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Jeremy asked-

"On a related topic, I wonder how often a yacht needs to correct its
compass? On a ship we are using the electro-mechanical gyrocompass that is
prone to getting a bit off kilter for a number of reasons, and therefore we
keep  a record on its error.  I would think that the deviation, once
determined,  would be fairly constant on a yacht as long as nothing metallic
is moved  around.  Am I correct in this?  I would think that swinging the
boat  on a range would be the best way to create a deviation table.  Of
course in  areas of "local magnetic anomalies" you might use a static
deviation table and  try to determine local variation for future reference,
but that is probably  fairly rare."

Only on a steel-hulled vessel will it matter much. For most fibreglass
hulls, such as mine, which has a little 7-horse diesel auxiliary about 6
feet diagonally below the compass, deviation tends to be ignored. I
occasionally check it on passage, but never find a compass error of more
than a degree or two, which is about the best I can hope for in a compass
observation anyway. And of course Jeremy is right; having done it once you
shouldn't have to do it again, unless someone leaves his knife by the
compass. The troublemaker's trick, on the old windjammers, was to conceal an
iron belaying-pin behind the compass binnacle.

 It can get more complicated someimes, though. I remember discovering severe
deviation on an elderly wooden yawl, which turned out to depend on the steel
steering-chain that was wound around a drum attached to the wheel, right by
the compass. And it varied with the rudder position, the amont of chain
strung out on each side depending on how much helm was called for.

Of course, in the old days, compass error in wooden vessels was checked to
get the local variation, in the days before it, and its annual changes, were
properly mapped. And in some places, as when approaching the Cape of Good
Hope, that could provide a useful clue to longitude, long before other
methods for longitude appeared.

A steel hull certainly needs to be swung, even (especially) when it's a
yacht. The simplest way to do that is to find a mooring, or drop an anchor,
in line with a known transit, then push the stern around in a circle, under
oars or outboard. There are fluxgate compass systems that claim you just
have to press a button, then sail in a circle, about which I am somewhat
sceptical; perhaps without good reason.

======================

George.

contact George Huxtable at george@huxtable.u-net.com
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.


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