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    Re: Deviation
    From: Lu Abel
    Date: 2006 Jan 29, 11:32 -0800

    As a quick answer, I have always been taught that easterly and westerly
    deviations should sum to zero (or, more correctly, if the axis of a
    plotted deviation curve should be at zero degrees).  In fact, if they
    don't then this is a signal the compass's lubber line is not aligned
    with the keel of the boat.
    The Handbook of Compass Adjusting (one of those wonderful, albeit hoary
    publications from the US DHMA (or NIMA, as they call themselves these
    days), http://pollux.nss.nima.mil/index/uncon/products/HoMCA.pdf,
    paragraph 318, gives a list of the types of errors one might see in a
    deviation curve.  If it were possible to reduce the deviation curve (eg,
    as plotted on a Napier Diagram) to a straight line and the straight line
    were off-axis (ie, giving a constant deviation regardless of the boat's
    heading) this would be called an "A" coefficient.  The possible causes
    for "A" coefficients are given as "misaligned compass," "incorrect
    observations" and "asymmetrical alignments of horizontal soft iron (rare)"
    Compass adjusting on steel vessels is an art unto itself, with Flinders
    bars, quadrantal compensators (the steel balls one sees in ships
    binnacles), "soft irons," etc.  Fiberglass or wooden boats pleasure
    boats have modern small-boat compasses that don't have "soft irons," so
    assuming you're talking about the world of pleasure boats, yes, easterly
    and westerly deviations should cancel.
    Lu Abel
    Pierre Boucher wrote:
    > Is it correct to say that if a compass is properly installed (align,
    > compensated, etc..) that the sum on the residual west deviation equals
    > the sum of the residual east deviation?
    > Pierre Boucher
    > ************************************************************************
    > Pierre Boucher N
    > formation en navigation de plaisance
    > Ste-Therese (Quebec) Canada
    > la "VOILE" ... le reste n'est que du vent...
    > "Sail, Sail !"... the rest is only wind...
    > EMAIL: pboucher@lavoile.com
    > http://www.lavoile.com
    > ************************************************************************

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