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    Re: Silicon Sea: Leg 71
    From: Dan Hogan
    Date: 2001 May 15, 09:59 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.
    > 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.
    > 5. Compass course to Aruba Gap is 286.
    > 6. Time enroute will be 49.5 hours.
    > 7. Speed made good to Aruba Gap should be 10.1 knots.
    > 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"
    Navigation-L List Owner

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