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Old 11-08-2012
HighFly_27 HighFly_27 is offline
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Now Blow (how to adjust compass) Me Down

I've been reading & reading, today, I got my 1st good book on -- Piloting & Seamanship by Chapman, 64th edition. I have a long way to go and will attend a 3 day sailing refresher course in December and follow on instruction from there.

Today's Question: Checking & Adjusting Mag. Compass on a Sailboat

Boat -- 1970 Islander, 37 Motor Sailor (MS)

Note: boat is on stands at my home in Bonifay, FL

I have two compass's (cabin & helm) and they are - 007 deg's. off from each other. I could see - 002 degrees for magnetic compass difference between the two of them, it appears that I'm - 005 degrees off but which one ____ ? is off.

Now, we have gotten away from the Old Time Rule... Use the boats compass & nav. charts as the 1st Nav. Method, Electronic Method as No# 2. I was trained this way in the army flight school in 1969. Currently, the army aviator depends on his tactical map to get him to the target (last 5 miles or so). Yes, the mil. has all the electronic gear, our military GPS will take you to a 10 ft. dia. area.

My Question -- I read (Chapman) how the compass is -- Checked & Compensation (adj'd.). I have done this in aircraft as part of the aircrafts annual certification requirement.

I see that the sailboat (Chapman) compass is checked (start) N./ S. and to heel the boat to one side and then the other to place the boat out of Horizontal Trim. We are heeling the boat to >> see the effect on the mag. compass. << I assume we are checking that -- the mag. compass does not hang up, rotates freely & maintains course or changes to course {proper operation). << This was the only thing that I was unclear about when I read about compass compensation (reading in Chapmans).

My last question, How do you know if your compass is accurate in the 1st place ? I used a certified ground rose (360 deg.) point or master (hand held) compass to check the aircraft mag. compass(s). I assume that (today) everyone use's a handheld GPS or Boats Nav. Sys. on board to verify the boats compass(s) accuracy. They would/ should check their local charts to record the magnetic variation to convert to true mag. from (if needed) the GPS heading. They would be dead on with what the mag. compass reading should be from the GPS readout. After the correct (mag. heading) value is known, then the boats compass(s) can be compensated with the adjustment screws N/ S & E/ W with a brass screw driver.

Closing -- I was/ am Not 100% on the boat compass -- adjustment (under way) or verification of accuracy issue. I do know that a compass that's more than -- 005 degrees off.. would not be a good thing. The allowable aircraft std. is 002 degrees ( +/ -) for mag. compasses. I assume that accurate compasses are needed for boats as well, otherwise ... bad things can/ will happen (off course & bang in the night, day time too) !

Last edited by HighFly_27; 11-08-2012 at 10:22 PM.
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