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    Plastic sextants. was: GPS shortcomings.
    From: George Huxtable
    Date: 2005 Jun 9, 21:13 +0100

    John Kabel wrote-
    >My own experience with plastic versus metal is backed up by about a
    >thousand sights with each.  The plastic arc was progressively eroded by the
    >metal worm screw over the first six months I owned it.  This problem was
    >compounded by occasional sand particles getting into the threads on the
    >arc, creating even more abrasion.  I was never able to get more than 30 %
    >of sights below a 5 mile error, while the majority of sights with an Astra
    >IIIB were below 5 miles, with about 50 % within 3 miles of actual position.
    > This was a static situation on a beach.  And it has nothing to do with the
    >fact that I was more expereinced by the time I bought the metal sextant.  I
    >can still do a run of sights with either and get roughly the same error
    >Plastic sextants are for emergency use only.  In fact, I would suggest that
    >even an experienced navigator would experience more wear on the plastic.
    >John Kabel
    Comment from George.
    I'm rather surprised that John Kabel experienced such large errors, when
    using his Astra from a beach (about 50 % within 3 miles). Was this
    shot-to-shot scatter? How repeatable was a series of repeated shots at
    close intervals? It could be explained by days of anomalous dip, but a 50%
    frequency seems absurdly high. Does John have an explanation?
    A more relevant question is how suitable is a plastic sextant for making
    altitude observations from A SMALL CRAFT AT SEA. Most of the time
    (excepting millpond conditions, that is) the precision is considerably
    worsened by the motion of the vessel underfoot, the heaving up and down in
    waves and swell, and the irregularity of a horizon that's made up from
    overlapping wave-tops. That worsening is just as bad for the most expensive
    metal sextant as it is for a cheap plastic one. John Kabel's beach
    observations are of little relevance in that context.
    If John made his observations at sea, rather than from a beach, he wouldn't
    get troubled by erosion from sand particles.
    My own experience, with Ebbco sextants, closely matches Lu Abel's, with
    Davis. But there could certainly be improvements, particularly with the
    optics and the shades.
    contact George Huxtable by email at george@huxtable.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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