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> As I read David's response and leaving all of David's
> arguments as valid, it seems to me that you arrive at
> an interesting picture that says: "In this test the
> plastic sextants were substantially less accurate than
> the metal sextants". Reason unknown.

Hello Mike

I think he additionally questioned whether index error was accounted for in
any of the sextants.

Most all of the intercepts were bad to plain horrible, metal or plastic.  I
just do not believe the metal or plastic could be that bad, or that plastic
could be that much worse than plastic in any hands.  In practiced hands a
plastic unit should meet or beat all but the C&P numbers from a beach.
Burch about equals all but the C&P on a boat in rough seas with plastic
units, and without slope fitting or his "good proceedures."  Espen Ore is
obviously producing excellent results on land with a natural and artificial
horizon and Davis Mark III.

It is just hard to imagine the CW observers were that bad.  I have handed my
Astra to a friend that had never held a sextant before (350 Catalina, 2-3
foot swells), explained the goal (kiss the Sun's lower limb *exactly* to the
horizon), put him within two degrees of touch and turned him loose.
Averaged and reduced his first three sights.  (No practice sights).  His
intercept was approx 7'--with a very hazy horizon!
>
> Does that mean that plastic sextants are bad? No it
> does not. Does it mean that you can't get accurate
> sights from them? No it does not - many list members
> and David Burch attest to that.

Agreed.  (Don't listen to the man behind the curtain.) I got intercepts at
the 5' and 7' level (two observations, each individually reduced) with my
cardboard unit my first time with both a natural horizon and on the water.
Catalina 34' under sail in confused 6-8 footers with some 12-foot swells and
haystacks, against a background of the Indiana Dunes.  As Ken Gebhart has
stated, folks make passages with nothing more than a sextant, globe and
almanac. As a list member pointed out, the pencil line on an ocean chart
might be wider than any sight error.

Burch's beach shots with a Davis MK15 were 4.0' Away plus/minus 2.3.  By his
own account the results could be improved by fitting to the slope and
following good proceedures later outlined in his Plastic Sextant PDF.
Causing me to feel he could easily have gotten in the 2'-3' range or better
from the beach.  The then 15-year-old Astra came in at 0.5' Away  plus/minus
0.6  Again he claims it could better with slope fitting (0.4 Away plus/minus
0.4) or better.

His beach results with the bottom-of-the-line MK3 were 1.6' Towards
plus/minus 1.6. About this he says, "This of course is most excellent, and
must be considered part luck. One should consider a consistent under 10
miles as good for this device. On the other hand, it is not surprising to us
to see the Mk 3 do as well as the Mk 15. We have noticed this in the past. I
am not sure it will hold up if we use good procedures with the Mk 15, See
Ref [1]. Part of the reason is you must always move the index arm in the
same direction. At least with my operation of it, the only way I can very
carefully squeeze and push it to a new angle is to push it down. So if I am
below the angle, I must crudely set it too high and then carefully push
it down. In other words, the very simple design of the instrument forces
users to operate it in a consistent way. And ? most important ? this is the
same motion needed to check the IC, so both are done in the same manner.
Also, since there are no optics and the arm is so difficult to set
carefully, one is forced to use the ?set and wait? method discussed in Ref.
[1] which is the best way to do plastic sextant sights."

Having read Burch's letter, and later your posting, but not the CW article,
it caused a few lights to go on.  Sounded like they could be the same.
Thought it best to determine if they were in fact the same, and you allow
you and others to draw your own conclusions about the numbers you posted.  I
feel the CW article is suspect.

If I sound like the newly converted, that is not the case. Please check my
Oct. 8, 2004 post in a long thread about "averaging."  Linear regression vs.
simple averaging was debated ad infinitum.  You will find my reference to
David Burch (and his fit-slope method), establishing a timeline for my
Burch-o-mania.

Putting it all in perspective, when I did my first cardboard sights and
reduced them, my long-time sailing buddies were having a good laugh at the
instrument and the 5-7 mile range.  What good is it, they chuckled?  I
pointed out that were in coastal piloting range.  If their GPS failed in the
middle of the Atlantic, would 5-7 miles be adequate?  They agreed it would
be.  Then I went below, fired up the seldom-used Loran (having checked it
out earlier) and asked them to compare it to the GPS.  It was 10 miles off!
This is what the boat owner planned to use as backup if his GPS went out. No
more laughter--except for the concept of getting the unit wet ;-)

Bill

   

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