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Subject: Re: Answers to Leg 84
From: Dan Hogan (dhhogan1@XXX.XXX)
Date: Thu Oct 03 2002 - 09:27:30 EDT
On 3 Oct 2002, at 15:56, Peter Fogg wrote:
> Q1) Can't get a morning ZT from 02:27 UT on 21/07/02.
> Can get 16:27 ZT on 20/07/02.
UT is not envolved in the answer. Time of sunrise + departure FIX ZT.
>
> Q2) By plot, 296nm. By calculation, 295.3nm.
OK
>
> Q3) 296nm / 9.3k = 31h 49m 40s.
OK
>
> Q4) Having the sun virtually directly overhead makes the sight
> difficult. This question indicates that the ZT for the departure should
> have been around 07:30, presumably on the 21/7. Have used this date from
> here on.
Right
>
> Q5) Fix at 19:30 ZT (N20d 59'.6 W156d 58'.6). The sights are taken over
> a period of more than 15 minutes, at 10 knots the boat travels a little
> more than 2.5nm during this time, so the later sights have been
> corrected back to the first one.
OK
>
> Q6) Because the fix position is so close to the Pailolo Entrance a small
> difference in the fix position will mean a large difference in the
> course. By plotting with an enlarged scale I get a TC of 098d and
> Distance of 4.3 nm. The current is running at almost a reciprocal
> direction, there seems to be less than a degree in difference with the
> CMG, say a TC to steer of 097d, and a SMG of 9.1k. Less the combined
> error of E10.5d gives a CC to steer of 86.5d
OK -- Based on your original time assumption.
>
> Q7) Now here's a tricky one. Which speed should we use, the speed
> through the water of 10k or the calculated Speed Made Good of 9.1k?
> Since navigation involves, by definition, finding position, courses,
> etc, by the most accurate methods available it would seem indicated to
> use the SMG, since we know the boat speed has been affected by the
> current. What is the point of knowing what our ETA might be if we
> travelled at some other, inaccurate speed? 4.3nm / 9.1k = 28m 21s.
It was a just a question. Calculate an answer only from the data provided.
>
> PS I was happy to see a rhumb line calculation (between passages 6&7)
> with an east/west course, as these can be tricky with some methods (but
> very easy by plotting). Let's have more of these. Another idea:
> something we never see is times taken from a less than accurate
> timepiece (aren't they all?) set to, for example, the ZT of the last
> port. This watch can't be altered - we'd lose the time! - so as time
> goes on the rate of gain/loss needs to be applied, then converted to
> present ZT then UT.
Well, they crop up only as the route calls for it. Setting up unusuall
questions eats up time and confuses me ;^)
Dan Hogan WA6PBY
C27 "Gacha"
dhhogan1@XXX.XXX
Nav-L Page: http://www.wa6pby.com
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