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    Re: How to Use a Plastic Sextant
    From: Lu Abel
    Date: 2011 Jan 15, 10:23 -0800
    I own a 36' sailboat.   To maintain my sextant skills and to experience taking sights at sea, I have taken shots from my boat.

    A 36' sailboat is far from a stable platform.   At almost any point of sail and offshore sea state, my boat roll and twists.   Except under strong wind conditions, it's a gentle roll and twist -- but enough to make it impossible to keep the horizon within view without raising and dipping the sextant itself (before someone asks, the period of the roll is short enough that one can't wait for the horizon to roll back into view).

    Net-net, it's extremely difficult to bring a body down to the horizon with any precision at all.   I've done runs of sights with five minute differences no matter how careful I am.  If I were trying to do true offshore navigation using celestial sights on my boat, I'd assume that any fix would have at least a five mile uncertainty.

    I also have used a Davis Mark 15 plastic sextant.  For the price, these are pretty remarkable instruments -- while also not being as accurate as a good metal sextant.  My experience is that one should check index error frequently to make sure it isn't drifting.   And, as I mentioned in a previous post, one should always bring bodies down or up in the same direction that IE was measured.  Even then, I'd assume a 0.2 minute uncertainty in my observation.

    That being said, though, if my boat's motion is making it difficult to get a shot with even four or five minute's accuracy with a perfect sextant, a 0.2 minute uncertainty from a plastic sextant is pretty insignificant.

    By the way, the Davis Mark 15-20-25 series plastic sextants are very different creatures from the Mark 3.   The former are plastic versions of decent micrometer drum sextants.   I wouldn't characterize the Mark 3 as a "toy," but it would fit more into the category of "lifeboat instruments" that have been discussed on this list than a "serious" sextant.

    Lu Abel

    From: John Brown <jdb0302@yahoo.co.uk>
    To: NavList@fer3.com
    Sent: Sat, January 15, 2011 2:37:54 AM
    Subject: [NavList] Re: How to Use a Plastic Sextant

    I have never owned or even had my hands on a plastic sextant, but I bought this book out of curiosity and fully endorse all the positive reviews posted here.

    As the owner of two good nautical sextants, a C.Plath and a Freiberger Yacht version, I think it would be interesting to see what sort of PRACTICAL results could be obtained at sea with a plastic sextant in a 36 foot sailing boat. On the "keep it simple" principle I would probably go for a Davis Mark 3.


    John B
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