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Subject: Re: Silicon Sea: Leg 80
From: Dan Hogan (dhhogan@XXX.XXX)
Date: Tue Oct 16 2001 - 09:32:07 EDT
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 @XXX.XXX
> >
> > 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 @ 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@XXX.XXX
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