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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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