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    Re: Should all electronic navigation devices on a yacht be set to True or Magnetic?
    From: George Huxtable
    Date: 2009 Nov 22, 12:15 -0000

    The topic under discussion has changed, somewhat, to whether one should
    blindly accept the feature claimed by many electronic compasses, of
    providing auto-correction for deviation. Here, we are presumably thinking of
    a non-steel vessel: steel construction calls for much more serious attention
    to deviation and its correction.
    
    Lu Abel correctly identifies one serious weakness: that such a scheme can't
    detect: an error in the alignment of the device when it was intially
    bolted-down to the vessel, so he concedes that at least an initial
    traditional deviation table is called for.
    
    He claims that - "  It's not a whole lot different from the way one creates
    a deviation table or a Napier diagram the old fashioned way." But as I see
    it, there's a great difference. With this procudure, the job is done with
    eyes closed, making no reference to observed azimuths or transits, but
    simply by timing, as the vessel turns in a circles.
    
    How is that done? By "Making just one reasonable assumption -- that the
    boat's angular velocity is constant "
    
    Well, the procedure stands or falls by the validity of that "reasonable
    assumption". How good is it, then? Has anyone made serious attempts to
    assess it?
    Presumably, the job has to be done under engine, with all canvas furled.
    Presumably the engine-speed will be kept constant, and the helm put over to
    some extent, and held at a chosen position, while a number of turns are
    made. Than might be reasonably easy to do on a wheel-steered craft; but how
    do you arrange to lock or lash a tiller to be sufficiently-well fixed? It's
    possible to do so, surely, but not trivial, depending on the degree of
    helm-constancy that's required.
    
    If a completely windless day can be chosen (and how rare are those?) it may
    be possible to do the operation with some precision. But otherwise, how
    precisely can it be claimed  that the "reasonable assumption of constant
    angular velocity" can be achieved?  What that means is that while making the
    required turns, the vessel's true heading is changing at a constant rate.
    
    That calls for the vessel's speed to remain constant throughout the circles
    that it's making, although some sections are upwind, others downwind. It
    calls for the radius of curvature of track to remain constant, although over
    part of the track the bow is being brought into the wind, against the
    natural inclination of many boats, and then turning off it, which many will
    do with alacrity, depending on balance of hull and topsides. We're all
    familiar with such effects when the wind is strong: they will be less
    obvious, but still there, in a lighter breeze. The point is that here,
    reliance is being placed on the constancy of that rate-of-turn, to a rather
    high precision. It may well work, in practice, for a big ship. Does it work
    for a small craft?
    
    Let's say there was no magnetic deviation at all, at the compass position of
    a vessel. Then, during the calibration, the observed magnetic course would
    change steadily, and linearly with time, as long as the true heading was
    changing at a constant rate. But if it wasn't changing at a constant rate,
    then the resulting non-linear change of magnetic course with time would be
    wrongly interpreted as deviation, and corrected-for forever after!
    
    In the end, the mariner is trusting his directions to corrections that have
    been stored inside a black box, and may have no knowledge of what those
    quantities are, and may not even be aware of the conditions under which they
    were obtained.
    
    The advantage of the method is that it's easy: easy for the manufacturer,
    and easy for the navigator, who doesn't need even to think about what's
    happening. We should distrust the easy unless we can be persuaded that it
    works just as well as the traditional way. I have seen no evidence, yet,
    that it does, and share Greg's doubts, but remain open to persuasion.
    
    George.
    
    contact George Huxtable, at  george{at}hux.me.uk
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    .com
    
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