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Old 16-08-2010, 23:25   #1
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Location: Otautahi, Aotearoa
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'Plastimo Contest' - Not Agreeing with 'Maximum Polyaxial'

I have a bulk-head mounted "Plastimo Contest".

A mate just dropped off another compass - has "Maximum Polyaxial" printed on the card.

I was thinking of mounting it in the cabin - somewhere I could see it from my bunk.

Yet it reads 10 degrees different from the bulkhead mounted "Plastimo Contest".

I'm tied up to a jetty at present and cannot move for some weeks.

The point of having a 2nd compass readable from my bed would be so that when single-handing - I could keep an eye on my heading when snoozing.

Guess with the 'difference' being constant that is still possible - - I will know my boats heading huh?
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Old 17-08-2010, 06:01   #2
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Compass deviation may (probably does) vary with mounting location. Thus two perfectly good compasses mounted in different locations on the vessel may show different headings.

To rationalize the differences you need to swing both compasses while mounted.

Do not count on the difference between the two to be constant. It probably is not.

She took my address and my name
Put my credit to shame
Sunspot Baby, sure had a real good time
Bob Seger
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Old 17-08-2010, 08:29   #3
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Magnetic deviation - Wikipedia, the free encyclopedia

Magnetic deviation is the error induced in a compass by local magnetic fields, which must be allowed for, along with magnetic declination, if accurate bearings are to be calculated.

In nautical terminology deviation is the magnetic compass error caused by magnetised iron within the structure of the ship.
Rotating the compass, as Sunspot Baby suggests, can correct the majority of the deviation. Compensating magnets do the rest. Large ship compasses have external compensators, while most compasses found on our boats have internal ones usually adjusted with screws. The owners manual for your compass should explain it.

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