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.
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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
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