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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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