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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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