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    Re: Plastic sextants. was: GPS shortcomings.
    From: John Kabel
    Date: 2005 Jun 10, 09:12 -0400

    There have been a lot of interesting comments in this and subsequent posts.
     A lot of food for thought.
    I have pulled my sight logs off the shelf (meaning I found them) and will
    do a bit of stats this weekend to report back to the list.  I have to
    segregate them by sextant and locate some spreadsheets that I used to do
    the original evaluations of the intercepts.
    More to come . . .
    John Kabel
    > 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
    > >results.
    > >
    > >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.
    > George.
    > ================================================================
    > 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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