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    Re: Plastic sextants and prejudice. was: GPS shortcomings.
    From: Dave Weilacher
    Date: 2005 Jun 10, 08:03 -0400

    The only thing a sextant does is measure an angle. (unless you are John Wayne)
    On a small boat (or a prudent navigator), you take a bunch of shots in order 
    to determine the likely best. You combine this with other lines of position 
    and a DR and all of it, as a whole, determines where you are, not the 
    composition of the sextant frame.
    They navigated for centuries with a gadget that you had to eyeball the horizon 
    and the sun (etc) with alternating eye movements.
    If you never use an angle closer than one minute and drop all precision in 
    table angles that is less than a minute plus exclude the z-correction and the 
    correction for minutes in declination.  The resulting lines of position you 
    achieve are still close enough to navigate offshore with. On a small boat, 
    you can even leave off the correction for dip and always take the 
    semi-diameter of the sun as 16 minutes, ignoring parallax entirely.  (This is 
    how I find my sight reduction errors when something goes amiss.  It amazes me 
    at just how close the result tends to be compared to a proper reduction.  I 
    am NOT advocating this in place of using any given precision.  I'm just 
    arguing that the quality of the cheapest plastic sextant is adequate to the 
    task and likely far superior to what was once used)
    I would far rather have a Davis Mark 3 than a backstaff.  BTW, I truly like my 
    Astra 3b. Compared to my Davis Mark 25, the optics are great.
    -----Original Message-----
    From: Robert Gainer 
    Sent: Jun 10, 2005 5:48 AM
    Subject: Re: Plastic sextants and prejudice. was: GPS shortcomings.
    My first sextant was a 13 dollar plastic Davis, the one with a solid frame
    and it was made before they even blackened the marks and numbers. I used
    that on my first Transatlantic on a 22-foot boat and made my landfall at
    Bishop Rock Light about ? mile to a mile north of my intended track. Used
    with care it is more then accurate enough to navigate within the limits
    obtainable from a small boat given the height of eye and movement of the
    The gray plastic English made Ebbco (East Banks Boat Company) is even better
    and more accurate then the cheapest Davis. The Ebbco even has a micrometer
    drum that the original Davis did not. I have not used the more expensive
    plastic sextants except to just hold them and look at them, so I can make no
    comment about them in use.
    I bought my first metal sextant because it was embarrassing to do a delivery
    or be the navigator during a race using a plastic sextant. Of course, I
    would never go back to plastic, but the addition of a metal instrument to my
    kit did not add anything to the precision of my landfalls. To me, this means
    either I am terrible at navigation or the Davis is equal to the job.
    I am not saying that the plastic is as good as the metal. Just that it is
    adequate and I would not be uncomfortable aboard a boat were the skipper
    used one as the primary means of navigation. My only question is about the
    skill of the user, not the accuracy of the instrument.
    Robert Gainer
    >From: Peter Fogg 
    >Reply-To: Navigation Mailing List 
    >Subject: Re: Plastic sextants and prejudice. was: GPS shortcomings.
    >Date: Fri, 10 Jun 2005 07:16:35 +1000
    >From George, with claws showing ever so slightly:
    > >
    > > I don't pretend that such sextants are perfect, and we have discussed
    > > defects more than once on this list. But Peter Fogg's comments read more
    > > as
    > > a rehearsal of prejudice than a reasoned evaluation.
    > >
    >There was no claim to a "reasoned evaluation", just a report of my limited
    >experience, given somewhat reluctantly after being specifically asked. Yes,
    >it was a Davis, I don't know which model - grey with a solid arc of
    >I'm prejudiced about very little and certainly not sextants - the doors of
    >my mind are blocked eternally wide open!
    >However, what I've just read here inspires even less confidence; it all
    >too much along the lines of: ' yes, they may have this and that and the
    >other shortcomings, but they're really not so terrible after all; given
    >this, that, and the other caveats'.
    >Much too defensive. Nevertheless, I'm not really all that into sextants as
    >such, and think it entirely appropriate that each navigator choose his own
    >instrument after careful reflection.
    >If I needed a hammer I would probably buy the cheapest on offer, why pay
    >more? If a carpenter chooses a hammer it is done with considerably more
    >care, and he may pay several hundreds of dollars. It makes sense from his
    >point of view; such things as the balance and the composition of the steel
    >are very important to him - just as a crude blunt instrument is all I need
    >from the same tool.
    Don?t just search. Find. Check out the new MSN Search!
    Dave Weilacher
    .IBM AS400 RPG contract programmer
    .USCG Master lic. 100 ton
    .ASA Sailing Instructor Evaluator

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