Some of the most useful compasses on the market today are the ex-R.A.F. series. If not the most suitable, they are certainly the cheapest and are normally sold for a mere fraction of their original cost. The most common types encountered are:—

1. The Marching Compass

This is a non-liquid single magnet compass in a black plastic box measuring approximately 3” x 2½” x 1”. The usual cost from ex-Government stores is about 17/6d. This makes a very useful emergency compass and I always carry one even when day cruising just in case of fog. There is a slit and notched ring for taking bearings the order of accuracy of which is ±5° — quite a useful means of ‘guestimating’ one’s position.

2. P10 and P11 Grid Steering Compasses

These are very accurate liquid filled bowl compasses marked in 1º intervals from 0 — 360° with a movable grid ring on which the course may be set. They are both damped and luminous. The only difference is one of size, the P10 bowl being larger than the P11. They normally run out at 30/— each, but with gimbals cost over £3. Since the compass card is self-gimballing up to 20° and dinghies should never be sailed at this angle it is doubtful if the gimbals serve any useful purpose. These compasses are accurate to ±2° which is much better than the average dinghy is capable of being sailed to. I have used one of these extensively, have found it most reliable and prefer it to the average mariners’ compass. The luminous “T” grid lines make night sailing much less arduous.

3. P12 Compass

This is an inverted compass designed to be viewed from below so unless you sail lying flat on your back (after a rally, perhaps) this is useless for dinghies.

4. E type Compass

This series is designed to be mounted on a bulkhead and they are therefore of limited use for dinghies although ideal for small cabin cruisers. They are in the main liquid filled in a hemispherical plastic bowl. The magnet is circular and the scale annular marked every 5° and numbered every 30°. Its accuracy is reputed to be ±5° but is usually considerably better than this. The bowl is about 2” in diameter and the compass is self-gimballing up to 20°.

5. The Astro Compass

Don’t be tempted to buy one of these. They are very complicated to manipulate and can only be used to obtain a relative bearing or for an astro heading check, both of which are of little use to the dinghy sailor.

Points to watch when buying a compass

a. Check the bowl for cracks or leaks, ensure that the liquid is clear and not discoloured and does not contain bubbles or sediment.

b. Inspect the graduation, lubber line, etc., for clarity and luminosity.

c. Using a magnet, deflect the needle first left and then right, allowing the needle to come to rest after each operation. The two readings should agree within 2°.

Siting the Compass

Keep the compass as far away as possible from ferrous metal in any quantity. In the dinghy the centreplate is normally the only thing to worry about. Provided the compass is situated 3 feet or more away from this or any other large steel object, the effect will be negligible. At the end of the centre thwart or on the stern bench is as good a place as any. Bear in mind however that the compass must be visible to the helmsman on either tack and should preferably be in a position where it won’t be trodden on too often.

Swinging the Compass

Having fixed the compass firmly in position the next thing is to check it and find the deviation if any. This is quite a simple procedure for a dinghy and all that’s required is a flat beach or sandbank and an accurate hand bearing compass. (The latter can usually be borrowed from a more prosperous friend).

Since dinghies are not steel built we can neglect the effects of hard and soft iron magnetism and need only correct for coefficient “A”.

The position of the lubberline in the boat is marked (on the P10 and P11 there is a graduated scale in degrees on the after fixing lug).

Starting with the boat approximately on north one walks 30 yards behind the boat, lines up the mast and forestay and reads off the bearing on the hand bearing compass. The heading on the boat compass is noted.

The boat is then turned and the process repeated, approximately every 45°, so that in all eight compass readings with the corresponding magnetic bearings are obtained.

In each case the magnetic bearing is subtracted from the compass reading algebraically. The results are added together and divided by 8. This gives coefficient “A”.

If it is positive turn the lubberline clockwise “A” degrees and anticlockwise if negative (i.e. make the compass bowl read A° less or A° more respectively. The deviation has now been reduced to a minimum. If the deviation was fairly constant A may be subtracted from each of the compass headings and a deviation card drawn up. If it varied considerably (more than 5°) the compass should be re-swung and the deviation card drawn up from the information.

A typical graph of compass heading against deviation is shown and the resultant deviation card (Figs. I & II)

If in doubt as to whether to add or subtract deviation while swinging, remember the old adage:— Deviation West Compass Best Deviation East Compass Least