Plastimo 135 compass correction

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jwilson

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I have a 20 year old Plastimo Olympic 135 compass - after some recent work on both steering gear and electronics near the compass the deviation has gone from 3 degrees max to almost 20 degrees. Today after a few hours replumbing water systems, I went out intending to adjust the built in compensators under the compass to find that whilst the Plastimo instructions say the two compensator screws are marked NS and EW there is not the slightest evidence of any marking - even with light and magnifying glass.

One screw is fore and aft aligned, the other is abeam aligned. I really would like to know which is which before adjusting one at random.. The compass looks exactly like the attached image - you can see the fore and aft aligned screw at the very bottom. Is this the N/S adjuster? Anyone know ?
 

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To remove /reduce deviation on N/S heading, use the compensator magnet which is abeam of the compass. To remove deviation on E/W heading, use the corrector which is fore and aft.
After your recent work, you may have casued Coefficient A, (error caused by unsymetrical magnetism). To check for this, note the deviations on N, NE, E, SE, S, SW, W, and NW and work out the average deviation, it should be zero,. If not, the only way to correct for this error is to rotate the compass. Once this is done, then correct the deviations on N/S and E/W. Remove the biggest error first.
Remember to allow for Variation.
 
nigel1 said:
To remove /reduce deviation on N/S heading, use the compensator magnet which is abeam of the compass. To remove deviation on E/W heading, use the corrector which is fore and aft.
After your recent work, you may have casued Coefficient A, (error caused by unsymetrical magnetism). To check for this, note the deviations on N, NE, E, SE, S, SW, W, and NW and work out the average deviation, it should be zero,. If not, the only way to correct for this error is to rotate the compass. Once this is done, then correct the deviations on N/S and E/W. Remove the biggest error first.
Remember to allow for Variation.
Click to expand...
Thank you very much. If I had been unable to get any answers I would have guessed that the fore and aft screw was N/S, which would have been completely wrong!
 
jwilson said:
Thank you very much. If I had been unable to get any answers I would have guessed that the fore and aft screw was N/S, which would have been completely wrong!
Click to expand...
Have a think about it. On a N/S heading, the compass needle will be aligned fore and aft. The fore and aft magnets will be under the needle and in line with it, and adjusting that magnet will have no or very little effect on the needle. On the other hand, the magnet which is abeam will be at right angles to the compass needle and will have maximum effect.

I was in the Merchant Navy for 47 years, 38 years as skipper, and I really enjoyed correcting the magnetic compass.

Cheers
Nigel
 
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Can I hijack your thread and ask Nigel (a man who clearly understands these things) another compass related question?. I have a 45 year old sestrel. Last year it developed a bubble. I have filled it and there was no bubble. Yesterday a bubble reappeared but today it has gone again. Any ideas? I can understand how air can get in but not how it can be expelled again.
 
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