A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Mark Coady
Date: 2015 Dec 13, 21:38 -0800
To be honest I had never heard of lubing a plastic sextant. I found, even knowing the tricks as described in David Burch's book on plastic sextants about always going the same way with the micrometer drum, that I was going insane trying to set my old plastic sextant to an even halfway constant index error.
In the end, by careful examination I found three things:
1) turning the drum, i would see tiny amounts of hesitation....drag.....hardly noticable...but if I tapped the sextant index pivot with a fingernail...it would move incrementally,,,and index error would be more uniform. My conclusion was that it was binding ever so slightly, and the plastic is much more flexible than metal so it distorts slightly before it moves. Lubrication ended the issue..and motion seemed more uniform.
Not saying you should or should not....it appeared to be clean. I bought it used I recall...and it may have been made stickier than normal by previous owner treatment or dirt? Don't know. Either way...lubrication seems to have restored uniform index arm sweep.
2) The second error was more sinister. I have big hands that have done a lot of handwork. My whole hand grip on the sextant I discovered was firm enough to distort things a few tenths of minute. I have since changed my plastic sextant grip to something intentionally more gentle, designed to eliminate the issue.
3) Lastly on prolonged experimental sessions on hot summer days, I felt temperature produced more variation in the plastic vs metal. If you look scientifically at the coefficient of thermal expansion, this makes sense. My solution was a thin white sheet. I would keep it shaded, take a sight, and then return it to shade. Reducing exposure to the hot summer sun.
Funny enough I still find the little cheap student split mirror Davis with a simple vernier essentially as accurate as the more sophisticated plastic metal sextant copies. Provided I had a good time piece.....even the vernier one will take you anywhere in the world. On an ocean voyage, I would never leave home without a second sextant such as the Davis, packed in my floating overboard bag for the liferaft. I am a firm believer in my own clumsiness, and the risk of dropping anything is directly proportional to its critical importance.
I don't think the plastic sextant versions are typically up to lunars with any great accuracy (at least not with my hands and eyes). That said, with either, if pushed to it, I would do it anyway. I would use sight averaging and the tricks I learned about always going one way for gear lash, grip, lube, etc.and just head to the known latitude a bit early rather than a direct shot. Approximate longitude is better than none in my book.
In truth, in this day and age, I can't imagine the sextant is not already the backup for GPS recievers, and anyone on an ocean voyage I would think would have both installed and a portable emergency floating GPS version.
That said I will never ever place infallable belief in any electronics. Back in the 1980's, (seems so long ago), I was on an ocean racing sailboat that had to sail up the Thames River into Lew London in in a sleet storm at night with dead batteries, blown diodes in the alternator, and only a few handheld battery lights carefully rationed... the can bouys were black back then...and stopwatch and paper were the name of the game. Old fashioned compass and off the paper bearings worked.
For now, its a hobby. My own little fish/dive boat I am sitting in isn't going far enough to be out of near coastal waters.....sigh...but I can still dream.... I keep studying hoping someday I can roam farther afield agian.....