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John:

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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