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I am perplexed by the series of references to taking a sextant in the
abandon ship bag.  Where and how on earth does everyone expect to navigate
to in a life raft?  That might have worked in a longboat in the late 1800's
but...

In a real abandon ship situation weight/space/time are valuable commodities
and I don't think the sextant, of any description plastic or metal, ranks
high on the list of items to bring.  I would appear to me that modern
abandon ship equipment using inflatable life rafts, EPIRB's GPS etc...are
based on the theory of survival until rescued by outside help, hence bring
stuff to maximize survival time, vice thinking about navigating to the
nearest land.


Jim Dullea
Tech Marine Business, Inc


-----Original Message-----
From: Navigation Mailing List [mailto:NAVIGATION-L@LISTSERV.WEBKAHUNA.COM]
On Behalf Of Trevor J. Kenchington
Sent: Tuesday, February 18, 2003 8:51 PM
To: NAVIGATION-L@LISTSERV.WEBKAHUNA.COM
Subject: Re: Davis Instruments Mark 15 Sextant

George Istok wrote:

> Just soliciting opinions on this sextant.  Any thoughts appreciated.


Mine is a Mark 25 but I doubt that there is much real difference from a
Mark 15.

Working on land, with a sea horizon, I am disappointed any time I get an
LOP more than a mile from my true position. The one time I fixed my
position in the open sea, I came out half a mile from the GPS position
-- though I did have the advantage of the stable deck of a big sailing
ship, plus GPS readings that allowed me to advance the morning LOP
without errors in dead reckoning. In short, the Davis plastic sextants
are quite accurate enough for practical use.

However, you do have to adjust the mirrors before every set of
observations, you have to check index error before and after each
individual sight, and you have to take multiple sights and average them
by plotting altitudes against time. If temperatures are extreme (as in
winter here in Nova Scotia), you have to leave the sextant in the open
air for a while before starting so that it can equilibrate to the
temperatures that the sights will be made at. Also, the optics are not
the best. The field of view is narrow and the light-gathering power is
lower than I would like, which makes sights of the fainter stars
awkward. (Efficient use of the brief period when stars and horizon are
both visible becomes critical.)

Gary Harkins has pointed out that, if you are making serious voyages, a
Davis sextant would be useful in an abandon-ship bag. It would also be
useful on days when spray is flying, accurate sights are impossible and
a precious metal sextant could be easily damaged. For those of us taking
sights more for fun than navigation, starting with a Davis will teach
you to be very careful and precise. After working with one, using a
"real" sextant should seem easy and efficient by comparison!


Trevor Kenchington


--
Trevor J. Kenchington PhD                         Gadus@iStar.ca
Gadus Associates,                                 Office(902) 889-9250
R.R.#1, Musquodoboit Harbour,                     Fax   (902) 889-9251
Nova Scotia  B0J 2L0, CANADA                      Home  (902) 889-3555

                     Science Serving the Fisheries
                      http://home.istar.ca/~gadus

   

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