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Subject: Answers to Leg 84
From: Peter Fogg (ffive@XXX.XXX)
Date: Thu Oct 03 2002 - 01:56:15 EDT
Q1) Can't get a morning ZT from 02:27 UT on 21/07/02.
Can get 16:27 ZT on 20/07/02.
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
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.
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.
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