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Subject: Re: Silicon Sea: Leg 72 -NEW
From: Dan Hogan (dhhogan@XXX.XXX)
Date: Sat May 26 2001 - 15:36:03 EDT
On 26 May 2001, at 13:43, Michael Wescott wrote:
> > Sight Reduction and Revised
> > Dead-Reckoning problem Leg 72 052301 Test your knowledge!
> > ---------------------- ------ ------ --------------------
> >
> > Our Last position FIX was (13d 00.0'N 61d 30.1'W) at 08:50:10UT on
> > 30/05/2001. We are bound for a point on chart INT 401 marked as Aruba Gap
> > (13d 50.0'N 70d 00.0'W). Aruba Gap provides a favorable westerly current of
> > 1.5 Knts.
> >
> > We have had overcast skys and have missed some possible sights. The winds
> > have been fluky and we have had to tack several times since our last FIX
> > position.
> >
> > Tack 1: CC 333.0d - 04Hrs @XXX.XXX5 Kts - Var 12dW Dev 1dE
> > Tack 2: CC 242.0d - 11Hrs @XXX.XXX5dE
> > Tack 3: CC 262.0d - 08Hrs @XXX.XXX5dE
> > Tack 4: CC 307.0d - 10Hrs @XXX.XXX0dE
> >
> > 1) What is the DR position at the end of the Tack 4?
> > -- -------------------------------------------------
> TC Dist D.Lat dep
> Tack 1: 319.0 30.0 22.6 -19.7
> Tack 2: 231.5 93.5 -58.2 -73.2
> Tack 3: 252.5 40.0 -12.0 -38.1
> Tack 4: 297.0 90.0 40.9 -80.2
> =============
> -6.7 -211.2
> D.Lo = -211.2/cos 13.4 = -216.8 = 3d 36.8'
>
> DR Position = 12d 53.3'N 65d 06.9W
OK
> > 2) What is the DR position local Zone Time(ZT) and Date?
> > What is the DR position GMT/UT time and Date?
> > -- -----------------------------------------------------
> 4 + 11 + 8 + 10 = 30hrs = 1d 6h
Uhh..33 Hrs
>
> DR UT = 17:50:10 31/05/2001
> DR ZT = 13:50:10 31/05/2001
>
> > 3) From the DR position, what is the TC to Aruba Gap
> > (13d 50.0'N 70d 00.0'W)?
> > -- -------------------------------------------------
>
> D.Lat = 13d 50.0 - 12d 53.3 = 56.7
> D.Lo = -70d 00.0 - -65d 06.9 = 4d 53.1 = 293.1'
> dep = 293.1 * cos 13d 20 = 285.2
>
> TC = 270 + atan(56.7/285.2) = 281.2d
OK
> > Our speed is 8.5 Kts, Var 10dW Dev 1.0dE. Current Drift 0.7 Kts Set 265.0d
> >
> > 4) What is the Compass Course(CC)/Course-to-Steer for Aruba Gap
> > from the DR position?
> > -- ------------------------------------------------------------
>
> CC = 281.2d + 10 - 1.0 = 290.2d
OK
> correction for current (by simple plot) adds a fraction more than
> one degree: CC = 291.5 and gives a speed over ground of about 9.0
>
> > The following morning the navigator see stars through breaks in the
> > clouds. He takes the following sights. Same data as for Q3 & Q4.
> >
> > Body ZT Hs
> >
> > UNKNOWN 05:39:00 13d 53.6' (Bearing 349.7 M)
> > Mars 05:39:11 18d 14.5'
> > UNKNOWN 05:40:03 54d 58.9' (Bearing 327.3 M)
> > Vega 05:41:06 40d 10.3'
> > Venus 05:43:33 35d 25.8'
> >
> > 5) What is the FIX position?
> > -- -------------------------
>
> 9 knots @ 281 for 16 hrs gives AP of about 13d 20'N 67d 30W
>
> Plotting the sights gives us a crossings all in the vicinty of 67d 00'W
> and 13d 30'N. A correction of 1.0' T gives a very small set of crossings
> at
>
> 13d 30.5'N 66d 59.5'W
OK
> Was the IC really -1.5' rather than the -2.5' given?
Yes. Thanks, I forgot to change it in the Master Problem NOTES.
> > 6) What are the names of the two unknown bodies?
> > -- ---------------------------------------------
>
> Unknown #1:
>
> 349.7 + 10 - 1 = 358.7d
>
> almost due north and altitude is about same as our latitude:
> got to be Polaris
Right
> Unknown #2:
>
> 327.3 + 10 - 1 = 336.3
>
> This is visually in the angle between Polaris to the north and Vega
> to the northwest. GHA Aries is about 34d and Long is 67W so zenith
> has SHA of about 33d. Looking at the starchart in the almanac, there
> is only one navigational star between Vega and Polaris: Deneb
Right
> > 7) What is the TC and Distance to the Aruba Gap (13d 50.0'N 70d 00.0'W)
> > waypoint from our FIX?
> > -- ---------------------------------------------------------------------
>
> dLat = 13d50.0' - 13d30.5' = 19.5'
> dLo = 70d00.0 - 66d59.5' = 180.5'
> dep = 180.5*cos 13d40' = 175.4
>
> TC = 270 + atan(19.5/175.4) = 276.3
> Dist = sqrt(19.5^2 + 175.4^2) = 176.5
OK
Dan Hogan WA6PBY
Catalina 27 "GACHA"
dhhogan@XXX.XXX
Navigation-L List Owner
http://www.wa6pby.com
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