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    Re: Answers to Leg 84
    From: Dan Hogan
    Date: 2002 Oct 3, 06:27 -0700

    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.
    > Q3) 296nm / 9.3k = 31h 49m 40s.
    > 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.
    > 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.
    > 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"
    Nav-L Page: http://www.wa6pby.com

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