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## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

**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