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    Re: Silicon Sea: Leg 72 -NEW
    From: Dan Hogan
    Date: 2001 May 26, 15:36 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 {at}XXX.XXX5 Kts - Var 12dW Dev 1dE
    > >     Tack 2: CC 242.0d - 11Hrs {at}XXX.XXX5dE
    > >     Tack 3: CC 262.0d - 08Hrs {at}XXX.XXX5dE
    > >     Tack 4: CC 307.0d - 10Hrs {at}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 {at} 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{at}XXX.XXX
    Navigation-L List Owner
    http://www.wa6pby.com
    

       
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