no problems in getting away to sea from it. A strange place, though, may require a little more careful study, and perhaps a written-down plan for leaving harbour with courses on leading marks, turning points, and clearing bearings taken off the chart and noted down for quick reference as you proceed to sea. It is a good thing to keep one section of your `navigator's notebook' (see next paragraph) reserved for leaving harbour plans of this nature. Once the problem has been
136 SMALL BOAT NAVIGATION I PRACTICAL NAVIGATION IN A BOAT I37
studied and the plan made, it can be used again at any time in the future with adjustments for different states of the tide, wind, etc. In power driven boats, the same plan can be used time and again without any alterations.
The Navigator's Notebook
It is a good practice when making a passage to keep a small notebook in which all important navigational data can be entered. It serves as the place in which alterations of course and speed, bearings taken for fixes, compass checks, etc., can be written. It is better to write such data down in a notebook rather than scribbling it on odd scraps of paper or on the chart where it stands in grave danger of being erased or lost or confused with other detail. One good reason for recording these facts on a systematic basis is because of the necessity sometimes to go back and check previous fixes, courses, etc. Suppose a mark is not sighted at the expected time; the wise sailor then goes quickly back over his previous work to make sure that no glaring errors have been made. If the facts have been recorded, an error may be found and corrected in time. If no obvious error has been made, then there is at least some comfort in that knowledge.
The notebook need not be an elaborate affair. A small pocketbook with a pencilled margin down the left-hand side of each page is all that is required ... time being inserted in the margin and relevant details across the rest of the page opposite the appropriate times.
An example of a typical page of a navigator's notebook is given opposite.
Taking Bearings for Fixes
Whether taking bearings by using the Azimuth circle on a compass or by means of a hand-bearing compass, it is necessary to observe them accurately—and preferably quickly. The best routine in this respect is:
a. Identify the objects you are going to use both visually and on the chart.
b. Write the names of the objects into your notebook.
c. Take the bearings of the objects in quick succession. If the boat is moving fast and the objects are close, take the bearing of
0 r'' I -f C4a2er . /5o C
/000 I G~.1a~cd ~ust`t~, . ~C' ed ,~. 3kts,
}--- means abeam to Port • —I means abeam to Starboard --~ means right-hand edge
138 SMALL BOAT NAVIGATION PRACTICAL NAVIGATION IN A BOAT 139
the object nearest the beam last; this is the bearing that will be changing most rapidly.
d. Do not write the bearing of each object down before taking the bearing of the next. This wastes time. Instead remember the last two digits of each bearing in your head, e.g. og, 24, 67 for bearings of 309, 024 and o67. Then go to the notebook and write the two figures against the appropriate objects. The first digit in each case can easily be added by inspection of the chart. It is considerably easier to retain six figures in your head than nine whilst taking the bearings.
e. If you are in a boat that is only fitted with a steering compass with no horizon view from it, and there is no hand bearing compass available, the boat must be pointed at each object in succession and the ship's head read off the compass when the bow is pointed directly at each object.
Speed through the Water
If you do not possess a log, judgement of speed or distance run through the water is largely a matter of experience with that particular craft. The amount of wake produced at varying boat speeds is normally a good clue. Sailing craft, too, have a fairly set maximum speed which is a function of their waterline length. In powered craft, the maximum speed is also well defined in calm water but more difficult to assess when heading into a lop. Intermediate speeds can also be pretty accurately estimated in power boats from the engine revolutions and a speed/revolution table devised (a measured mile in your locality?).
Particularly when the boat is moving fairly slowly, quite an accurate judgement of speed can be made by using the `Dutchman's Log'. If the time taken for a piece of wood or other marker to travel from the bow to the stern is measured, the boat's speed can be calculated. If, therefore, you can persuade your wife to throw her handbag in the water (latest hat will not do—too much windage) abreast the bow and clear of the hull and, with a stop-watch, time the period elapsed until you fish it out of the water exactly abreast the stern, you will obtain an estimate of the boat's speed. The shorter the boat, of course, the more critical the timing becomes from all points of view.
In a boat of 30 foot length, suppose the marker thrown over parallel with the bow takes 5 seconds to reach the stern. Then:
Speed (knots) = Distance (miles) Time (hours)
or in this case Speed = 3o x 6o x 6o = 3.6 knots. 6000 c
Surface Drift
When the wind has been blowing steadily over the sea for some time, the water surface responds to the frictional drag of the air by moving in the general direction towards which the wind is blowing. This drift current, or surface drift as it is called, is deflected by the Earth's rotation to the right of the wind direction in the Northern Hemisphere and to the left in the Southern. The amount of deflection can be as much as 45°. The strength of these currents varies greatly depending on how long the wind has been blowing, its strength, and the amount of fetch (distance over the sea) it has had. An average figure for the current's rate is usually taken to be somewhere about I/5oth of the wind speed. Therefore a 20 knot wind blowing in a steady direction for a few hours could be expected to produce about I a knot of surface drift.
The major currents in the open oceans are, of course, caused by winds such as the Trade Winds blowing consistently over vast areas of ocean for many months of the year. The surface drift mentioned here is a much more local effect.
Leeway
In any boat, but particularly sailing craft, leeway (movement bodily downwind) has to be taken into account when estimating the course made good. This is over and above any current or tidal stream that may be present. The amount of leeway a boat makes is governed by its hull form, the relative wind direction and the boat's speed. Boats with deep keels or large centreboards will make less leeway than shallower, less stiff craft. Again, judging the amount of leeway a boat will make under given conditions is largely a matter of experience with that boat —and the helmsman! The angle the wake makes with the boat's heading is a good indicator; generally, the faster the boat is moving in a forward direction, the less the leeway made.
I40 SMALL BOAT NAVIGATION PRACTICAL NAVIGATION IN A BOAT I4I
.Navigating out of Sight of Land
It is is not the intention in this book to include anything about the taking and working of astronomical sights, frequently the only method of obtaining a reliable fix out of sight of land. A knowledge of astro-navigation is essential if you are contemplating any really long voyages in the open sea.
A point that might be made here is that the accuracy of your D.R. or Estimated position is bound to degrade with time—unless you are very lucky and all the drift and current errors conveniently cancel each other out; sadly, this does not often happen! The staleness of the D.R. or E.P. (since the last fix) is thus a big factor in any estimation of how closely you know your true position. The longer time away from a fix, the bigger the guess.
When the shore and known fixing marks are visible, it is not always necessary to keep a record of every tack or alteration of course as a fix can be taken when necessary to re-establish the boat's position. When out of sight of land, however, there is a real need to keep as accurate a D.R. or E.P. as possible. This can be done by plotting on the chart every course and every distance run on that course. If, when sailing, frequent tacking is necessary to make ground to windward, the plotting of each tack may be a tedious business. The alternative is to use a Traverse Table.
These are contained in most sets of nautical tables. Basically, all they do is to give a tabular solution for a right-angled triangle, the hypoteneuse being the Distance Run, the other sides being the distance resolved North–South (D' Lat.) and the distance resolved East–West (Departure). The course steered is the angle (or go° minus the angle) between these two sides (Figure 25).
Suppose a boat is steering o600 and runs on this course for 3 miles. Solving the triangle by plane trigonometry:
Sine 300 = D' Lat. Therefore D' Lat. = 3 Sine 30 3
= 3 X 0.5
= I.5 miles or minutes of latitude.
Cos 30° Departure
= Therefore Dep. = 3 Cos 30
3
= 3 x o.866
= 2.598 miles East (or West) .
Note. The Departure can be plotted directly on the chart—using the Latitude scale for its measurement—or converted into a difference of Longitude (D' Long.) by the formula:
D' Long. = Departure X Secant Middle Latitude.
departure
Figure 25
A Traverse Table, such as that given in Inman's Nautical Tables, may be arranged as on page 142.
The arrangement of this table is such that if the course angle is less than 450 the table is entered from the left-hand edge and the column headings at the top used for D' Lat. and Dep. If the course is greater than 450, the table is entered from the right-hand edge and the column headings at the bottom used. For example:
D' Lat. Dep.
Course 031°—Distance 2.98 miles 2.55 N 1.53 E
Course o6o°—Distance 3.00 miles 1.5 N 2.598 E
For courses between 0900_I 8o°, the table is used as if the course was South ... ° East; a course of 176° is the equivalent of
Course Dep. D'Lat. Dep. D'Lat. Dep. D'Lat. Dep. D'Lat. Course
South 40 East, therefore the D' Lat. and Dep. for a distance of 3.o miles are 2.933 S and 0.209 E respectively. Note that the D' Lat. and Dep. must be South and East in this case as the course has a Southerly and Easterly component. Obviously the D'Lat. is going to be much the larger as a course of 176° is within a few degrees of due South. Similarly, for courses between I8o°270°, the course is considered as being South ... ° West, and between courses 2700-3600 North ... ° West.
The following example illustrates the use of the Traverse Table in practice. A boat starts in position 50° 10.0' N 4° o6.o' W. It then steers these courses:
D' Lat. Dep.
N S E W
29.93 - 2.09 ‑
- 25.44 15.90 ‑
- 29.98 - 1.05
20.07 - - 22.29
50.00 55.42 17.99 23.34
PRACTICAL NAVIGATION IN A BOAT 143
As a result of these manoeuvres the boat has travelled:
50.00 N 17.99 E
55.42 S 23.34W
5.42 S 5.35 W
In other words, it finishes up 5.42' or minutes of Latitude or miles South of the starting position and 5.35 miles West. The simplest way of re-plotting the boat's position on the chart is to set these 'Northings' and 'Eastings' successively on the dividers-Departure is in miles East or West and therefore the Latitude scale can be used for both quantities-and plot the appropriate distances South and West from the initial position. Alternatively, the Longitude of the final position can be found by converting Departure into D' Long.-by a table or:
D' Long. = Departure Sec. Lat. Therefore D' Long. = 5.35 Sec. 500 05' = 5.35 X 1.55
= 8.3' West.
Starting Position 500 10.0' N 40 o6.o' W
Difference o° 05.4' S o° 08.3' W
Final Position 50° 04.6' N 4° 14.3' W
In practice, there would be no point in working to two places of decimals of a minute; that was done for illustration. The final position can then be plotted on the chart quite normally using the Latitude and Longitude scales. To simplify matters, a table for converting Departure into D' Long. is usually placed next to the Traverse Table in the book.
Having worked the courses and distances run in one of these ways to give the D.R. position, allowance must then be made for leeway, tidal stream and current to arrive at the Estimated Position. As for leeway, it is probably easier to allow for it by judging the difference it will have made on each of the courses steered and then working the adjusted courses in the Traverse Table instead of the straight courses steered. The effect of tidal stream or current over the whole period can be estimated and applied as one correction on the chart.
Course 004° for 30 miles Course 148° for 30 miles Course 182° for 30 miles Course 312° for 30 miles
I44 SMALL BOAT NAVIGATION
Making a Landfall
The acid test! But there is nothing more satisfying than to arrive at the right place at the right time in spite of all the quirks of wind, tide and weather.
The careful work required in keeping an accurate E.P. in the open sea, aided by soundings and radio aids if you have any to use, will now begin to pay off. On the other hand, a bit of careless work out in the middle may result in a good deal of nail-biting and the aspect of a selection of objects which bear no resemblance to the chart around your destination.
To take the worst case first; suppose that you have been at sea for a couple of days or more without a proper fix—it has been blowing hard and the visibility is not good. Your destination is sited on a low-lying, pretty featureless coast with no conspicuous off-shore marks; there are rocks well to seaward and the current is strong. Under these conditions, the wisest course of action would be to wait or anchor until the visibility improved. However, there may be urgent reasons for an early arrival in harbour. As the D.R. position may be well out, it is
ilk
too optimistic to expect a neat arrival straight off the harbour entrance. The first objective must be to get a positive identification of some land or sea mark as early as possible. Consequently, although the coastline may appear to be absolutely featureless from the chart, an exhaustive study of the Sailing Directions (Admiralty Pilot) may have revealed some clues which would help, i.e. the colour and appearance of the coastline along particular stretches, any small hills or dunes that are slightly more easily seen than than the rest. Having then amassed as much local information as possible, aim to close the coast in an area where the offshore dangers are least and where there is a good chance of identifying some feature on the shore from a good range. An alternative course of action is to deliberately aim to miss your destination on one side or other by an amount at least equal to your probable reckoning errors. Then, on finally making the coast, there can be no doubt in which direction to turn parallel with the coastline in order to reach harbour.
At other times, you may be making a landfall on a coastline which has plenty of artificial or natural features, but with tricky approach channels through shallows before the harbour can be reached. A bold headland is easy to identify, but a
PRACTICAL NAVIGATION IN A BOAT 145
coastline backed by broken country may prove more difficult to sort out. If there is a major lighthouse in the area, it may be preferable to approach the coast in the dark to gain a positive identification of the light by its characteristics. It is often more difficult to pick out a lighthouse by day, even if the visibility is good, if it does not stand out well against the background. Conversely, off some major rivers and ports, the host of navigational marks and shore lights may tend to confuse by their sheer weight of numbers. At night, it may not be easy to pick up the smaller navigational lights against a background of street lighting, traffic moving on shore, etc. Such considerations as these should be taken into account when approaching a strange harbour from seaward. Glean as much information as you can beforehand from the chart and Sailing Directions and then use common sense in planning the approach taking into account the possible inaccuracies in your Estimated Position. A lot of landfalls are quite straightforward but there may be the odd one which may cause some anxiety unless the appropriate precautions have been taken.
Approaching the Coast in bad visibility
As has already been said, the golden rule if visibility is bad and you are uncertain of your position is to anchor as soon as the water becomes sufficiently shallow—preferably clear of shipping lanes. It is most dangerous to press on in the blind faith that the mark which should have appeared some time back will sooner or later appear. Another `must', if it is possible, is to get a good fix before the visibility completely closes down; this applies when making a passage along the coast as well.
Anchoring in a chosen spot
There are many factors to be taken into account when choosing an anchor berth. The principal ones are:
(i) What is the direction of the prevailing wind? (Avoid anchoring if you can possibly do so on a Lee shore—if the anchor drags you will end up on the rocks, or at best have a most uncomfortable time.)
(ii) Is there sufficient room to swing safely round the anchor without danger to yourself or other craft?
146 SMALL BOAT NAVIGATION PRACTICAL NAVIGATION IN A BOAT 147
(iii) Is it reasonably easy to get out of your selected berth if the wind or current changes direction or strengthens?
(iv) Is the holding ground good?
(v) Is the water too deep for comfort?
There are a lot of supplementary considerations (or perhaps not so supplementary?) such as nearness to landing places, bright lights, etc., but the first requirement for an easy mind is to ensure the boat's safety. A swinging time ashore is slightly spoilt if on your return you find that the boat has had ideas of its own.
No anchor, however good its design, will hold properly if an insufficient length of cable or anchor warp is veered. The holding power of an anchor depends to a large extent on the angle the chain makes with the stock of the anchor whilst it is lying on the bottom. If the length of chain near the stock is lying flat, as the strain comes on, the initial pull on the anchor is applied to it almost horizontally. The anchor then provides the maximum resistance to being dragged along the bottom, and the additional cable acts as a spring (due to its weight) preventing any sudden jerk on the anchor. On the other hand, if only just enough cable is let out to allow the anchor to reach the bottom, the pull coming onto it is applied almost vertically and as a sudden jerk; thus the anchor is easily pulled out of its ground and has little holding power.
The amount of cable that should be veered depends on a number of factors such as the depth of water, strength of wind, duration of stay in that berth, etc. As a good general rule, if you intend to remain at anchor for an appreciable length of time, it is advisable to let out a length of cable equal to Eight Times the Depth of Water, i.e. in a depth of 5 fathoms veer 40 fathoms of anchor chain. This is a generously safe allowance under normal conditions. Some people might say that this is far too much, but if there is otherwise plenty of swinging room, it is much better to err on the safe side. Should a gale blow up and you intend to remain at anchor, it would be advisable to let out all the cable you have got—the more out, the better the chance of the anchor holding.
The quality of the various holding grounds has been discussed in a previous chapter. Sand and mud are the best, rock andweed the worst. Apart from the danger of an anchor slipping and dragging quickly across a rocky bottom, there is also the risk of it jamming in a crevice when you least want it to do so. If you have to anchor on a rocky bottom, it is a wise precaution to buoy the anchor before letting it go—this means attaching a rope (of greater length than the depth of water!) to its crown. Before the anchor is let go, the anchor buoy is thrown clear over the side. If the anchor subsequently jams on being weighed, the buoy can be recovered and a pull put on the anchor from a different direction to try and pull it out. If this in turn fails, the next recourse is to try and sail it out, i.e. heave in until the cable is taut, secure it and then gather headway until the cable draws aft. A strain from a different direction is thus applied, and this can be tried successively on different headings. If this also fails, and you are in a hurry to get clear, the final solution is to slip the cable, buoying it from the inboard end before you do so. This end of the cable can then be recovered on return to the berth.
The amount of swinging room to be allowed when coming to a single anchor depends on the length of cable out, the depth of water, and the length of the boat. The proximity of other craft and dangers must be taken into account. For example, a 27 foot boat anchoring in 4 fathoms might veer 32 fathoms of cable. The theoretical radius to be allowed for the swinging circle is therefore:
27 feet +6 (32 —4) feet = 27 +6 X 28 = I95 feet or 65 yds. However a safety margin should be allowed (over and above this) of at least two boat's length from the nearest underwater hazard. When anchoring near other boats, it may not be possible to allow this comfortable combined margin, but it is as well to remember that although normally they will all swing to the current or wind at about the same time, sometimes things do not work out so neatly. It is not unknown for two boats anchored close to each other to swing to a shift of wind in the opposite directions. Complications can result!
Tidal stream or current has a greater effect than wind on the direction at which a boat will lie at anchor, particularly when the boat is deep in the water. A I knot of current may have a greater effect than a 15-20 knot wind if they are in opposition.
148 SMALL BOAT NAVIGATION
The approach to the Anchor Berth
There is sometimes a bit more to the actual business of anchoring than just dropping the anchor over the bow, securing the cable and then reaching for the brandy and ginger ale. Such methods may suffice in the summer in the Mediterranean, but are not liable to lead to security of tenure under English weather conditions. The aim on anchoring must be to lay the cable out across the bottom in a straight line, then any strain applied to the anchor and cable at a later time is taken up steadily. If, on the other hand, the cable is piled up in an untidy heap, coil upon coil, any subsequent boat movement will rapidly unwind the coil until a sudden jerk is imparted to the anchor which may break it out. The same disadvantage lies, to a lesser extent, in laying the cable out in a bight (loop) across the bottom.
There are basically two ways of laying the cable out in a straight line on anchoring—by means of a dropping or running moor. The former is principally used when it does not matter a great deal whether the boat is precisely in its correct berth; the latter when it is important to anchor bang in the right spot. Whichever method is used, it is always preferable to anchor heading into the wind or tidal stream. In a dropping moor, the anchor is let go as soon as the chosen berth is reached; the boat is then immediately allowed to drop back downwind or current laying out the cable as it goes. The cable will then be laid out straight and the boat drops gently back, its speed being controlled by the rate of veering the anchor cable. No jerk need be imparted to the cable at any stage.
With a running moor, the boat must have a certain amount of headway as the berth is reached so that control of the boat's heading can be maintained right up to the last moment. This allows a more precise positioning of the anchor on the bottom. For example the intention is to anchor precisely in the position shown in Figure 26.
The wind is from the NNE and there is no tidal stream. The approach course is from the SW on a heading of 030° True. First, a look at the chart may show a conveniently placed object which can be used as a headmark during the final approach; here the church is well placed to give a good lead into the anchorage and one which will clear the rocks on both sides of the entrance. (Very rarely does it happen that absolutely no
PRACTICAL NAVIGATION IN]A BOAT 149
No mast
'(A) __ —+ Church
2c ~I~I i Jct r LCI
3c oeo"T ~c~
oso~r r , , , °°° ;; ~e i r
! 7~ ighthouse. °°°°°~~\ ; i ~/
Bn
Figure 26
(The anchoring position is marked by the symbol under (A))
suitable mark is available—in this case, if the church had not been there, the point just to the South of it on the shore would have served just as well on a slightly different bearing; though it might have been more difficult to identify.)
With a parallel ruler set between the church and anchor berth, lay off the bearing on the chart and measure it from the compass rose (o8o° T). If the church is kept bearing o8o° True during the run in, the boat must be on the right line of approach to the anchoring position.
However, the boat has to get onto this line from its approach heading of 030° and this leads to a word about `turning bearings'. When using a bearing to decide the moment to turn onto a new course, always use the bearing of an object as near as possible parallel to the new course. Doing this much reduces the chance of ending up off the intended track due to compass error or some other reason. In the above diagram, it would be much better to use a bearing of the church for the final turn rather than a bearing of the lighthouse.
Next, distance to go, or distance along the track is required. This is obtained by selecting charted objects on the beam during the final approach and measuring the bearings these should subtend at given distances from the berth. In the sketch, the 3 cable, 2 cable, I cable, and I cable (ioo yards) distances to go have been marked, with finally the L.G. ('Let go') position shown. To be clinically accurate, the distance from the bow to
150 SMALL BOAT NAVIGATION PRACTICAL NAVIGATION IN A BOAT 151
the compass should be laid back from the berth in marking the L.G. position (to allow for the length of the boat between anchor and compass) but this is peanuts in short boats—in the Queen Elizabeth II it would make quite a difference.
Using the parallel ruler on the chart, the individual beam bearings from each object at each distance to go are then measured and tabulated. Before writing the bearings into the table, they should be corrected for Variation and Deviation (for the final approach course) :
Total correction I o°W.
Run o800 True—ogo° Compass on Puddlecome Church
Distance Lighthouse Ro. Mast Beacon
3 C. 195° Compass — 125° C.
2 c. — 070° C. 140° C.
i C. — 055° C. 165° C.
I C. — 038° C. 182° C.
L.G. — 007° C. 196° C.
This preparatory work does not take long and with the facts written into a notebook, a perfectly safe and accurate anchoring can be achieved without any need for fixing.
If the church is kept bearing ogo° by compass by adjustment of the course to allow for leeway, the boat can be brought in at the right speed for anchoring by checking the beam bearings. It is wise to have more than one set of beam bearings up your sleeve, as one can then be checked against another; also it may be found that one mark is obscured, at an important time, by other craft or be difficult to identify against the background.
With a running moor, if there is no wind or current, headway can be maintained until the right amount of cable is out. If there is a strong headwind or current, however, it is better to take the way off the boat as soon as the anchor is down and then drop slowly back on the chain. Whichever way the approach is made, if it is impossible to anchor into wind or tide, the anchor should be dropped on the weather side (i.e. up wind or current). When the way comes off the boat, the anchor chain will then lead out clear as the boat drops to leeward. If the anchor is let go on the lee side, the chain may foul the keel or scrape paint from the bow.
Entering Harbour
Shiphandling is a subject in its own right, and it is not the intention to discourse on it here. However, one or two small navigational hints may be of some help. On entering a harbour, always keep an eye on other craft at anchor and buoys to see in which direction the current is setting. Then identify the berth for which you are making as early as possible and approach it, if you can, against the current. The same applies when aiming to pick up a mooring. In sailing boats, the direction of your final approach to a berth is inevitably determined by the wind direction. In power boats, if the wind is blowing off a jetty, the approach should be made at a wider angle than normal and the boat turned to parallel up with the jetty at the last moment. If the wind is blowing onto ajetty, a shallower approach aiming a short distance off the jetty should be made.
In any type of boat bigger than dinghy size, always have an anchor ready for letting go when entering (or leaving) harbour. You never know when this may come in useful and it is always a good precaution even when you intend to berth alongside or at buoys. In a strange place, you may not know what the berthing arrangements are in that port on first entering, and therefore keep a good look out for groups of other yachts, customs posts and harbour offices where you may be able to obtain quick enlightenment (or a rude answer).
Finally to emphasise once again the importance of looking up all the information you can on the tides, currents, depths, conspicuous objects before entering a strange place. A frantic thumbing-through the Pilot with one hand, whilst steering with the other, and holding the sheets with your big toe may not breed confidence amongst your fellow men and girls.
ii iI ifl[II' Iii
Any boat owner must have a working knowledge of the International Regulations for the Prevention of Collision at Sea. Otherwise, he may become something of a menace to others on the sea.
The Rule of the Road at Sea is published, in this country, under an Order in Council entitled `The Collision Regulations (Ships and Seaplanes on the Water) and Signals of Distress (Ships) Order 1965'. This is obtainable from H.M. Stationery Office for the princely sum of 71 new pence.
By international agreement, the rules were slightly changed in 1960, principally to take into account the fitting of radar into a large number of ships since the war. There are altogether some 3I Rules covering—the lights and shapes to be carried by day and night—the sound signals to be made in poor visibility—and the Steering and Sailing rules. Some of them have been re-grouped for convenience and paraphrased here, without altering their sense; and some of the more important have been quoted verbatim—where this is the case, they are shown within inverted commas. Some commentary is also given on specific rules. These alterations and additions are not `official' but merely aimed to help interpretation; needless to say, I disclaim any responsibility for accidents that might be deemed to have arisen as a result of reading these pages!
The Rules should not be confused with the `Racing Rules' which are only in force when boats are actually competing, or about to compete, in a race. The Racing Rules are themselves based on the Rule of the Road but have major additions and modifications to the basic rules.
Rules r and 3o—Preliminary Statements and Definitions
(i) The International Regulations do not interfere with LocalTHE RULE OF THE ROAD AT SEA 153
Authority rules made for particular harbours, rivers, and inland waterways. (Rule 30.)
(ii) The rules concerning lights must be complied with from sunset to sunrise in all weather conditions. Additionally, these lights may be shown in daylight hours in restricted visibility, or in any other circumstances when it is considered prudent to do so. No other lights should be exhibited which can be confused with the prescribed lights. (Rule x.)
(iii) A power-driven vessel is a vessel that is using an engine at the time, whether or not this is her principal means of motive power, whether or not she is also sailing, and whether or not it is an inboard or outboard motor. (Rule I.)
(iv) If a boat is using an engine with sails also hoisted, it should, by day, carry a black cone, point downwards, in the fore part of the boat where it can best be seen. The cone should be at least two feet in diameter at its base! (Rule I4.) (This, you may imagine, is not always complied with, but in the event of an accident...)
(v) A vessel is considered to be underway as soon as it is no longer attached by any means to the shore or bottom; i.e. it is underway as soon as the last line is slipped from a jetty or a buoy, and as soon as the anchor is clear of the bottom when weighing. A boat aground is obviously not underway! (Rule I.)
(vi) Vessels are considered to be in sight of one another only when one can be observed visually from the other. (This is put in principally to disqualify radar contact as a `sighting'.) (Rule I.)
(vii) Regarding lights, `Visible' means visible on a dark night in a clear atmosphere. (Rule i.)
(viii) The term `engaged in fishing' means fishing with nets, lines or trawls but does not include fishing with trolling lines (streamed when underway over the stern or from outriggers). (Rule i.)
Rules 2 to I4—Lights and Shapes
POWER DRIVEN VESSELS UNDERWAY
I. A power driven vessel underway and over 150 feet in length must carry:
(a) Two White Masthead Lights. These must be sited on different mastheads or parts of the ship's superstructure and
154 SMALL BOAT NAVIGATION
the forward one must be at least 15 feet lower than the after. Both must be visible from 5 miles over an arc of visibility stretching from two points (221°) abaft the beam on one side, through the bow, to two points abaft the other beam, i.e. from the relative bearings of Red I I2 to Green 1124°. (Rule 2.)
(b) Bow Lights. On the port bow, a RED light showing from right ahead (but not across the bow) to two points abaft the port beam. On the starboard bow, a Green light showing from right ahead to two points abaft the starboard beam. The visibility distance of both lights must be 2 miles. (Rule 2.)
(c) Overtaking Light. This is a white light, carried on the stern, visible over 12 points (135°) through the stern, i.e. from 67 on one quarter—through the stern—to 671° on the other quarter. Visibility distance 2 miles. (Rule I o.)
Summary
A ship underway over 150 feet in length seen from dead ahead will show both masthead lights, one above the other, and both bow lights. As you cross its bow from port to starboard (at a goodly range, I trust), the red light will go out, the green light remain visible, and the two white lights separate. At long range, the bow lights may not indeed be visible and the masthead lights, by their aspect, will give the first indication of the ship's course.
If your relative position is more than 224° abaft the beam of the other ship, its white overtaking light only can be seen.
2. A Power Driven Vessel less than z5ofeet in length must carry:
One White Masthead Light, Bow Lights, and an overtaking light. It can carry a second white masthead light (as above) but is not obliged to do so. All these lights have the same characteristics as those defined in paragraph (I) a, b,
and c, for ships over 150 feet long. (Rule 2.) f'.
3. A Power Driven Vessel less than 65 feet long must carry:
(a) A White Masthead Light not less than 9 feet above the gunwale and visible 3 miles. (Rule 7.)
(b) Bow Lights showing RED and GREEN over the same arcs as those of a ship but need only be visible at I mile. (Rule 7.) or (c) The bow lights instead of being individually sited one
THE RULE Off' THE ROAD AT SEA 155
on each side may be combined into the same lampholder (Combined Lantern) but they must show over the same arcs as before (Red—Port: Green—Starboard) and the combined lantern should be not less than 3 feet beneath the white mast-headlight. (Rule 7.)
(d) Overtaking Light. In small vessels, it may not be possible on account of bad weather or other sufficient cause for the overtaking light to be a permanent fixture. However, if a permanent overtaking light is not carried, a white portable light or electric torch should be kept at hand ready for use and be shown in sufficient time to prevent collision. (Rule Io.)
4. Power Driven Vessels less than 4ofeet long may carry:
(a) The White Masthead Light at less than 9 feet above the gunwale but not less than 3 feet above the sidelights or combined lantern. (Rule 7.)
(b) Separate Sidelights or a Combined Lantern. (Rule 7).
(c) A fixed or temporary overtaking light under the same conditions as for a boat of 65 foot length. (Rule I o.)
SAILING VESSELS UNDERWAY
These follow the same lights, depending on their size, as the equivalent length large power driven vessels with the important exception that they never carry the white masthead lights of the power vessel. They may carry instead on their foremast a RED light over a GREEN light both visible from Red 1121° through the bow to Green 112-°.
These foremast lights should be visible at 2 miles. (Rule 5.)
A large sailing vessel underway may therefore be showing a Red over a green at the foremast, its bow lights, and an overtaking light. The latter lights are obligatory, in accordance with the rules for craft of their size, even though the foremast lights are not.
Sailing Vessels and Boats under oars less than 4o feet long must carry:
(a) Bow lights. If they are not fitted with the normal side‑
lights, they must carry, where it can best be seen, a combined
lantern Red to port and Green to starboard, visible I mile. If
it is not possible to fix this light permanently, it should be
kept ready for immediate use and exhibited in sufficient time
156 SMALL BOAT NAVIGATION THE RULE OF THE ROAD AT SEA 157
to prevent collision. (You will get no marks under these rules if Red is allowed to show to starboard or Green to port.) (Rule 6.)
(b) Overtaking Light. The normal overtaking light. Again, if it is not possible for this light to be fixed, a torch or lantern showing a white light should be kept at hand ready for immediate use. (Rule I o.)
Small Rowing Boats under Oars or Sail:
The full concessions are allowed here. They need only have an electric torch or lantern showing a white light available which must be exhibited in time to prevent collision. (Rule 7.)
Notes on Sailing vessels and Rowing Boats
As can be seen, there is no stipulated maximum length, under the 40 foot limit, above which the use of the single white light is forbidden. This is left to the discretion of the owner, noting that it is in his own interest to carry the proper lights if it is practicable so to do.
VESSELS TOWING OR BEING TOWED
(a) The Towing Vessel. A Power-driven vessel if towing (or pushing) other craft, and if the length of tow exceeds 600 feet, must carry:
Three White Masthead Lights in a vertical line on the same
mast and visible from two points abaft the beam, through the bow, to two points abaft the other beam. These, therefore, show over the same arc as the normal masthead lights but are carried instead of them. (Rule 3.) (By day, the towing vessel shows a black diamond shape.)
Bow Lights. As normal.
Overtaking Light as normal, or a small white light abaft the funnel or superstructure for the tow to steer by, but this light must not show forward of the beam. (Rule 3.)
(b) Power Vessel Towing—with a length of tow of less than boo feet must carry:
Two White Masthead Lights again in a vertical line and
having the same characteristics as the 3 Masthead Lights mentioned above. (Rule 3.)
Bow Lights.
Overtaking Light or a small light for the tow to steer by.
(c) Power Driven Vessels of less than 65 feet in length Towing. When towing or pushing other vessels, they must carry:
The Two White Masthead Lights not less than 4 feet apart,
and one of them must be in the same position as the ordinary white masthead light, i.e. an extra masthead light is needed. (Rule 7.)
Bow Lights or a Combined Lantern.
Overtaking Light or a small white light for the tow to steer by. (Rules 7 and Io.)
The Vessels being Towed
A vessel being towed must carry bow lights (or a Combined Lantern) and if it is the last vessel in the tow, an Overtaking Light. Craft in the middle of the tow must carry Bow Lights, but need not carry a proper overtaking light, but may show a small white light aft. (Rules 5 and Io.)
Vessels being Pushed by a Tug or another Boat
(a) A single vessel being pushed carries the normal sidelights or a Combined Lantern.
(b) A group of boats being pushed must be lit as one vessel, i.e. the port bow light on the port outer craft and the starboard bow light on the starboard outer craft. (Rule 5.)
LIGHTS AND SHAPES FOR SPECIAL CIRCUMSTANCES
A Vessel Not Under Command
A vessel is said to be `Not under Command' when she has had some sort of failure such as a steering or engine breakdown which precludes her from manoeuvring; she cannot therefore keep out of the way of other ships or craft. At night she shows, where they can best be seen, TWO RED LIGHTS vertically one above the other, visible all round the horizon at a distance of at least 2 miles. A power vessel will switch off her masthead lights, but the sidelights should be kept on in both power and sailing craft, under these circumstances, if the vessel is making way through the water. By day, a vessel not under command hoists
L
158 SMALL BOAT NAVIGATION
two black balls in lieu of the red lights (colloquially known as N.U.C. Balls). (Rule 4.)
Vessels Carrying out Special Operations
Vessels engaged in special tasks such as laying Telegraph Cables or navigational marks, surveying, carrying out underwater operations, warships replenishing at sea or operating aircraft, cannot easily get out of the way of other vessels approaching them. Instead of the normal masthead lights, at night they carry 3 lights in a vertical line—RED WHITE RED—visible all round the horizon for at least 2 miles.
By day, these lights are replaced by shapes in a vertical
line—RED BALL WHITE DIAMOND RED BALL. (Rule 4.)
Minesweepers
When engaged in minesweeping operations or exercises, a minesweeper carries at night a GREEN all round light at her fore truck, and a GREEN light at the yard on the side on which danger exists. If the minesweeper has sweeps out on both sides, therefore, a TRIANGLE OF GREEN LIGHTS will be visible; these lights show all round the horizon; and are in addition to her ordinary lights.
By day, black balls are shown in lieu of the lights. (Rule 4.)
The showing of these lights or shapes by sweepers indicates that it is dangerous for other craft to approach closer than 3000 feet astern or 1500 feet on the beam on the side (or sides) from which the sweeps are streamed. (Rule 4.)
Note: When any of these vessels (Not under Command, conducting special operations or minesweeping) are actually making way through the water they continue to show their bow lights and overtaking light.
Pilot Vessels
Steam Pilot Vessel. When on her station and underway at night will show her bow lights, overtaking light, and a special signal of an all round WHITE LIGHT over an all round RED LIGHT at the masthead. She should also show an intermittent all round white flare or light every ten minutes or so. When not under way, she switches off her bow and overtaking lights. If she is not engaged on pilotage duty at the time, she merely shows the ordinary lights for a similar vessel of her length. (Rule 8.)
Ii
THE RULE OF THE ROAD AT SEA 159
Sailing Pilot Vessel. When on her station on pilotage duty under way, she carries her bow and overtaking lights plus an all round WHITE LIGHT at the masthead, and burns one or more white flares or lights at intervals not exceeding Io minutes. If not engaged on pilotage duty, she shows the lights appropriate to a vessel of her size only. (Rule 8.)
Fishing Vessels. Rule 9
When they are not actually engaged in fishing, they should show the normal lights appropriate to a vessel of their length. When they are fishing, they carry the following lights:
Trawlers. These craft drag a dredge net or other gear along or near the sea bed; when trawling they carry:
GREEN LIGHT over a WHITE LIGHT visible all round the horizon.
Masthead Light. The ordinary masthead light may also be carried, but it must be lower and abaft the green and white lights.
Bow Lights and Overtaking Light. These are shown, as normally, if the trawler is making way through the water; if she is stopped, they are switched off.
Fishing Vessels Other than Trawlers. They carry:
RED LIGHT over a WHITE LIGHT visible all round the horizon.
Bow Lights and Overtaking Light if making way through the water.
Fishing Vessels with gear extending more than 500 feet horizontally into the Seaway carry:
RED LIGHT over a WHITE LIGHT.
An additional all round WHITE LIGHT in the direction in which the gear is extended. This extra light should show above the sidelights but not at a greater height than the other all round white.
Bow and Overtaking Lights if they are making way through the water.
A drift net vessel may well be showing lights of this nature.
Note. Fishing vessels may use a flare or searchlight to attract the attention of approaching vessels endangered by or en‑
16o SMALL BOAT NAVIGATION
dangering their gear. They also sometimes use clusters of bright working lights.
By day fishing vessels over 65 feet long when engaged in fishing carry two black cones point to point where they can best be seen. If they are less than 65 feet long, they may carry a basket instead. If their outlying gear extends more than 500 feet horizontally, they should carry an additional cone—point up— in the direction of the outlying gear.
Vessels at Anchor. Rule I I
A Vessel more than zso feet long when at anchor must show, at night, an all round white light forward and an all round white light aft; the latter being lower than the forward light. Both should be visible at 3 miles.
A Vessel less than i jo feet long need not carry the after light, and the required visibility range of the forward anchor light is 2 miles.
By day, the anchor lights of vessels of any size are replaced by a black ball carried in the forepart of the craft.
A Vessel Aground. Rule I I
Must show at night:
Anchor Lights appropriate to her size.
TWO RED LIGHTS one above the other, visible all round the horizon.
By day, she should show three black balls in a vertical line.
Rules 15 and i6—Sound Signals and Conduct in Restricted Visibility
A power-driven vessel 40 or more feet in length must have a whistle or siren, a mechanical foghorn and a bell.
A Sailing vessel 40 or more feet in length must have a mechanical foghorn and a bell.
A vessel of less than 40 feet in length, a rowing boat (or a seaplane!) shall not be obliged to make any of the following signals in poor visibility, but must make some other efficient sound signals at intervals of less than one minute.
This is the gist of the slightly ambiguous statement in Rule 15 (ix) but it means that, in bad visibility, as in good, the onus for avoiding collision is on all seafarers and not only on those in large ships. In any sort of cruising boat, 40 feet long or not, there should be some form of foghorn or claxon. The investment
it
THE RULE OF THE ROAD AT SEA 161
of a few shillings in one of these will seem justified if you find yourself in the middle of a busy shipping lane in dense fog.
Means
The signals described below for vessels underway should be given by:
(i) power driven vessels on a whistle or siren.
(ii) sailing vessels on the foghorn.
(iii) vessels being towed on a siren, whistle or foghorn.
Duration of Blasts. Rule I
A `short blast' means a blast of about I second's duration. A `prolonged blast' (or `long' blast) means a blast of between 4-6 seconds' duration.
Sound signals in Bad Visibility
In fog, mist, falling snow, heavy rainstorms or any other condition similarly restricting visibility, whether by day or night, the following sound signals must be made:
(The signals are listed here in tabular form for ease of reference, although in the Rules they are written out in full.)
Made by Made on Interval Signal
Vessel more than Bell forward 350 feet in length Gong aft
AT ANCHOR
Vessel less than Bell 350 feet long
AT ANCHOR
Every Rapid ringing of
minute bell for 5 seconds, followed by gong aft
Every Rapid ringing of
minute bell for 5 seconds
Siren, whistle As requisite or foghorn
Bell forward Every (Gong aft) minute
One short—one prolonged—one short blast ('R' in Morse)
3 distinct strokes on the bell, followed by 5 seconds' rapid ringing, followed by 3 strokes. (Gong as before for large ship)
Any vessel AT ANCHOR sensing danger from an approaching vessel
A vessel AGROUND
162 SMALL BOAT NAVIGATION
Made by Made on Interval Signal
Conduct of Vessels in Fog or Poor Visibility. Rule 16
A great number of the collisions at sea occur in fog or in conditions of poor visibility, and Rule 16 is one of the most important of the `Rules of the Road'. It is therefore quoted here in full:
(a) `Every vessel, or seaplane when taxi-ing on the water, shall, in fog, mist, falling snow, heavy rainstorms or any other
THE RULE OF THE ROAD AT SEA 163
condition similarly restricting visibility, go at a moderate speed, having careful regard to the existing circumstances and conditions.
(b) A power-driven vessel hearing, apparently forward of her beam, the fog signal of a vessel the position of which is not ascertained, shall, so far as the circumstances of the case admit, stop her engines, and then navigate with caution until the danger of collision is over.
(c) A power driven vessel which detects the presence of another vessel forward of her beam before hearing her fog signal or sighting her visually may take early and substantial action to avoid a close quarter situation but, if this cannot be avoided, she shall, so far as the circumstances of the case admit, stop her engines in proper time to avoid collision and then navigate with caution until danger of collision is over.'
Many court cases have been won and lost on the careful (or otherwise) observance of this rule. Paragraph (c) was inserted in the 1960 version of the Regulations to take into account the detection of other vessels by radar, but it does not absolve the radar fitted vessel from stopping engines if a close quarter situation develops.
From the yachtsman's point of view, it must be borne in mind that the very conditions which produce bad visibility, heavy rain for instance, can also degrade radar performance due to the attenuation of radar signals by the water droplets in the atmosphere. The degree of attenuation suffered depends in part on the frequency of the individual radar set. At worst, small craft may escape detection at all by a ship's radar, particularly if the sea is rough and the small echo from a yacht or boat becomes lost amongst the signals reflected by the waves (this is usually called `Sea Clutter'). At best, a boat which only presents a small reflecting area to the radar pulse cannot be detected at long range. It is, therefore, foolish to assume that a small boat or yacht is bound to be detected by a ship's radar and to take any unnecessary risks based on this assumption; for example trying to get across the bows of a ship in fog.
Sound Signals made by Vessels in Sight of one another. Rule 28
Certain other sound signals are restricted for use when vessels are in sight of one another by day or by night. It is important that,
Power vessel
Siren or
Every two
One prolonged
MAKING WAY
whistle
minutes
blast
through the water
Power vessel
Siren or
Every two
Two prolonged
under way but
whistle
minutes
blasts with an
STOPPED
interval of one
second between
Sailing vessel on
Foghorn
Every
One blast
the STARBOARD
minute
TACK
Sailing vessel on
Foghorn
Every
Two blasts
the PORT TACK
minute
Sailing vessel
Foghorn
Every
Three blasts
with the wind
minute
164 SMALL BOAT NAVIGATION
when taking avoiding action in close quarter situations, one
boat's intentions should be made clear to the other. This can
and must be done by the use of sound signals which are tabu-
Y g
lated below:
Made by Made on Signal
A power driven vessel about to ALTER Siren or One short
P
COURSE TO STARBOARD whistle blast
A power driven vessel about to ALTER Siren or Two short
COURSE TO PORT whistle blasts
A power driven vessel about to PUT Siren or Three short
ITS ENGINE(S) ASTERN whistle blasts
A power driven vessel having the Siren or At least
right of way over another vessel, but whistle 5 short and
in doubt whether the other vessel is rapid blasts
taking sufficient action to avert a
collision. (This signal is optional)
Note. Strictly speaking the last signal only applies to one power
vessel having the right of way over another. However, there are
other Rules (zo and 25) which state that a power driven vessel,
of less than 65 feet in length, or a sailing vessel must not hamper,
in a narrow channel, the safe passage of a ship which can only
navigate inside the channel. A liner or heavy tanker entering or
leaving harbour may well be unable to alter course to avoid
small craft in the channel and may use this signal to indicate
`Get
that she is concerned, i.e. out of my way—I cannot get out
of Yours'.
SOUND SIGNALS—INTERNATIONAL CODE
These are not taken from the Regulations for the prevention of
Collisions at sea, but from the International Code of Signals.
They are included here for easy reference.
Signal Morse Letter Meaning
S.O.S. ...---... Distress Signal.
F • . -. I am disabled. Communicate k
with me.
K –•– You should stop your ship
instantly.
L –•• You should stop. I have something
important to communicate. THE RULE OF THE ROAD AT SEA 165
0 – – – Man overboard.
P - -. Your lights are out or burning
badly.
R – • The way is off my ship. You
may feel your way past me.
– (Could be used in fog.)
U You are standing into danger.
V • • • - I require assistance.
W • - – I require medical assistance.
These signals may be made by flags, morse, semaphore, or on
a siren, whistle or foghorn in any weather.
THE STEERING AND SAILING RULES (I-3o)
Green to Green and Red to Red
Perfect safety, go Ahead,
If to starboard Red appear
'Tis your duty to keep clear.
A well known doggerel verse which summarises some of the
main rules regarding power vessels. This section of the Regula‑
tions is, of course, the vital section. Those rules already covered
about Lights, Shapes, and Sound Signals are designed' to assist
r comply with main Steering and Sailing
the mariner to com w t the
~ Y g an
Rules which follow here.
They cover the actions to be taken by individual vessels
approaching one another where a risk of collision may exist.
They tell the seaman which vessel must give way under various
circumstances and when the other vessel may be expected to
take the appropriate avoiding action.
Some of the basic principles are:
(i) A vessel must establish first that, if she holds her present
course and speed, a risk of collision with another vessel exists.
(ii) Power-Driven (Steam) vessels generally give way to Sail
or boats under oars.
(iii) When two Sailing Vessels approach each other, the
vessel which, owing to the wind direction, would lose more
ground by an alteration of course has the right of way.
(iv) In all circumstances where a risk of collision exists, the
vessel that has the right of way (sometimes called the `privileged'
vessel) should hold her course and speed whilst the other vessel
ABAFT the beam
A vessel TOWING,
Siren, whistle
Every
One prolonged
picking up
or foghorn
minute
blast followed by
CABLES, or NOT
two short blasts
UNDER COMMAND
('D' in Morse)
A vessel being
Siren, whistle
Every
One prolonged
TOWED, or if
or foghorn
minute
blast followed by
more than one
three short blasts
vessel is being
('B' in Morse)
towed, the last
vessel in the tow
A vessel ENGAGED
Siren, whistle
Every
One prolonged
IN FISHING, when
or foghorn
minute
blast followed by
under way or at
two short blasts
anchor. (Does not
('D' in Morse)
apply to vessels
trolling, which give
normal signals)
A power driven
Siren or
As requisite
The normal fog
PILOT VESSEL
whistle
signal followed
when engaged in
by four short blasts
PILOTAGE
(`H' in Morse)
166 SMALL BOAT NAVIGATION I THE RULE OF THE ROAD AT SEA 167
(sometimes called the `Giving Way' or `Burdened' vessel) must alter course and/or speed. However, if at the last moment collision appears to be inevitable, both vessels must do their best, by altering course or speed, to avoid it if at all possible or at least lessen the impact.
(v) When obeying these rules, the action taken by the burdened vessel must be positive and taken in ample time.
(vi) An Overtaking vessel, whether power or sail driven, must always avoid the vessel being overtaken. This overrides all the other rules.
(vii) All vessels not engaged in fishing (except those `Not under Command', engaged in special operations, etc.) must keep out of the way of vessels actually engaged in fishing. This does not, however, give fishing vessels the right to obstruct a fairway.
Sailing Vessels. Rule I
~ 17
This Rule is quoted here in toto:
`(a) When two sailing vessels are approaching one another, so as to involve risk of collision, one of them shall keep out of the way of the other as follows:
(i) When each has the wind on a different side, the vessel which has the wind on the Port side shall keep out of the way of the other.
(ii) When both have the wind on the same side, the vessel which is to Windward shall keep out of the way of the vessel which is to Leeward.
(b) For the purposes of this Rule, the windward side shall be deemed to be the side opposite to that on which the mainsail is carried or, in the case of a square-rigged vessel, the side opposite to that on which the largest fore-and-aft sail is carried.'
Interpretation
This rule was reworded in the 1960 Revision, and is now stated in much simpler form than it used to be. It is quite straightforward—Port Tack gives way to Starboard Tack when the boats have the wind on the opposite sides. The Windward boat gives way to the Leeward boat when they are both on the same
tack.
Some examples of these rules, applied to particular situations,°~ w
i
N D
Figure 27
are given in Figures 27A. In all the diagrams, the `burdened' or `giving way' vessel is shown shaded.
The Overtaking Rule (Rule 24) is over-riding; in Example (d), the Leeward boat is overtaking the other from a relative position more than two points abaft the Windward boat's beam, and in this situation the Leeward boat has to give way. (The obligations of vessels overtaking others are further expanded in the section under Rule 24.)
Power-Driven Vessels. Rules 18 and I
The basic rule is quite straightforward and is quoted here in full: `When two power-driven vessels are crossing, so as to involve risk of collision, the vessel which has the other on her own starboard side shall keep out of the way of the other.'
L
A
168 SMALL BOAT NAVIGATION i THE RULE OF THE ROAD AT SEA 169
In the normal course of events, the `burdened' vessel, which has the other to starboard of her, will alter course to starboard to avoid crossing ahead of the other ship.
The end-on situation is usually more difficult; this is covered in Rule 18 and this is quoted in full here (apart from the paragraph covering seaplanes) :
`When two power-driven vessels are meeting end on, or nearly end on, so as to involve risk of collision, each shall alter her course to starboard, so that each may pass on the port side of the other. This Rule only applies to cases where vessels are meeting end on, or nearly end on, in such a manner as to involve risk of collision, and does not apply to two vessels which must, if both keep on their respective courses, pass clear of each other. The only cases to which it does apply are when each of the two vessels is end on, or nearly end on, to the other; in other words to cases in which, by day, each vessel sees the masts of the other in a line, or nearly in a line, with her own; and by night, to cases in which each vessel is in such a position to see both the sidelights of the other. It does not apply to cases in which a vessel sees another ahead crossing her own course; or by night, to cases where the red light of one vessel is opposed to the red light of the other or where the green light of one vessel is opposed to the green light of the other or where a red light without a green light or a green light without a red light is seen ahead, or where both green and red lights are seen anywhere but ahead.'
Interpretation
The italics in the above text are mine, but see to what great (and almost exhausting lengths) the Rule goes in defining the `end on' situation. This is because it is one of the trickiest for assessment of the correct action to be taken. There is also always the doubt as to what action the other ship is going to take in these circumstances.
Suppose, for instance, two ships or boats sight each other at comparatively long range very fine on each other's starboard bow. Each knows that if a true end on situation exists, each should alter course to starboard, viz. Figure 28A.
But if the other is not exactly dead ahead, the two vessels may pass clear starboard to starboard, viz. Figure 28B.0~ ~ i 0
30
0* 0*
3 Y
Figure 28
Dangerous doubts can arise if the situation is allowed to develop into a close range situation when neither is sure whether the other will hold her course with the hope of passing clear to starboard or alternatively is about to alter course thinking it to be too risky to hold her present heading (Figure 28c).
In ship `X', the skipper is doubtful whether by continuing on his present course he will risk a head-on collision with `Y'. The same thoughts may well be in `Y's mind. For both of them at this late stage there are four possible alternatives:
(a) To hold their present course and speed in the hope that they will pass clear.
(b) To alter course to starboard.
(c) To alter course to port.
(d) To take the way off the ship by going astern.
A
C
I70 SMALL BOAT NAVIGATION THE RULE OF THE ROAD AT SEA 171
Of these alternatives:
(a) May well be a vain hope and is probably too risky.
(b) Is a viable action if there is room enough for `X' to cross `Y's track before they get too close to each other and similarly for `Y' to cross `X's track.
(c) An early alteration to port on both ships' parts at long range would have prevented this situation from developing, but a late alteration to port on either ship's part would be extremely dangerous. Suppose `X' at the last moment decided he was not going to clear on his present course and made an alteration to port. Simultaneously `Y', worried by the closeness `X' is likely to pass on his present heading, decides to alter course to starboard. The result is very probably a nasty collision. (See Figure
28D.)
(d) Taking the way off by going astern may be a way of preventing collision (or minimising its results) if it is done in time.
Avoiding the Close Quarter Situation
The main point to remember in any interpretation of these rules is that the close quarter situation should never be allowed to develop. An early, bold alteration by either ship in a doubtful situation will clarify it from the start and remove the dangerous elements of doubt inherent in late, hasty avoiding action. This applies to all combinations of circumstances and not just to the particular case quoted above.
A good indication whether a close quarter situation is likely to develop later is given by carefully watching the compass bearing of every approaching vessel. If the compass bearing does not change, and both vessels maintain their present courses and speeds, the two will collide. If the bearing is only changing slowly, the two craft may approach each other later at too close a range for comfort (the rate of change of bearing is, of course, a function of the range between the two ships). In the long range nearly head-on situation, the watching of the other ship's bearing can be the only reliable way of judging the relative motion of the two ships, particularly if the other ship is yawing in a seaway and it is difficult to determine its mean course.
At long range, the compass bearing is normally taken of the other ship's bridge or mast. At short ranges, it may be necessary to take the bearing of the right section of the other vessel; forexample, if you are trying to cut under a super-tanker's stern, the bearing of its stern, and not its bridge, must be watched (cutting across such a ship's bows is in no way recommended—bearings or no) .
Power Driven Vessel's Meeting a Sailing Vessel. Rule 20
`When a power driven vessel and a Sailing vessel are proceeding in such directions as to involve risk of collision, except as provided for in Rules 24 (the Overtaking Rule) and 26 (Vessels engaged in fishing) the power driven vessel shall keep out of the way of the Sailing Vessel.
This rule shall not give the Sailing vessel the right to hamper, in a narrow channel, the safe passage of a power driven vessel which can navigate only inside such channel.'
Interpretation
This rule is quite simple. Normally, sailing vessels have the right of way over power-driven vessels—except when they are overtaking, meet power driven vessels engaged in fishing, in a narrow channel, not under command, etc.
Holding Course and Speed. Rule 2I
`Where by any of these rules one of two vessels is to keep out of the way, the other shall keep her course and speed. When, from any cause, the latter vessel finds herself so close that collision cannot be avoided by the action of the giving-way vessel alone, she also shall take such action as will best aid to avert collision (see Rules 27 and 29).'
Interpretation
It is important that the `privileged' vessel holds her course and speed, otherwise early avoiding action by the `burdened' ship may be nullified and the situation confused.
Avoiding Crossing Ahead. Rule 22
`Every vessel which is directed by these rules to keep out of the way of another vessel shall, so far as possible, take positive early action to comply with this obligation, and shall, if the circumstances of the case admit, avoid crossing ahead of the other.'
I72 SMALL BOAT NAVIGATION THE RULE OF THE ROAD AT SEA 173
Slackening Speed. Rule 23
`Every power driven vessel which is directed by these rules to keep out of the way of another vessel shall, on approaching her,
if necessary, slacken her speed, or stop, or reverse.' X 1121 A
The Overtaking Rule. Rule 24
'NOTWITHSTANDING ANYTHING CONTAINED IN THESE RULES, every
vessel OVERTAKING any other shall keep out of the way of the overtaken vessel.
Every vessel coming up with another vessel from any direc‑
tion more than 222° (2 points) abaft her beam, i.e. in such a t
°2Z 0
position with reference to the vessel she is overtaking that at D B
night she would be unable to see either of that vessel's sidelights, X
shall be deemed to be an overtaking vessel; and no subsequent alteration of the bearing between the two vessels shall make the overtaking vessel a crossing vessel within the meaning of these Rules, or relieve her of the duty of keeping clear of the overtaken
vessel until she is finally past and clear. Y
If the overtaking vessel cannot determine with certainty y
whether she is forward or abaft this direction from the other
vessel, she shall assume that she is an overtaking vessel and keep Figure 29
out of the way.'
her beam; she must also be going faster than `X', for otherwise her bearing as seen from `X' would be drawing aft.
Interpretation `Y' is now an overtaking vessel, being more than 2 points
The capitals in this extract from the rules have been added for abaft `X's' beam, and is obliged to alter course to avoid `X' and
emphasis, as this is one of the most important regulations. Its keep out of `X's' way until she is completely clear ahead. If `Y'
application hinges on the relative direction of approach is doubtful when first seeing `X' whether or not she is more than
between the two vessels, two points abaft `X's' beam, she must assume that she is an
In Figure 29A, two power driven vessels are on converging overtaking vessel.
courses but `X' has just sighted `Y', and `Y's' bearing appears to
be steady. A risk of collision must therefore exist. .Narrow Channels. Rule 25
`X' must keep clear of `Y' in this case, as `X' has the other `In a narrow channel every power-driven vessel when proceed‑
}I
on, or just before her beam on the starboard side. `X's' best ing along the course of the channel shall, when it is safe and
action would be to alter course to starboard to pass under `Y's' } practicable, keep to that side of the fairway or mid-channel
stern, whilst `Y' must hold her course and speed. Alternatively which lies on the starboard side of such vessel.
`X' could reduce speed to allow `Y' to pass unhindered ahead. Whenever a power-driven vessel is nearing a bend in a
Suppose, however, a slightly different situation exists (Figure channel where a vessel approaching from the other direction
29B) when `X' is first aware of `Y's' presence. The bearing is' cannot be seen, such power-driven vessel, when she shall have
again steady, but `Y' is now approaching `X' from well abaft arrived within one half mile (? mile) of the bend, shall give a
M
174 SMALL BOAT NAVIGATION
signal by one prolonged blast on her whistle which signal shall
g y p
g g
be answered by a similar blast given by any approaching power-driven vessel that may be within hearing around the bend. Regardless of whether an approaching vessel on the farther side of the bend is heard, such bend shall be rounded with alertness and caution.
In a narrow channel a power-driven vessel of less than 65 feet in length shall not hamper the safe passage of a vessel which can navigate only inside such channel.'
Interpretation
(a) Keep to the starboard side of a channel. ~.
(b) Sound one long blast when going round the bend! (only applies to power boats—and the mad presumably).
(c) Self evident.
g
Fishin Vessels. Rule 26
`All vessels not engaged in fishing, except vessels to which the provisions of Rule 4 (vessels not under command, etc.) apply, shall, when under way, keep out of the way of vessels engaged in fishing. This rule shall not give to any vessel engaged in fishing the right of obstructing a fairway used by vessels other than fishing vessels.'
Limitations of Craft. Rule 27
`In obeying and construing these rules due regard shall be had to all dangers of navigation and collision, and to any special circumstances, including the limitations of the craft involved, which may render a departure from the above rules necessary in order to avoid immediate danger.'
Neglect of Seamanlike Precautions. Rule 29
`Noting in these Rules shall exonerate any vessel, or the owner, master or crew thereof, from the consequences of any neglect to carry lights or signals, or of any neglect to keep a proper lookout, or of the neglect of any precaution which may be required by the ordinary practice of seamen, or by the special circumstances of the case.'
Interpretation
So Watch Out!
THE RULE OF THE ROAD AT SEA I75
Local Harbour Regulations. Rule 30
`Nothing in these rules shall interfere with the operation of a special rule duly made by local authority relative to the navigation of any harbour, river, lake, or inland water, including a reserved seaplane area.'
Distress Signals. Rule 31
`When any vessel ... is in distress and requires assistance from other vessels or from the shore, the following shall be the signals to be used or displayed by her, either together or separately, namely:
(i) A gun or other explosive signal fired at intervals of about a minute. (Not normally a thing to hand!)
(ii) A continuous sounding with any fog-signalling apparatus.
(iii) Rockets or shells, throwing red stars fired one at a time at short intervals.
(iv) A signal made by radiotelegraphy or by any other signalling method consisting of the group • • •---• - • (S.O.S.) in the Morse Code.
(v) The International Code signal of distress indicated by
N.C.
(vi) A signal consisting of a square flag having above or below it a ball or anything resembling a ball.
(vii) A signal sent by radiotelephony consisting of the spoken word "Mayday".
(viii) Flames on the vessel (as from a burning tar barrel, oil barrel, etc.
(ix) A rocket parachute flare or a hand flare showing a red light.
(x) A smoke signal giving off a volume of orange-coloured smoke.
(xi) Slowly and repeatedly raising and lowering arms outstretched to each side.
The use of any of the foregoing signals, except for the purpose of indicating that a vessel or seaplane is in distress, and the use of any signals which may be confused with any of the above signals, is prohibited.'
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