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    Re: GPS shortcomings.
    From: John Kabel
    Date: 2005 Jun 9, 08:27 -0400

    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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