Compass Construction, Installation and Care
Practical, survey-style questions: what is inside the bowl, where the binnacle may stand, and what checks prove the compass is still trustworthy. Answers must be concrete about components and inspection points.
Construction of the magnetic compass
- Bowl, liquid, expansion chamber or bellows, card with its system of magnets, pivot and jewel, gimbal rings, lubber line, azimuth mirror and a non-magnetic binnacle.
- The liquid damps the card and takes most of its weight off the pivot; the expansion chamber absorbs volume change with temperature.
- The binnacle carries the corrector magnets, soft-iron spheres, Flinders bar and heeling magnet, and usually a red hooded light to protect night vision.
Installation
- On the fore-and-aft centreline at the steering position, with a clear view of the horizon and the lubber line parallel to the keel.
- Away from electrical machinery, loudspeakers, VHF and radar cables, steel structures, funnel heat, and magnetic cargo.
- Accessible for adjustment and reading, with the correctors reachable and the card level.
Care and inspection
- Check for bubbles or discoloured liquid, a sluggish or sticking card, worn pivot and jewel, stiff gimbals, cracked glass, failed illumination and an out-of-date deviation card.
- Re-swing after welding, drydocking, a lightning strike, a change of magnetic cargo, or work on degaussing.
- Gyro side: repeater alignment with the master, settling time, latitude and speed input, and a usable magnetic compass as backup.
Worked example
A transit of two charted objects bears 068 T. The steering compass reads 062 C on the same transit and variation is 5 E. Compass error = 068 - 062 = 6 E. Deviation = error - variation = 6 - 5 = 1 E. The card for that heading states 4 E, so the compass is 3 degrees from its last swing. That indicates a change on board, such as magnetic cargo, welding or a steel item left near the binnacle, not a change in variation. Re-swing and reissue the card.
Traps
- Placing a magnetic-base lamp, radio or tools near the binnacle, or painting it with magnetic paint.
- Ignoring a bubble or a slow-swinging card as cosmetic.
- Adjusting correctors before removing local magnetic interference.
- Using an old deviation card after cargo, welding or docking changes.