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    Re: [Fwd: [Fwd: Davis Sextants]]
    From: Bob Trenkamp
    Date: 1999 Jan 25, 21:00 EST

    At 05:08 PM 1/25/99 -0800, John Brenneise wrote:
    >Has anyone subjected any of the sextants to which this thread of
    >discussion refers to a test of  thermal expansion issues?
    Yep - when you leave a MK-3 on the coachroof for an hour at midday, you
    have to recompensate for index error.
    >Have any other quantitative measurements been made?
    #1: experiments with a binary result - it gets me where I've been trying to
    go.
    #2: experiments with a somewhat continuous result - from a fairly-stable
    deck just south of Long Island, I seem to have gotten a quasi-normal
    distribution of error (wrt the Loran readings) from LOPs, with the mean at
    1.2 miles, std dev = .3 mi. (I just checked the log on this & did a quickie
    on excel, but I'm not about to go back through my log and crunch all the
    data. My sense is that this particular experiment yielded somewhat typical
    results.
    #3: experiments in the real world offshore - I just checked a few old
    plotting sheets and it seems that I generally get a cocked hat with a max
    dimension of 2-3 miles offshore. The LOPs used to construct the cocked hat
    are usually a mixture of advanced LOPs and current observations, though
    occasionally I can get 2 stars to mix with the moon.
    >There also is a practical issue of the mass of the sextant when
    >trying to take a sight from the deck of a pitching and rolling boat.
    Amen.
    >In practice, what precision do the MK15 et. al. deliver?
    >With my Astra IIIB and an artificial horizon consisting of
    >plate of oil placed upon the south facing deck (of my
    >condominium), I was able to get a fix that landed within
    >four nautical miles of that reported by my hand held GPS
    Wow, John...have you considered changing your oil...or perhaps your GPS
    batteries? (grin)  Last time I used an Astra III, it beat my MK-3 by a long
    shot. It's weight made all the difference in trying to get good shots from
    a 38' sailboat. At the same time, I'd not be willing to part with the extra
    money to get that improvement. (Ritual and reverence are two separate
    concepts, please.)
    Regards,
    bob
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