NavList:
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Re: Silicon Sea: Leg 71
From: Michael Wescott
Date: 2001 May 12, 14:24 EDT
From: Michael Wescott
Date: 2001 May 12, 14:24 EDT
> Sight Reduction and > Dead-Reckoning problem Leg 71 050801 Test your knowledge! > ---------------------- ------ ------ -------------------- > At 18:27:00ZT @XXX.XXX6'N > 61d 30.1'W) as we passed between the Lessor Antilles islands of St, Lucia and > St. Vincent. We are bound for, Mid Ocean Point(MOP1)(12d 30.0'N 70d 00.0'W), > in water of more than 1000 mtrs North of Aruba. This is to avoid unfavorable > currents along the coast. > Dev 1.5dE, Var 12dW. Speed is 8.7 Kts, Current Drift 0.7Kts, Set 270.0d. > TC 263.7d. > 1) At the log reading of 89.6Nmi, what is the DR position? > -- ------------------------------------------------------- Using mid-Latitude computation (since Lat is low and course is almost due west, error due to choice of middle latitude is low as well). dLat = 89.6 * cos 263.7 = -9.8 dLo = (89.6 * sin 263.7)/cos 13d 20.0 = -91.5 Lat = 13d 25.6'N - 09.8 = 13d 15.8N Lon = -61d 30.1' - 91.5 = 63d 01.6W > 2) At the DR position what is the celestial phenomena that is starting > at this DR Zone Time(ZT)? > -- ------------------------------------------------------------------- Time enroute = 89.6/8.7 = 10h 18m ZT = 18:27:00 + 10:18:00 = 04:45:00 30/05/2001 Nautical Twilight. > NOTE: Sextant Index correction is on the arc 1.5d. > We then take the following sights: > Body UT Hs > Fomalhaut 08:50:10 40d 54.7' > Polaris 08:50:55 13d 17.3' > Deneb 08:51:05 56d 56.3' > Mars 08:52:03 25d 35.0' > Venus 08:53:22 28d 13.8' > Vega 08:53:45 45d 55.6' > 3) What is the FIX position? > -- ------------------------- Polaris is almost due north, and Venus due east. From the Polaris table in the back of the almanac we find a Lat of 13d 01.5'N given an AP of 13d 15.8N and 63d 01.6'W. For Venus we get: UTC 08:53:22 Hs 28 13.8 Ic -1.5 Dip -2.9 ========= Ha 28 09.4 R -1.8 Add Cor 0.2 ========= Ho 28 07.8 GHA-hr 345 25.3 GHA-ms 13 20.5 v 0.3 AP-Lon -63 01.6 ========= LHA 295 44.5 Dec-hr 7 16.5 d 0.5 ======== Dec 7 17.0 AP-Lat 13 15.8 Hc 26 38.5 Delta 89.3 T Z 88.4 Inspection shows that a closer AP would be about 90 nmi to the east: 13d 00.0N 61d 30.0'W Working the sights based on new AP: Fomalhaut Polaris Deneb Mars Venus Vega UTC 08:50:10 08:50:55 08:51:05 08:52:03 08:53:22 08:53:45 Hs 40 54.7 13 17.3 56 56.3 25 35.0 28 13.8 45 55.6 Ic -1.5 -1.5 -1.5 -1.5 -1.5 -1.5 Dip -2.9 -2.9 -2.9 -2.9 -2.9 -2.9 ========= ======== ======== ======== ======== ======== Ha 40 50.3 13 12.9 56 51.9 25 30.6 28 09.4 45 51.2 R -1.1 -4.1 -0.6 -2.0 -1.8 -0.9 Add Cor 0.3 0.2 ========= ======== ======== ======== ======== ======== Ho 40 49.2 13 08.8 56 51.3 25 28.9 28 07.8 45 50.3 GHA-hr 7 54.0 7 54.0 7 54.0 101 22.5 345 25.3 7 54.0 GHA-ms 12 34.6 12 45.8 12 48.3 13 00.8 13 20.5 13 28.5 v 2.7 0.3 SHA 15 34.4 49 37.7 80 45.0 AP-Lon -61 30.0 -61 30.0 -61 30.0 -61 30.0 -61 30.0 -61 30.0 ========= ======== ======== ======== ======== ======== LHA 334 33.0 319 09.8 8 50.0 52 56.0 297 16.1 40 37.5 Dec-hr -29 36.9 45 16.8 -25 47.4 7 16.5 38 47.0 d - 0.1 0.5 ========= ======== ======== ======== ======== ======== Dec -29 36.9 45 16.8 -25 47.5 7 17.0 38 47.0 AP-Lat 13 00.0 13 00.0 13 00.0 13 00.0 13 00.0 Hc 40 49.6 13 00.3* 56 51.5 25 31.5 28 07.2 45 50.3 Delta .4 A .2 A 2.6 A .6 T 0.0 Z 150.4 348.6 232.8 88.6 313.2 *Lat from polaris table Plotting this out gives a fix 13d 00.3 N 61d 29.4 W 09:00:00 30/05/2001 > The navigator has been examining nautical chart INT 400. It shows an area > labeled the Aruba Gap (13d 50.0'N 70d 00.0'W) with a favorable current of > 1.15 Kts. He decides to make for this waypoint. Var 12dW, Dev 1dE. > Current Set 275d Drift 0.8 Kts. Speed 9.3 Kts. > 4) What is the TC and Dist from our FIX to the Aruba Gap > (13d 50.0'N 70d 00.0'W) > -- ------------------------------------------------------ dLo = -70d 00.0 - -61d 29.4 = -510.6' dLat = 13d 50.0 - 13d 00.3 = 49.7' dep = 510.6 * cos 13d 25' = 496.7 TC = atan2(-496.7, 49.7) = 275.7 Dist = sqrt(49.7^2 + 496.7^2) = 499.2 nmi TC = 275.7 D = 499.2 with current directly behind us there's no need to adjust for drift > 5) What is the Compass Course(CC)/Course-to-Steer for Aruba Gap? > -- ------------------------------------------------------------- CC = 275.7 + 12 - 1 = 286.7 > 6) What is the time enroute to Aruba Gap? > -- -------------------------------------- T = D/S = 499.2/(9.3 + 0.8) = 499.2/10.1 = 49.5 hr > 7) What is the anticipated Speed Made Good(SMG) to Aruba Gap? > -- ---------------------------------------------------------- Speed is 9.3 with drift 0.8 in the same direction so SMG = 9.3 + 0.8 or 10.1 knots