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    Re: Answers to Leg 84
    From: Arthur Pearson
    Date: 2002 Oct 4, 18:29 -0400

    1) ZT = 16:27 on 20/7/02
    2) Using Mercator calculations in CelestNav, total distance of all legs
    is 303.
    3) 303/9.3 = 32.6 hrs = 32 hrs and 36 min.
    4) Sight directly overhead is very difficult, hard to know where to
    bring it down. That said, declination on this day = 20d 31' which is
    therefore our approximate latitude, which corrects or DR latitude by 7
    miles)
    5) Solving the sights in Celestnav and plotting them yields 26d 00'N and
    157d 00' W.  CelestNav fix calculation yields the same position exactly.
    6) Mercator calculations in CelestNav give TC = 102, D = 4.8nm. Current
    has negligible effect on course to steer, corrections of Var. and Dev.
    give CC = 90.5 (anyone who can steer within 0.5 degrees, please email me
    with instructions)
    7. Speed adjusted for current = 9.1. Divided into distance yields T = 32
    minutes. Look over the bow and you won't need the compass.
    
    -----Original Message-----
    From: Navigation Mailing List
    [mailto:NAVIGATION-L{at}LISTSERV.WEBKAHUNA.COM] On Behalf Of Peter Fogg
    Sent: Thursday, October 03, 2002 1:56 AM
    To: NAVIGATION-L{at}LISTSERV.WEBKAHUNA.COM
    Subject: Answers to Leg 84
    
    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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