Don't call me a "senior"!
Join Date: Feb 2014
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Re: DIY Compass Calibration
The "factory calibration" was probably worthless the moment the compasses were installed. Even a GRP (fiberglass) boat has enough metal bits and pieces to give it its own magnetic environment. That environment has to be compensated for by adjusting the compass(es) after installation.
You might try fine-tuning the calibration by anchoring away from any docks or other boats (i.e., away from anything else that might have an influence on the compass). Then using a GPS and charts to find some landmarks as close to the cardinal compass points as possible (but as far away from your location as possible), swing the boat around to the necessary headings and use a second (and maybe even a third) anchor to steady it as you follow the adjustment procedure recommended by the manufacturer. You may have to swing the boat through 360˚ a couple-three times to get the deviation as small as possible. Once you're satisfied, swing it one last time and note the error every 45 or 90 degrees. That's your deviation card. Don't worry too much about deviations for intervals less than 45˚, that's what GPS is for. You only really need a complete deviation card (every 15˚) if you are exclusively using traditional navigation techniques.
One last thing. Try swinging the compass with everything turned off, and then with everything (including the engine) turned on. You might find that you need two deviation cards; one for sailing, and one for motoring.
"If a man must be obsessed by something, I suppose a boat is as good as anything, perhaps a bit better than most." --- E.B. White