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    Re: Silicon Sea : Leg 69
    From: Dan Hogan
    Date: 2001 Apr 10, 10:12 EDT

    On 9 Apr 2001, at 21:13, Sam Chan wrote:
    > ----- Original Message -----
    >
    > > >
    > > > > 3)  What is the DR position?
    > > > > --  ------------------------
    > > >
    > > > S = 10.7 C = 288.9 T,
    > > >
    > > >   20/05/2000 12:13:00 ZT = 20/05/2000 16:13:00 UT
    > > >                           -18/05/2000 22:20:00 UT
    > > >                          ========================
    > > >                             1 day     17:53:00
    > > > or 41.88 hrs
    > > >
    > > > Dist = 41.88 * 10.7 = 448.1 nmi
    > > >
    > > > D.Lat = 448.1 * cos 288.7 = 145.2 = 2d 25.2'
    > > >
    > > > DR Lat = 9d 50.5' + 2d 25.2' = 12d 15.7'
    > > >  dmp = mp(12d 15.7') - mp(9d 50.5')
    > > >      = 147.0
    > > > D.Lo = 147.0 * tan(288.9)
    > > >      = -429.4
    > > >      = -7d 09.4
    > > > DR Lo = -53d  00.0 - 7d 09.4'
    > > >       = -60d 09.4
    > > >
    > > > DR  12d 15.7'N    60d 09.4'W
    > > >
    > > > Actually this is too far off to make any sense.
    > > > We seem to be well past Barbados.
    > >
    > > It is off. If you are calculating use a Traverse Table interpolation may
    > > be accumlating an error. A better method would be to use one of the
    > > Sailings methods for DR. For sight reduction at least using a Law of
    > > Cosines formula. If you insist on working it "longhand" download
    > > 4mulas.zip from the web site.
    > >
    >
    > 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 same
    > result doing the calculations by hand. I entered the same problem into the
    > 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.
    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 Var
    > 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 arrived
    > at a CTS of 324T.
    Using several checks I keep getting 332.0d. But the DR being off would
    account for the difference.
    Dan Hogan WA6PBY
    Catalina 27 "GACHA"
    dhhogan{at}XXX.XXX
    Navigation-L List Owner
    http://www.wa6pby.com
    

       
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