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    Re: Fluxgate compass
    From: George Huxtable
    Date: 2002 Feb 3, 00:21 +0000

    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."
    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
    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.
    George Huxtable, 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    Tel. 01865 820222 or (int.) +44 1865 820222.

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