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    Re: Manual calculation of compass variation
    From: Henry Halboth
    Date: 2004 Oct 16, 22:24 -0400

    My reference was specifically to the metallic spreader employed in a
    naval or merchant marine officers/ratings cap - some yachtsmen used to
    wear them also, but I think have since converted to baseball caps. Lecky,
    some time ago, wrote on this subject and I experimented while taking
    azimuths, thereby finding the possibility of inducing up to 3-degrees of
    compass card deviation. I really have no idea as to the hats worn by
    Brother Cook and his crew, but the assortment of buttons, belt buckles,
    sword slings, etc., could easily have the same effect should the wearer
    not be alert to the possibility. I am not recommending that the azimuth
    taker strip, but s/he should be very conscious of any metals brought
    close-by the compass - even the binnacle light, if not properly wired,
    may cause deviation while the current is flowing.
    
    Little has been said on this List regarding the matter of compass error,
    its determination, and application - a matter which seems surprising,
    given that many smaller vessels employing satnav, radar, loran, et al,
    still rely on the magnetic compass as their primary directional
    instrument; even the lengthy discussion on lightning did not seem to
    consider its potential affect on the magnetic compass, whether direct or
    through reversal of the vessel's magnetic character. It certainly appears
    that some discussion with respect to the determination of deviation, as
    well as its application, would prove instructive the many who do not have
    an instinctive grasp of the subject.
    The compass, the lead, and the log, as well as the lookout, remain the
    most basic tools of navigation - generally speaking, if you use them
    properly your chances of staying afloat are pretty good.
    
    I fully realize that it would be possible to get along with a severely
    impaired compass if outfitted with the electronic aids mentioned above,
    but the ability to maintain an independent dead reckoning account would
    be compromised and a basic back-up tool to this technology ignored. Flux
    gate, gyro, and azimuth compasses are there for the asking but also can
    suffer derangements.
    
    On Sat, 16 Oct 2004 20:17:16 +0100 George Huxtable
     writes:
    > Henry Halboth pointed out-
    >
    > >George - the discrepancy you describe is the compass error to which
    > >deviation must be applied to obtain variation. Perhaps Brother Cook
    > was
    > >not too much troubled with deviation, but it's there nevertheless;
    > even
    > >the metal ring in an officer's hat while taking an azimuth can
    > deviate a
    > >standard magnetic compass as much as 3-degrees, as I have proven by
    > >experiment. Please don't construe that I am suggesting Cook's hat,
    > >whatever that may have been, as the start of another thread, but I
    > am
    > >saying that, without a knowledge of the deviation the variation
    > recorded
    > >must be suspect.
    >
    > ===================
    >
    > Henry is quite correct, as usual. I fell into the mistake of
    > assuming that
    > for Cook's wooden vessel, there would be no deviation, but should
    > have
    > known better. I've recently been studying a voyage to the "Greenland
    > Whaling" (West of Spitzbergen) in which the log notes that on
    > certain
    > courses, the two compass positions in the binnacle differed by a
    > whole
    > point (11 and-a-quarter degrees).
    >
    > Two compasses were placed a few feet apart in the wide binnacle
    > (somewhat
    > like a domestic sideboard), presumably so that the helmsman could
    > read a
    > compass straight-on, wherever he needed to stand. Checks had been
    > made to
    > ensure that it wasn't the effect of interaction between them. It
    > must have
    > been deviation resulting from some local ironwork, to which the
    > compass
    > would be very susceptible in such high latitudes. Such problems were
    > only
    > poorly understood in the early 1800s.
    >
    > I'm interested in Henry's account of the effect of the "metal ring
    > in an
    > officer's hat". Unfortunately, most such portraits in my books of
    > Cook's
    > period are posed hatless, but none of the others show such a ring.
    > What was
    > it FOR, and how big was it, I wonder? Some sort of buckle, perhaps,
    > for a
    > chinstrap to hold down such awkward headgear, with its immense
    > top-hamper,
    > in a blow? But an IRON ring (which it would have to be to affect the
    > compass)? I can imagine it being of ivory, perhaps, or brass, or
    > even
    > silver; all non-magnetic. But an iron ring doesn't sound right to
    > me, even
    > for an officer's working headgear. Perhaps Henry will explain
    > further.
    >
    > Of course, on Lisa's steel vessel, compass deviation and its
    > correction is
    > a serious matter, and in her case it's likely that compass bearings
    > of low
    > celestial bodies would be useful, not for checking variation, but
    > instead,
    > taking compass variation from the chart, for checking deviation, and
    > how
    > well the compass has been corrected..
    >
    > George.
    >
    > ================================================================
    > contact George Huxtable by email at george---.u-net.com, by
    > phone at
    > 01865 820222 (from outside UK, +44 1865 820222), or by mail at 1
    > Sandy
    > Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    > ================================================================
    >
    
    
    

       
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