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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Plastic sextants. was: GPS shortcomings.
From: George Huxtable
Date: 2005 Jun 9, 21:13 +0100
From: George Huxtable
Date: 2005 Jun 9, 21:13 +0100
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. ================================================================