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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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