NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Answers to Leg 84
From: Dan Hogan
Date: 2002 Oct 3, 06:27 -0700
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. 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@earthlink.net Nav-L Page: http://www.wa6pby.com