# NavList:

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

**Re: Silicon Sea: Leg 77**

**From:**Michael Wescott

**Date:**2001 Jul 25, 8:29 PM

> Promptly at Sunrise on 24/06/2001 we raise anchor at Isl Flamenco > (08d 55.2'N 79d 31.6'W) and head for our departure point at Isla > San Jose Light(08d 10,5'N 79d 31.6'W) motoring. Is Isla San Jose Light truly due south of our anchorage? > Speed = SQRT LWL x 1.34 > 1) What is the Zone Time (ZT) of our Estimated Time of Arrival(ETA) > at the Isla San Jose Light? > What is the UT/GMT ETA? > -- ---------------------------------------------------------------- Sunrise at Isl Flamenco should be: 0541 (Almanac entry for 10d N) + 2 (Interpolation for Latitude) + 18 (4d 31.6'west of center of zone) ==== 0601 Speed is 1.34 sqrt(39) = 8.4 kn Dist = 44.7 mi (by inspection) Time en route = 44.7/8.4 = 5.32 hr (5h 19m) ETA = 1120 ZT (GMT-5) = 1620 UTC > On our ETA at Isla San Jose we take a departure FIX off of the light that is > (08d 55.2'N 079d 29.4'W). We set our course for the Mid-Ocean Point(MOP) at > (03d 00.0'N 110d 00.0'W). Been motoring in circles, have we? We're only a couple of miles from our anchorage. > Variation 2.0dW, Dev 0.5dE. Drift 0.6 Kts Set 285.0d. Speed 10.1 Kts. > 2) What is the TC and Distance from Isla San Jose Light to the MOP? > -- ---------------------------------------------------------------- dLat = 355.2' dLo = 1830.6' dep = dLo * cos 6 = 1820.6 arctan(355.2/1820.6) = 11d sqrt(355.2^2 + 1820.6^2) = 1854.9 C=259T, D=1854.9 > We plot our track on INT Chart 811 and see that we can not head direct for > the MOP(03d 00.0'N 110d 00.0'W) without crossing land above Punta Mala in > Panama. So we alter course to MOP2 at (05d 00.0'N 80d 00.0'W). > Variation 2.0dW, Dev 0.5dE. Drift 0.6 Kts Set 285.0d. Speed 10.1 Kts. > 3) What is the True Course and distance to MOP2? > What is the Compass Course(CC)/Course-to-Steer? > -- ----------------------------------------------- Assuming the course changed at 08d 55.2'N 079d 29.4'W, i.e. immediately. dLat = 235.2 dLo = 30.6 dep = 30.6 * cos 7 = 30.4 arctan (235.2/30.4) = 82.6 sqrt (30.4^2 + 235.2^2) = 237.2 D = 237.2 C = 187.4T = 189.4M = 188.9C Correcting for current (plotting sheet) C = 184T = 186M = 185.5C > At 14:45:30UT the next morning we take a SUN LL sight. Other Data as above. > Hs 47d 54.6'' > 4) What is the DR position? > -- ------------------------ dTime = 2400 + 1445 - 1620 = 2225 = 22.4 hr D = 10.1 * 22.4 = 226.24 dLat = cos 187.4 * 226.24 = 224.4'S dep = sin 187.4 * 226.24 = 29.1 mi W dLo = 29.1/cos 7 = 29.3'W DR = 5d 10.8'N 79d 58.7'W > At 22:45:23UT we take another sight of the SUN LL > Hs 15d 34.3' > 5) What is the DR position? > -- ------------------------ dTime = 2245 - 1445 = 8hrs D = 10.1 * 8 = 80.8 mi dLat = cos 187.4 * 80.8 = 80.1'S dep = sin 187.4 * 80.8 = 10.4 mi W dLo = 10.4/cos 4.5 = 10.4'W DR = 3d 50.7'N 80d 09.6'W > 6) What is the Running FIX(RFIX) position at the 22:35:23UT sight? > -- --------------------------------------------------------------- Which is wrong, this time or the 22:45:23 above? At 22:35:23, I get a Ho = 12d 31.7' at the DR. At 22:45:23, I get a Ho = 10d 14.0' at the DR. Something is way off. Was the time actually 22:25:23? > Checking our Running FIX against the position of MOP2(05d 00.0'N 080d > 00.0'W) it seems we have passed MOP2. So we alter course to our original Mid- > Ocean Point which we re-name MOP1(03d 00.0'N 110d 00.0'W). > Speed 10.5 Kts. Current Set 285, Drift 0.6 Kts. Var 1dW Dev 2dE. > 7) What is the TC and Dist to the MOP1(03d 00.0'N 110d 00.0'W)? > What is the Compass Course(CC)/Course-to-Steer? > -- ------------------------------------------------------------- Form DR 3d 50.7'N 80d 09.6'W to MOP1: dLat = 50.7' dLo = 29d 50.4' = 1790.4 dep = 1790.4 * cos 3.5 = 1787.1 arctan(50.7/1787.1) = 1.6 C = 268.4T = 269.4M = 267.4C D = sqrt(1787.1^2 + 50.7^2) = 1787.8 Correcting for current (plotting sheet): C = 267T = 268M = 266C