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Chuck Taylor wrote:

>In the course of teaching celestial navigation courses
>for the U.S. Power Squadrons, I have had the
>opportunity to examine more than a few Davis plastic
>sextants (maybe about a dozen different examples).  My
>observation is that they vary in quality, even within
>the same model.
>
>
I agree with this and from my own experience I can add to Chuck's list
of defects:

On one Mark 15, the sun shades are so loose that they don't stay in
position and fall down when swinging the arc or when a wave hits. There
is no way to adjust the tension (friction) in a controlled manner.
Different brands of honey that I poored into the bearing react different
to heat and spray. Fortunately, the shades have a tendency to fall into
the line of sight and not out of it!

On the same model, one focuses the telescope by pushing the inner tube
with the ocular into and out of the outer tube which carries the
objective. This in itself already poses a problem since the friction is
not strong enough to keep the two tubes in place when the sextant bumps
lightly against the face, which is unavoidable in any kind of wave
action. To make matters worse, the fit of the tubes is nearly air tight
so that the whole system acts like a dampened air spring. Setting the
focus becomes an elaborate task.


Chuck also pointed to

>Starpath has a discussion about and pictures of the
>currently available Davis plastic sextants on their
>web site
>

I am wondering about Starpath's recommendation of the Mark 3 as

"... the sextant of choice for coastal piloting."

while they are at the same time admitting that in celestial applications

"With good procedures you can obtain some 5 to 10 miles position
accuracy, providing you follow the careful procedures outlined in our
course materials."

What applications do they have in mind? In coastal navigation one often
does not have the luxury of unlimited time to follow "careful
procedures" (=averaging?) or a geometrically satisfying configuration of
targets. The quoted accuracy of the instrument (worse than 10')
precludes all applications where the angle is small, but critical; in
particular the use of all vertical angles such as, for example, the one
required for finding the distance from a lighthouse of given height.
This leaves problems involving rather large horizontal angles, such as
establishing a maximum angle between two suitable objects for avoiding a
danger zone. But is it not quicker to use a bearing compass for this?

I am probably missing the obvious. Can someone give me a good use for a
sextant of limited accuracy in coastal navigation?

Herbert Prinz

   

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