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Subject: Re: Silicon Sea: Leg 71
From: Michael Wescott (wescott_mike@XXX.XXX)
Date: Sat May 12 2001 - 14:24:21 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
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