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    Re: New Member, No Assumed Position Watches
    From: David Romer
    Date: 1996 Dec 04, 07:00 EST

    Doug Dotson wrote:
    >        1. No Assummed Position techniques are not really practical in
    > most cases, which is pretty obvious to me and apparently many
    > others (thankfully).
    I can't really agree, Doug. There are a variety of No-Assumed-Position
    techniques that have been widely and successfully used, including Pepperday's
    nine-page S-tables and most of the navigational calculators out there. The
    problem isn't with Triangle's ESAE (or whatever they call it) method but with
    their promulgation of the idea that their method can give an accurate lat/lon
    fix based on two near-simultaneous sights of the same body.
    You could use _any_ sight reduction method to produce two azimuths and
    intercepts from two nearly simultaneous sights of the same body and then find an
    "accurate" lat/lon using the resultant azimuths and intercepts and formulae in
    the Nautical Almanac. However, such a fix couldn't be relied upon . . . even if
    produced by Triangle's method. Regardless of method, it is still important to
    select sights such that the azimuth's of the sighted body are appropriately
    spaced.
    Doug continues:
    > 2. The old battle of plastic vs. metal sextants.  I bought what I think is a
    > good plastic sextant (Davis a little less than $200). Personally I think
    > plastic makes a lot of sense because:
    >
    >        a. Less physical distortion due to temperature changes.
    >
    >       b. If dropped a plastic sextant will not bend permanently (assuming
    > it does break). Of course the optics will have to be realigned but that's
    > true of the brass models as well.
    Well, Doug, let me first say that there isn't anything wrong with a good plastic
    sextant. In fact, I own a Davis Mk 25 (in addition to a Kelvin Hughes and
    Cassens & Plath, both brass instruments), and it is perfectly capable of being
    used for practical navigation.
    However, your assertion that the plastic instrument suffers less physical
    distortion due to temperature changes than the traditional brass instrument
    doesn't hold true in the real world. I think you have it backwards. The fact is
    that plastic sextants suffer large temperature-change-induced distortion, much
    more so than metal instruments. The result is that index error (as well as other
    mirror adjustments) can vary widely, even within the short span of a single run
    of sights. In contrast, metal instruments "hold" their adjustments and if
    properly handled rarely require adjustment.
    This isn't all bad. Too often beginning navigators are so paranoid about doing
    something wrong while adjusting their sextant that they avoid properly adjusting
    the instrument. Because a plastic unit will require frequent and constant
    adjustment, they soon get over the fear.
    I'd hate to drop _any_ sextant. As Bruce Bauer says, they don't bounce very
    well. But if I had to select between dropping my Davis or my Cassens & Plath, I
    agree with you: I'd rather drop the Davis . . . not because it wouldn't bend,
    but because if it were damaged I could replace it at a fraction of the cost of
    the C&P.
    >From my perspective, the Davis has only one significant advantage: It costs less
    than a good-quality metal instrument. The fact that it weighs less might also be
    an advantage to those people (such as my wife) who find holding a four- or
    five-pound instrument difficult.
    Regards,
    -- David
    

       
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