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    Re: Automatic deviation calculation by electronic compasses
    From: Lu Abel
    Date: 2009 Nov 22, 11:36 -0800

    First of all, I have no personal experience with auto-correcting
    electronic compasses, so what I write is hearsay.  At the same time, if
    auto-correction didn't work reasonably well, I suspect that
    manufacturers would have been found out and forced to withdraw the
    feature from their compasses.
    Second, we seem to be in agreement that the careful mariner will check
    his/her compass's opinion of magnetic direction.   But I think any
    careful mariner will do this occasionally even with a conventional
    compass to insure that some unknown change won't have affected his/her
    deviation table.  I personally don't do this as a matter of a special
    activity, but rather as I'm out sailing I simply verify that courses
    between known marks agree with the expected compass course.   (I learned
    this the hard way many, many years ago when a guest brought aboard
    several tins of canned goods for that night's dinner and stowed them in
    a convenient locker whose role on my boat was to conceal the back side
    of my bulkhead-mounted compass!)
    Third, I think we disagree on the ease of turning a recreational vessel
    in a 720 degree turn at a constant angular velocity.  I don't know where
    you sail, but having sailed on both coasts of the US, periods of no wind
    here are less than "rare" as you fear.  In fact, isn't that the concept
    of a "harbor" where one is sheltered from the wind and weather?   As to
    tiller-steered vessels, there are very, very few sailboats (and no
    powerboats) sold (in the US, at least) that might be candidates for an
    electronic compass (say, a vessel over 30') that do not have wheel
    steering.   Even so, I suspect someone who is far less clever than you
    are could figure a way to lash or otherwise hold the tiller in a fixed
    position.   And every boat I've owned has enough friction in its
    throttle mechanism that one can set the throttle at a fixed position and
    expect it to stay there.  As a double-check one could simply monitor the
    engine's tachometer.  So I don't see making a 720-degree turn at a
    constant angular velocity to be anywhere near the problem you fear it to
    Last but not least, there is a "reset" button on these compasses that
    will zero out the deviation table.   So it it doesn't work -- disable it!
    I for one feel much more comfortable trusting a technology where there
    is a way of double-checking it, whether that technology be an
    auto-correcting compass, GPS, or even a chronometer.   But I also think
    that eschewing a new technology via a blanket "don't trust new
    technology" is not a responsible way to navigate.   Your mileage may
    George Huxtable wrote:
    > 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@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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