200253 Compass Reliability

Compass Reliability
Report No. 200253

I am glad that the author of MARS 200221 (SEAWAYS May 2002) included the word "almost" when referring to the reliability of the standard compass as, on three occasions, I have experienced unexpected errors. The first occasion was in a wooden hull vessel based on the "Ham" class minesweeper after spending a full night alongside a much larger steel vessel. We were fitted with an Admiralty Gyro Magnetic Compass (AGMC) which, for those who are unaware of its characteristics, contain a stabilising (as opposed to north-seeking) gyro, limiting the precession rate of the repeaters to 40/min. The error would have taken roughly 30 minutes to clear itself and therefore a large proportion of error was still present when we wished to steer a compass course once we were clear of the magnetic influence. On subsequent nights, having learned from experience, we slipped our moorings with the compass switched to ALIGN position which enabled the repeaters to precess at 400/min, thus clearing the error in only 3 minutes.

The second occasion was in my own (wooden) vessel. We were acting as "stand on" vessel during a "bump" transfer with a vessel of roughly similar size and design and thus had to maintain a steady course. As the other vessel approached, we veered all over the place and it was not until afterwards that I remembered that the other vessel was the only one of our group of five which was built of steel.

The third was when I was the Navigating Officer of HMS DAMPIER and I have no guilty conscience about this one as it took me several days to convince the Captain that it was, in fact, the magnetic compass and not the gyro that was in error. Whilst at anchor overnight, we were struck by lightening - something that only happens to "other people". In the morning, there was a difference of roughly 15 degrees between the gyro and magnetic compasses. We were on the East coast of Borneo and the nearest professional Admiralty compass adjusters were in Singapore. To get there we had to pass through a narrow, winding channel where it was obviously desirable to have gyro and magnetic compasses as near in agreement as possible. We first removed the soft iron spheres and the Flinder's Bar and heated them to remove any possible permanent magnetism which might have been induced. This had little effect so we took the "bull by the horns" and repositioned the permanent corrector magnets. We reduced the maximum error to about 1½ degrees - not bad for an amateur in the field. Taking bearings with one hand while holding the manual with the other hand to see what to do next! This was considered good enough to get us to Singapore.
As I am long retired, the anonymity given by MARS is of no consequence to me. Perhaps we could have a new style of report call WATER - Wise After The Event Report (I like this idea - Ed.).