NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Plastic sextants. was: GPS shortcomings.
From: John Kabel
Date: 2005 Jun 10, 09:12 -0400
From: John Kabel
Date: 2005 Jun 10, 09:12 -0400
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. > ================================================================