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Subject: Re: Silicon Sea: Leg 71
From: Dan Hogan (dhhogan@XXX.XXX)
Date: Sat May 12 2001 - 21:25:01 EDT
On 12 May 2001, at 14:24, Michael Wescott wrote:
> > 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
OK
> > 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
>
OK, BUT *MORNING* 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
OK. All that work reminds me of when I was using H.O. 208.
> > 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
OK
>
> 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
OK. But the following current will boost Speed Made Good(SMG) which will
affect time enroute..
> > 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
OK
Dan Hogan WA6PBY
Catalina 27 "GACHA"
dhhogan@XXX.XXX
Navigation-L List Owner
http://www.wa6pby.com
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