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Old 11-08-2012
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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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Old 11-08-2012
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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!

T. P. Donnelly
S/V Tranquility Base
1984 Islander 30 Bahama
Pasadena, MD
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Old 11-09-2012
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Re: Now Blow (how to adjust compass) Me Down

Timid Virgins Make Dull Company

True + Variation = Magnetic + Deviation = Compass

East is least (subtract), West is best (add)

Obviously this is the other way around to Decap 06 so the ad/subtract thing is opposite - to me it's just easier to remember


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Old 11-09-2012
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Re: Now Blow (how to adjust compass) Me Down

You have a substantial boat so why not invest in the services of a professional compass adjuster? Do only after all your electrical and electronic changes are complete, at least for the forseeable future. Using a GPS to adjust or compute deviation is a little risky. It only works with no wind or current. GPS shows Course Over Ground(COG) not Heading(HDG). You can use GPS by setting a visible landmark as a waypoint and pointing the bow at it. The difference between course to waypoint and compass is then deviation. As to the adjusting, we cannot get the paint to stick out on the water for a compass rose. We use shore references or a gyro to establish the actual magnetic heading of the vessel. One colleague of mine likes to use Sun azimuths.
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Old 11-10-2012
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Re: Now Blow (how to adjust compass) Me Down

Bearing & All,

I have adjusted compasses in aircraft since 1973, it's one of the items we do as a maint. test pilot. Adjusting a sailboats compass was different for me. I did not have a compass rose or hand held master compass to check accuracy. I have re-read all the info. provided in this post and Chapmans section on it. I went back out to the boat and checked the two compasses out.

I removed the stern compass from its mount (just lifted out) and saw the 2 adjustment (N/ S & E /W) screws. I'm famiiar with using these compass adj. screws. I just need to plan out my procedure. I have learned here or relearned things that will effect the compass accuracy. The area I check it in (mag. grn. fault issues) can affect me. I need to turn on -- all the boats equipment that's on normally or not turned on (normally) when under way. When I check an aircraft compass... we turn everything on too. The heated windshields cause the worst problems.

I plan on using my hand held GPS and take along (master compass) a hand held compass to swing my 2 compasses. I have aircaft (compass) work sheets but do not (rarely) fill them out to make adjustments. I do 1/2 's adjustment(s) at NW & SE headings. I quicky .. dial the compass in to 002 deg. error. I turn to N record, E. S. W then back to North. I compare the (aircraft/ boat ?) to master compass/ GPS readings. This was my 1st step to establish (see) how much I'm off from the master compass/ GPS track results.

Here's where I would normally adjust the compass using the N/S E/W screws per the adjustment work sheet results. I look at the results and adjust the compass to be dead on at -- 330 degrees and again at -- 120 degrees & then back to 330 to make a 3rd adj(if needed). After I have brought these two headings into 002 degree accuracy; I recheck N, E, S, W, for results. Most of the time... I can (now) complete the dev. card with saved time & good accuracy.

I have found that these two (NW & SW) headings give me fastest results and (rarely) very little back & forth for a 3rd or 4th adjustment. I was showed this method by a very expereinced maint. test pilot. I have found a handful of compasses that were not adjustable to 002 ( +/ - ) deg. standards. I replaced them with new ones, and they adjusted out or checked out fine.

Last edited by HighFly_27; 11-10-2012 at 12:00 PM. Reason: mistakes
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Old 11-12-2012
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Re: Now Blow (how to adjust compass) Me Down

Regarding your question, "How do you know if your compass is accurate in the 1st place?" A highly reputed compass adjuster told me that he established true north by the sun and an almanac. I believe this this discussed in American Practical Navigator (H.O. Pub. No. 9). US Government Printing Office : Washington, DC, 1966 As I recall he would develop the magnetic north through the use of a variation chart
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