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    Re: Silicon Sea: Leg 71
    From: Paul Hirose
    Date: 2001 May 15, 00:52 EDT

    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.
    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.
    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.
    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.
    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.
    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".
    paulhirose{at}XXX.XXX (Paul Hirose)

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