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    Re : Silicon Sea: Leg 70
    From: Sam Chan
    Date: 2001 Apr 22, 16:54 EDT

    Here are some of the numbers I came up with:
    1. Distance to Cristobal. I computed the distance between the various way
    points using three methods: (1) traditional Mercator sailing using tables of
    Maridional Parts, (2) Mercator sailing using a table of Latitude Parts which
    accounts for the eccentricity of the earth when calculating the meridian
    distance from initial to final latitude, and (3) the Starpath Starpilot
    which uses mid-latitude sailing. Only the distance is different between
    these three methods. The course are identical.
    Way Point                          D by (1)         D by (2)        D by (3)
    Course (T)
    Bridgetown
    St Lucia/St Vincent Pass          84.8              84.2            84.5
    286.8
    MOP1                                  522.0            518.8          523.2
    263.4
    Cristobal                               618.4            614.6
    616.9                 252.4
    Total                                   1225.2          1217.6        1224.6
    Enroute time {at}XXX.XXX8m
    2. Days to Cristobal. To the nearest tenth, 6.8 days.
    3. ETA in Zone time at the St. Lucia/ St. Vincent Pass waypoint.
    Sunrise on 29/5/2001 at Bridgetown 13d 05.5'N, 59d 47.4'W is 0532Q. The
    central meridian for the time zone is 60d W and the 12.6' of longitude east
    of the central meridian did not affect the SR time significantly. So
    departure is at 0532Q.
    There are two ways to get at the ETA. The course to the waypoint is 286.8.
    If we go with the boat speed of 8.1 kts, and the distance is 84.8nm, the
    enroute time is 10h 28m and the ETA is 1600Q. However, given the current set
    of 270, drift of 0.7 Kts, and steering 288 T, the resulting SOA is 8.8 Kts
    on a Track of 286.8 T. The enroute time is then 9h 38m with an ETA of 1510Q.
    4. Compass course/Course-to-steer (CTS). Setting a course so that our Track
    is 286.8 T, the CTS is 303 C. When the current is not accounted for, the CTS
    is 302 C. This is to be expected since the current set is so close to the
    desired track.
    5. Departure point position to nearest minute.
    St. Lucia at 292.2 d C is 279.2 T
    St. Vincent at 129.7d C is 116.7 T
    By plotting out the fix on a plotting sheet, the fix is at 13d 25.5' N, 61d
    29.8' W.
    6. TC and distance to MOP1 from departure point from 5.
    The course is 263.7 T. The distance is 502.8 NM using method 2.
    7. CC/CTS for MOP1.
    The set of the current is so close to the desired track that it has very
    little impact on the CTS. So CTS is 274.5 C.
    Sam Chan
    

       
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