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Old 11-08-2012
dacap06
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Re: Now Blow (how to adjust compass) Me Down

Swinging the ship, eh? Well, in small boats some compasses are adjustable and some aren't. Add new gear in your cockpit, and you'll need to do it all over again. Mine isn't adjustable, so instead I computed a compass deviation table.

To find your deviation, use a hand-held GPS to set your course to a true bearing near slack tide when there is little wind. Measure what your compass read and write it down. Do that for every 15 degrees. Adjust the magnetic bearing for each true bearing you sailed using magnetic variation in the area where you're sailing. The difference between the magnetic bearing and the compass reading is your compass deviation for that bearing. You can use that to figure out what to steer. You can use linear interpolation for the bearings between the 15 degree points.

Can Dead Men Vote Twice At Elections?

Compass + Deveation == Magnetic; Magnetic + Variation == True; Add East, Subract West.

Hope this helps!

Tom

T. P. Donnelly
S/V Tranquility Base
1984 Islander 30 Bahama