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    Re : Silicon Sea: Leg 70
    From: Dan Hogan
    Date: 2001 Apr 23, 13:51 EDT

    Sam:
    Sorry for the delay on my response. I had to unscramble the layout. When
    layouts exceed the width of the screen adding the reply symbol messes it up.
    On 22 Apr 2001, at 13:54, Sam Chan wrote:
    > 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
    OK. Distances will vary depending on what method is used for calculation.
    Occasionally TC will vary +/_ a degree depending on the method and direction
    > Enroute time {at}XXX.XXX8m
    >
    > 2. Days to Cristobal. To the nearest tenth, 6.8 days.
    OK
    > 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.
    OK. Out of curiosity, what program are you using for your calculations?
    >
    > 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.
    OK.
    >
    > 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.
    OK.
    > 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.
    OK.
    > 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.
    OK.
    > Sam Chan
    Dan Hogan
    dhhogan{at}XXX.XXX
    

       
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