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Lu Abel said-

>If one plots deviation as a function of compass direction it should look
>like a sine wave (for fiberglass and wood boats at least, not sure about
>steel).  If the sine wave is "off center" (ie, the plot shows more westerly
>deviation that easterly or vice versa) it is an indication that the lubber
>line of the compass is not aligned with the ships keel.
>
>After I learned this fact I plotted the deviation table for my 36' sailboat
>and -surprise- it was uneven.  Even though it's a high quality boat, the
>compass had been fastened to the steering pedestal with the lubber line of
>the compass pointing about 2 degrees to starboard.  Hard to discover, easy
>to fix once I had done so.
>
>What I suspect that any decent deviation-determining software does is
>simply to even out the deviation readings so the sine-wave is "balanced."
>
>Lu

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If a plot of deviation shows an "off-centre" average, as Lu describes, that
can arise from two possible causes,

1. As he says, a misalignment of the sensor with the axis of the vessel.

2 An asymmetrical distribution of ferrous material, subject to induced
magnetism.

Unfortunately, the usual sail-in-a-circle technique for correcting compass
errors is quite unable to correct for errors of this type, or even to
determine that they exist at all.

George Huxtable.


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george@huxtable.u-net.com
George Huxtable, 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
Tel. 01865 820222 or (int.) +44 1865 820222.
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