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    Re: Silicon Sea: Leg 80
    From: Dan Hogan
    Date: 2001 Oct 16, 7:32 AM

    On 16 Oct 2001, at 7:36, Noyce, Bill wrote:
    
    > Better late than never, I hope.
    >
    > >The weather has been overcast for the past several days. Seas lumpy,
    > >and wind variable. On 03/07/2001 at 01:37:00ZT and a distance of 754.0 Nmi
    > >on a TC of 267.7d from our last position FIX at (03d 30.0' 097d 30.0'W)
    > >30/06/2001 {at} 11:53:00UT the navigator orders a turn to a TC of 307d.
    > >
    > > 1)  What is the DR position at the time of the turn?
    > > --  ------------------------------------------------
    >
    > TC 267.7d = W 2.3d S   Dist 754.0 nmi
    > dLat = 30' S  =>                  DR Lat =   3d 0' N
    > dep = 754 nmi => dLo = 753' W =>  DR Lon = 110d 3' W
    
    OK
    
    >
    > > At 09:27:06ZT that same morning we manage a Sun Lower Limb (Sun LL)
    > sight of:
    > >     Hs    42d 01.5'
    > >
    > > TC 307d, Speed 10.4 Kts.
    > >
    > > 2)  What is the DR position at the time of the Sun sight?
    > >     What is the UT/GMT time of the sight?
    > > --  ------------------------------------------------------
    >
    >
    > TC 307d = W 37d N  time = 7h 50m 06s => dist = 81 nmi
    > dLat = 48' N  =>               DR Lat =   3d 48' N
    > dep = 65 nmi => dLo = 65' W => DR Lon = 111d  8' W
    > Zone is 7 hours west of Greenwich => UT = 16:27:06
    
    OK
    
    > Working the sight using the DR position as the Assumed position gives:
    > GHA 178d 56.1'  dec 22d 54.8' N  Ho 42d 10.6'  Hc 42d 12.2'
    > Intercept: 1.6 nmi Away from 062.3d
    >
    >
    > > At a DR position of (04d 10.1'N  111d 32.5'W) at 12:30:22ZT we manage
    > > another Sun sight.
    > >
    > >     Hs    Sun LL: 71d 05.6'
    > >
    > > 3)  What is 12:30:22ZT Sun sight called?
    > >  --  ------------------------------------
    >
    > It's obviously close enough to local noon to give a LOP oriented
    > mostly east/west, so could be called a "Latitude sight".
    > A bit of work with the Almanac shows the sun's GHA is within
    > a fraction of a minute of our estimated longitude, so this is
    > definitely a "LAN sight" or a "Noon sight".  As such, it gives
    > Latitude directly with little calculation:
    >
    > Hs  71d  5.6'   decl 22d 54.1' N
    > -IC      2.5'   -ZD  18d 44.5'
    > -Dip     3.0'   --------------
    > -Refr    0.3'   Lat   4d 9.6' N
    > +SD     15.7'   ===============
    >    ----------
    > Ho  71d 15.5'
    > ZD  18d 44.5'
    
    OK.
    
    >
    > > 4)  What is the Running FIX(RFIX) position?
    > > --  ---------------------------------------
    >
    > For the RFIX, we must advance the earlier LOP to account for the
    > motion from 9:27 to 12:30.  We do this by advancing the Assumed
    > Position from which the LOP is drawn.  One way is by advancing
    > the position by the given course, speed, and time.  This gives
    > a new Assumed Position of 4d 5' N, 11d 34' W.  Since the earlier
    > sight was worked with Apos = DR pos, we can simply take the new
    > DR position as the place from which to draw the LOP.  Which is
    > better depends on why the new DR pos differs from the one I
    > assumed for the earlier sight.
    >
    > Using 4d 5' N,    111d 34' W   => RFIX = 4d 9.6' N, 111d 38.5' W
    > Using 4d 10.1' N, 111d 32.5' W => RFIX = 4d 9.6' N, 111d 34' W
    >                                   ============================
    >
    > For the remainder of the problem, I'll use the second solution
    > (111d 34' W).
    
    OK
    
    > > 5)  What is the True Course(TC) and distance to the Hawaii landfall
    > >     point at (19d 30.0'N  154d 45.0'W)?
    > > --  ---------------------------------------------------------------
    >
    > It's a long way! dLat = 15d 20.4, dLon = 43d 11'
    > Even though it's not appropriate, I tried a Mid-Lat solution just to
    > see what would happen, and got dist = 2710 nmi, TC = 289.7d.
    >
    > Using Great Circle, I get      dist = 2687.4 nmi, TC = 296.7 (initial)
    >                                =============================
    >
    >
    > The GC course is closer to the pole (expected), but I was surprised
    > that the distances came out so close to equal.
    
    OK, but Great Circle is not of much use since on a small sailboat it is just
    about impossible to sail it. But you have an idea of how many days to go. Try
    using Mercator/Rhumbline calculations, the distance is a bit far for accuracy
    from Mid-Latitude.
    
    > > The sky remains a heavy overcast. Winds are variable and indicate a
    > slow backing. We maintain at True Course of 289.9d.
    > >
    > > At the Running FIX position(RFIX) 03/07/2001 {at} 19:30:22UT, Current
    > Drift 0.7 Kts, Set 095d. Var 9dE, Dev 1.5dE. Speed 10.0 Kts.
    > >
    > > 6)  What is the Compass Course(CC)/Course-to-Steer?
    > > --  -----------------------------------------------
    >
    > Not sure whether you're asking for TC 289.9d to be translated to a CCTS,
    > or for my Hawaii course (TC 296.7d) to be translated to a CCTS.
    > I did the latter, and came up with
    > Steer 1.4d further West, and SOG = 9.3 kts
    > TCTS = 295.3 => TVMDC => CCTS = 284.8d
    
    With Silicon Sea always assume that you are in the cockpit navigating using
    traditional navigation methods and equipment.
                             =============
    OK. You would think that after all this time I would have learned how to
    write a question.
    
    > > We travel for a distance of 646.6 Nmi on a TC of 289.9D.
    > >
    > > 7)  What is the Dead Reckoning position at this point?
    > >     What is the Zone Time(ZT) and Zone Date(ZD) of the DR
    > >     position?
    > > --  -------------------------------------------------------
    >
    > Using the given distance and TC directly for DR:
    > 289.9d = W 19.9d N
    > dLat = 230'  =>                DR Lat =   7d 59.6' N
    > dep = 606 nmi => dLo = 604' => DR Lon = 120d 38' W
    >                                =====================
    > I assumed the distance was distance through the water (measured
    > by a log), not distance over the ground, so the time is found
    > by dividing by 10.0 kts.  64.66 hrs = 2days 16h 40m
    >                                    + July 3 19:30:22 UT
    >                                    --------------------
    > We're now 8 hours west               July 6 12:10:22 UT
    > of Greenwich ==========>             July 6 04:10:22 ZT
    >
                                         ==================
    OK
    
    > > 8)  What is the UT/GMT time and date at the DR position?
    > > --  ----------------------------------------------------
    >
    > July 6 12:10:22 UT
    
    OK
    
    
    Dan Hogan
    dhhogan{at}verionmail.com
    

       
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