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    Re: Silicon Sea : Leg 69
    From: Sam Chan
    Date: 2001 Apr 10, 19:25 EDT

    > > Dan,
    > >
    > > I find no problems with the details and results from Mike Wescott. The
    > > calculation he shows is the Mercator sailing as shown in Cotter, the
    > > Elements of Navigation and Nautical Astronomy, Chapter 13. I got the
    > > result doing the calculations by hand. I entered the same problem into
    > > Starpath Star Pilot and got 12d 15.3N, 60d 09.6W.
    > The answer I got for Q3. DR= 13d 16.4'W  59d 32.3'W. The program I use
    > corrects for ellipsoid eccentricity and is a Rhumb-line formula. Mike's
    > working is OK, but I think the elapsed time is off.
    Using a new course of 298T, I get the following results using a method that
    account for the ellipsoid eccentricity:
    DR = 13d 21.8N, 59d 44.3W. I used a table of Latitude Parts to calculate the
    difference in latitude parts in place of the difference in latitude. The
    table is similar to the table of Meridional parts. The table of Latitude
    parts was generated using the eccentricity of WGS84. What method is used in
    your program?
    > In setting up the problems I work some backwards from the answer I want.
    > > For question 7, I got an answer of CTS 338 C with a SMG of 10.9 Kt. The
    > > Star Pilot gave an answer of CTS 324.4T and SMG of 10.9. Applying the
    > > 15dW (add), and a Dev 1dE (subtract), I arrive at a CTS of 338.4 C. I
    > > double checked the Star Pilot with a plot on the Mo Board and also
    > > at a CTS of 324T.
    > Using several checks I keep getting 332.0d. But the DR being off would
    > account for the difference.
     Question 7 is a problem in current sailing and I don't see how the DR
    position would affect it other than the magnetic variation which was given
    in the problem. Using the Mo Board, I constructed the CMG at 318T. I then
    construct the current set and drift at 270T /1.5 kt. Both vectors starts at
    the center of the board. I then took a divider set at 10 kts, the boat
    speed, place one end at the head of the current vector and the other end on
    the CMG line. The line connecting the two points was my CTS. This is how I
    arrived at a CTS of 324T. The correction to compass course is the standard
    TVMDC AW. See any problems?
    Sam Chan

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