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    Re: Leg 83
    From: Arthur Pearson
    Date: 2002 Sep 22, 22:33 -0400

    Peter et. al.
    Many thanks for these exercises, they are a great project now that the
    boat is put away for the winter and we have to find another way to keep
    the skills fresh.
    I am consistently getting 290d True for Q2 below. I am using 1958
    Bowditch Traverse tables and I have worked the problem as a Mercator
    Sailing (looking up the MP's of the two latitudes to get m, dividing
    that into DLo, using that ratio to get a NW bearing) and as a Mid
    Latitude Sailing (using DLo at 19d and 20d to interpolate for departure
    (p) and dividing by change in latitude (l) and using that ratio to get a
    NW bearing. Both methods bring me to 290d as does my PalmPilot running
    CelestNav for a Mercator Sailing. In no case can I find my way to 285d.
    I concur that the adjustment for current is about -1d and the combined
    variation and deviation is -11d, bringing me to a course to steer of
    Where have I strayed?
    -----Original Message-----
    From: Navigation Mailing List
    [mailto:NAVIGATION-L@LISTSERV.WEBKAHUNA.COM] On Behalf Of Peter Fogg
    Sent: Saturday, September 21, 2002 11:52 PM
    Subject: Leg 83
     Q1)          navigator (9ft) to horizon      3.51nm
    Mauna Loa (4169m) to horizon         +133.66 nm
          navigator to top ML on horizon    137.2 nm
    Q2) First calculate TC along rhumb line from (N18d 37'.2  W152d 13'.6)
    to (N19d 30'.0  W155d 36'.0)
           by MP&MD  285.3d
               MP           285.3
           PS (ML)        285.4
           by plot          285d
    Then, by plotting, with fast speed and weak contrary current,  I get CMG
    286d and SMG 9.3k,
           thus TC to steer  284d
    combined error     E   -11d
    CC to steer                273d
    Q3) Time Zone at (W150d +/- 7.5d) is West 10 hours, 10 hours behind UT.
    Q4) traverse from N18d 37'.2   W152d 13'.6    at 290d  for 140 nm.
           MP&MD       N19d 25'.3   W154d 32'.5
            ML              N19d 25'.1   W154d 32'.0
            plot              N19d 24'      W154d 31'.5
        elapsed time is 140 nm / SMG 9.3k = 15h 03m 14s
    Q5) Am a little unsure what is meant to be done here, assume what is
    needed is the CC to steer from
        calculated DR (N19d 25'.3  W154d 32'.5) to DO point (N19d 30'.0
    W154d 41'.6).
        First calculate TC along rhumb line   (distance
              MP&MD 298.6d                           9.8 nm
                MP       298.5                             9.8
             PS(ML)   298.7                             9.8
                plot      299.0d                          12.0 nm - from
    plotted DR)
        Then another vector diagram, this time with current setting to the
    west. TC to steer 300d
    combined error     E   -11d
    CC to steer                289d
        elapsed time is 9.8 nm / SMG 10.5k = 00h 56m 00s
    Q6) rhumbline calculation for distance from (N19d 30'.0  W154d 41'.6)
    to (N19d44'.0  W155d 04'.0)
        by MP&MD  25.3 nm
             MP          25.4
             PS (ML)  25.3
                 plot     25.8
    There is no information on current for this leg so shall assume speed is
    elapsed time is 25.3nm / 10k = 02h 31m 48s
    Q7)     For the ETA at Hilo add the time for each leg to   01:44:36 UT
    on 16/07/2001
    + 15:03:14
    + 00:56:00
    + 02:31:48
    = 20:15:38 UT on 16/07/2001
    = 10:15:38 ZT on 16/07/2001
    MP&MD     Meridional Parts and Meridional Distances
    MP             Meridional Parts
    PS (ML)      Plane Sailing (Middle Latitude)
    As all the distances involved in these rhumb line and traverse
    calculations are fairly short ones in low latitudes there is little
    difference between the different calculations.

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