# NavList:

## 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 09, 23:01 EDT

On 9 Apr 2001, at 11:43, Michael Wescott wrote: > I tried sending this once before but it seems to have gotten lost ... > > Well this one has me stumped. I'm either missing something, or I'm > so rusty that I'm a danger to myself and others should I get on my > boat anytime soon. > > > > > Dead-Reckoning problem Leg 69 040401 Test your knowledge! > > ---------------------- ------ ------ -------------------- > > > > The last DR position was(10d 27.9'N 52d 26.4'W) on 18/05/2000 > > @XXX.XXX Dev > > 1.5dE. Current Set 285d Drift 0.5Kts. > > Figure roughly 10hrs @XXX.XXX Using traverse tables > for 289 d we get > > D.Lat = 34.8 > Dep = 101.2 > D.Lo ~= 101.2 / cos 11 = 103.1 > > 10 27.9N 52 26.4W > 34.8 103.1 > ========= ========= > 11 02.7N 54 09.5W My computed answer DR--11d 01.7'N 54d 06.3'W You are OK a good answers when using Traverse Tables. > > Using A.P. of 11 00.0 N and 54 00.0 W. > > > On that evening we take the following sights: > > > > Body UT Hs > > > > Sirius 22:15:30 28d 16.3' > > Spica 22:17:02 42d 20.6' > > Avior 22:19:00 15d 55.3' > > Bellatrix 22:19:45 13d 26.5 > > Dubhe 22:20:12 37d 56.1' > > > 1) What is our FIX position? > > -- ------------------------- > > Using AP 11N 54W: > > Sirius Spica Avior Bellatrix Dubhe > UT 22:15:30 22:17:02 22:19:00 22:19:45 22:20:12 > > Hs 28 16.3 42 20.6 15 55.3 13 26.5 37 56.1 > IC -2.5 -2.5 -2.5 -2.5 -2.5 > Dip (9') -2.9 -2.9 -2.9 -2.9 -2.9 > ======== ======== ======== ======== ======== ======== > Ha 28 10.9 42 15.2 15 49.9 13 21.1 37 50.7 > R -1.8 -1.1 -3.4 -4.0 -1.2 > ======= ======== ======== ======== ======== ======== > Ho 28 09.1 42 14.1 15 46.5 13 17.1 37 50.7 > > GHA-A(:00) 206 53.2 206 53.2 206 53.2 206 53.2 206 53.2 > GHA-A(inc) 3 53.1 4 16.2 4 45.8 4 57.1 5 03.8 > SHA 258 43.0 158 41.8 234 22.5 278 43.3 194 04.0 > Long -54 00.0 -54 00.0 -54 00.0 -54 00.0 -54 00.0 > ========== ======== ======== ======== ======== ======== > GHA-star 415 29.3 315 51.2 392 01.5 436 33.6 352 01.0 > > Lat 11 00.0 11 00.0 11 00.0 11 00.0 11 00.0 > Dec -16 43.2 -11 09.8 -59 30.9 6 20.8 61 45.3 > > Hc 28 32.3 40 51.1 14 56.3 14 21.0 38 54.8 > Z 243.9 115.4 196.2 173.8 004.8 > > Delta(dir) 23.2 A 83.0 T 50.2 T 64.0 A 65.3 A > > Plotting these gives a fix near 10 00.0 N and 53 00.0W. Reducing sights > again with A.P. of 10 N and 53 W: > > GHA-star 416 29.3 316 51.2 393 01.5 437 33.6 353 01.0 > Lat 10 00.0 10 00.0 10 00.0 10 00.0 10 00.0 > Dec -16 43.2 -11 09.8 -59 30.9 6 20.8 61 45.3 > > Hc 28 05.1 42 10.1 15 37.1 13 18.0 37 59.6 > Z 244.8 115.1 196.7 274.2 004.2 > > Delta 4.0 T 4.0 T 9.4 T 0.9 A 10.1 A > > Plotting these gives a good fix at about 9d 50.5'N 53d 00.0' W > > This is a bit far off course. What did I get wrong? My computed answer-- 9d 50.0'N 53d 00.2'W Don't see anything wrong. Your answers are within 0.5' of accuracy. > Every thing gets further askew from here on... > > > 2) What is the TC and distance to Barbados(13d 10.0'N 59d 20.0'W)? -- > > ---------------------------------------------------------------- > > Hmmm ... isn't that a few miles east of Barbados? Yes. I am using approach positions given in Ocean Passages for the World and Cornell, World Cruising Routes. You don't want to run up on the island. > > Barbados 13 10.0 59 20.0 > Fix - 9 50.5 - 53 00.0 > ======== ======== > D.Lat = 3 19.5 D.Lo = 6 20.0 > = 199.5' = 380.0' > > dmp = mp(13d 10.0') - mp(9d 50.5') > = 202.3 > > TC = 270 + arctan(202.3/380.0) > = 270 + 28 > = 298d > > Dist = 199.5/sin 28 > = 424.5 Very OK here. > > We continue sailing maintaining the same speed and True Course with an > > overcast sky. Then, with the navigator on deck waiting for an opening > > for a Sun shot, we see a spot of land on the horizon ahead. It is > > 20/05/2000 @XXX.XXX > > > 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. > > 4) What day of the week is this? > > -- ----------------------------- > > >From the almanac -- Saturday. Right > This I'm (almost) sure I have right :-) Your procedure seems OK. It may be the method you are using. Dan Hogan WA6PBY Catalina 27 "GACHA" dhhogan@XXX.XXX Navigation-L List Owner http://www.wa6pby.com