Deviation After a Refit: Correct or Relocate?
After a refit, a 60 m standby vessel reports the following deviation, measured by swing on compass headings: N +9°, NE +4°, E −6°, SE −8°, S −10°, SW −2°, W +7°, NW +8°. Before the refit the card was N +2°, E 0°, S −1°, W +1°. The refit added a steel locker 1.2 m from the binnacle and two large batteries beneath the chart table. The owner wants the compass adjuster to ‘remove all deviation’ with the binnacle magnets before the vessel sails. The adjuster says the pattern is not a simple semicircular error and that corrector magnets alone will not be enough. The vessel’s trading area has variation 4°E. You must decide what the table shows, what correctors are appropriate, and whether the compass position or the new steelwork must be changed before adjustment.
1. Interpret the deviation table. Which components dominate, and why can the old card not be corrected by a single magnet setting?
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Use the standard approximation δ = A + B sin θ + C cos θ + D sin 2θ + E cos 2θ, with θ measured from north. From the cardinal headings:
- C ≈ (N − S)/2 = (9 − (−10))/2 = +9.5°.
- B ≈ (E − W)/2 = (−6 − 7)/2 = −6.5°.
- The average offset is about −0.5°, and the E/W pair gives a smaller additional term. This is a large semicircular pattern: N and S are opposite in sign and differ by 19°, and E and W differ by 13°. That is why the old card and a single magnet setting cannot be reused. Semicircular deviation is corrected with permanent fore-and-aft and athwartships magnets. The intercardinal values do not fit a smooth semicircular curve, so there is also a quadrantal/soft-iron component; that is corrected with soft-iron correctors such as Kelvin’s spheres, not with the permanent magnets alone. The adjuster must separate the components before turning magnets.
2. What should be done about the new steel locker and batteries before the compass is adjusted?
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Do not start by adjusting magnets. The refit introduced a steel locker only 1.2 m from the binnacle and batteries beneath the chart table. A binnacle magnet can cancel a steady field at one heading, but it cannot stop the field changing when the locker door is opened, tools are put in, or battery current changes. First relocate or replace the locker with non-ferrous construction, move the batteries or verify their field at the compass, and route heavy-current cables away/twisted. Then swing the compass with all equipment in normal operating states. If the source cannot be moved, accept that the compass position is poor and consider relocating the binnacle; otherwise the deviation card will be invalid as soon as the locker or electrical load changes.
3. The owner demands zero deviation on every heading. Is that a realistic acceptance criterion, and how should the final swing be verified?
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Zero deviation on every heading is not a realistic acceptance criterion and should not be promised. After correctors are set, the compass is swung through at least eight headings, with the vessel on an even keel and all normal equipment running; each heading is compared with a known true bearing or azimuth. Opposite headings are checked, and the residual deviation is recorded on a new card. The acceptance judgement is whether the residual is small and stable enough for the intended navigation, not whether it is zero. The card should state the conditions (equipment on/off, heading references) and the swing should be repeated after any further structural or electrical change.