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Subject: Re: Silicon Sea: Leg 71
From: Dan Hogan (dhhogan@XXX.XXX)
Date: Tue May 15 2001 - 09:59:20 EDT
On 14 May 2001, at 21:52, Paul Hirose wrote:
> 1. DR position at log reading 89.6 is 13 17 N 63 02 W.
OK
> The problem doesn't say when the log was zeroed. I assume that happened
> when departure was taken.
>
> There's enough info to compute the estimated position (allowing for
> current) too: 13 16 N 63 09 W.
I don't make trick questions(intentionally), so no need to answer more than
what the question asks for.
)
> 2. Lacking an almanac, I don't know what celestial phenomenon is
> occurring at the DR time. Could probably figure it out from the USNO Web
> site, but that's too easy.
Well an Almanac or Almanac program would give you the answers for Sunrise,
Sunset, twilight and time of LAN problems.
> 3. Fix May 30 0852 Z is 13 02 N 61 29 W. This is 24 miles south of the
> departure position, yet we were supposed to have sailed 90 miles west!
> I'm puzzled that two other people got results substantially identical to
> mine but didn't comment on this gross discrepancy in the DR.
Your latitude is off by a degree. I checked my data and it looks ok.
> The sights are so close in time I decided to ignore vessel's movement
> and just use the mid time of the sights as the fix time. LOP
> intersections were a bit sloppy, probably due to the long intercepts.
> Given the realities of small craft celestial, I didn't think it
> worthwhile to re-reduce from a better assumed position.
>
> > NOTE: Sextant Index correction is on the arc 1.5d.
Right, it should read on the arc 1.5'.
>
> I got confused over the above. Of course common sense tells me it's got
> to be 1.5 minutes, not degrees. But it's not clear what sign to use for
> the correction when it's described that way.
A menomic to use is "On the arc off(-), off the arc on(+)."
> For a more realistic problem, don't tell us the IC. See if we know how
> to get it. E.g., "Just before the round of sights, the images of the
> horizon are brought to coincidence to determine index correction. The
> sextant reads 1.5'."
>
>
> 4. Aruba Gap is 502 miles and 275 true from the fix.
OK
> 5. Compass course to Aruba Gap is 286.
OK
> 6. Time enroute will be 49.5 hours.
OK
> 7. Speed made good to Aruba Gap should be 10.1 knots.
OK
>
> Used the USNO celestial nav Web page to get celestial body coordinates,
> but performed sight reductions with HO 229. Did all plotting on pieces
> of typing paper. Time/speed/distance computations performed on an E-6B
> aviation "whiz wheel".
Explains your plotting. I still have my original plastic Weems E-6B and a
metal "Aerodyne" issued by the company. In practical use I only interpolated
to the nearest 0.5 Degree.
Dan Hogan WA6PBY
Catalina 27 "GACHA"
dhhogan@XXX.XXX
Navigation-L List Owner
http://www.wa6pby.com
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