A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Bill Morris
Date: 2021 Oct 18, 15:04 -0700
I am flattered to think my opinion may count for something and wonder if we are not straining out gnats.
I have always thought that a case which allows the index arm to be left in any position would be good, as the sextant can be put away with the reading of the last shot showing, in case its value comes into question later. It also means one less thing to do. On my limited budget over the years, I have acquired quite a few homeless sextants, and when making a new case have always tried to arrange things so that the index arm can be left anywhere. It is a nuisance too, to have to fold down shades in a specific position, as they are so easily damaged.
EBCO was not alone in having a specified value set in order that the case might close. The US Navy Mark III, although housed in a very large case, had to have the index arm left at 20 degrees, not just so that the sextant would fit in together with lots of odds and ends, but so a keeper on the lid could pass through the frame to bear on the handle.
Garry's worry about electrolytic corrosion is a valid one, but does not seem to happen in practice, as the rack and worm should be protected by an oil film, even if by neglect it has dried out to leave a gummy deposit. The rack is usually of bronze, or alluminium alloy in the case of several late C20 sextants, and the worm is also a bronze, or, if of white metal, probably of nickel silver (a brass in which about 20 percent of the zinc has been replaced by nickel). All are resistant to corrosion in a marine environment. I did once come across a USN Mark III in which the frame had visibly corroded near the zero, but it had been salvaged after a period under water. Otherwise, I have oobserved corrosion only in other parts, e.g.a well-used Freiberger yacht sextant had corrosion around the steel pins that held the arc in place for when the glue gave up, and corrosion is common around screws in the horizon and index mirror brackets.