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    Re: GPS shortcomings.
    From: Lu Abel
    Date: 2005 Jun 9, 08:01 -0700

    I can't deny your real-world experience, but I do have to respectfully
    ask, as I did Peter Fogg, "what brand of sextant?"   On my Davis the
    worm screw is completely enclosed so I can't tell if it's metal or
    another material.
    I can also tell you that I personally and dozens of my students from
    celestial courses I have helped teach have gotten sights within the USPS
    limits of a 3 mile error on land with Davis sextants.
    I think we also have an emotional reaction to "plastic"   For many years
    "plastic" was synonymous with cheap and shoddy.  And to some extent it
    still is; I can go to a discount store and find lots of cheap, shoddy
    goods made of plastic.  But let's remember that Kevlar is a plastic; I
    don't think a police officer or soldier whose life has been saved by a
    Kevlar bullet-proof vest would think "cheap and shoddy" about it.
    Lu Abel
    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
    >>Peter Fogg wrote:
    >>>>On Behalf Of Lu Abel
    >>>>We're talking about the sextant being a backup navigation instrument, not
    >>>>the primary one, so I must ask why you think a Davis Mk 15 or Mk 25
    >>>>wouldn't be up to the task.
    >>>Metal is more reliable. Won't warp and is less liable to temperature
    >>>effects. Nothing new or controversial here.
    >>Do you speak from experience or just theory about "plastic?"
    >>My understanding is that Davis's are made out of a form of "engineered"
    >>plastic that is very strong and has virtually no temperature coefficient of
    >>expansion.   I'll agree "metal is more reliable," but by how much?  I have
    >>co-taught USPS's celestial courses for many years and about 90% of our
    >>students have used Davis sextants.  In years of checking sights I've never
    >>run across one that was off because the sextant was off -- any errors have
    >>always been traced to student error.  My own Davis Mark 15 is almost two
    >>decades old and has never given me a problem.
    >>Lu Abel

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