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    Re: Logs
    From: Trevor Kenchington
    Date: 2003 Jun 10, 22:18 -0300

    Bill Noyce wrote:
    > The exact time for your sandglass isn't critical, but the spacing of the
    > knots has to be made to match.  I believe the classic sandglass was 28
    > seconds.  Leave some blank line so the chip can get settled and out of the
    > vessel's wake, then insert a starting mark (at which the man controlling
    > the line would shout, "Turn!"), and space the knots at 6080*t/3600 feet,
    > to measure nautical miles (6080 ft) per hour based on feet traveled in t
    > seconds. (Hope I got the formula right.)
    Traditionally, log lines were _not_ marked as Bill describes but had
    less distance between knots. Opinions are divided as to whether that was
    to allow for some movement of the chip (pulled along behind the ship) or
    because most navigators most of the time would rather have their
    reckoning run ahead of their true position than the other way around.
    Smyth (whose dictionary was published in 1867, though much was out of
    date by then) said that the common practice was to have 42ft between
    knots, when 47ft 4in "should" have been used (which means that he was
    thinking in terms of the traditional 28-second glass).
    Smyth also warned that any chip log was notoriously inaccurate. He much
    preferred patent logs.
    George Huxtable noted:
    > Faster vessels might have a 15-second glass, and
    > closer knots, to minimise the length of line required.
    I think that misunderstands the usual practice: Fast ships would carry a
    14-second glass as well as the usual 28-second but they only needed one
    log line. When making a good clip, the standard log line was used with
    the fast glass and the number of knots that passed over the taffrail was
    doubled to get the speed. A similar practice might well help with the
    limited space and relatively high speeds of modern sailing yachts.
    Finally, Vic Fraenckel asked:
    > Any idea how long a nautical mile was in
    > Aubrey's time (ca 1800). I am sure it has been standardized since then.
    > Perhaps it was defined as some number of fathoms, which I assume was 6 feet.
    > Any ideas?
    I think the nautical mile was defined as one minute of latitude until
    the U.S. settled on 6080 feet for a standard mile, Britain on a lesser
    amount (6076?) and the Gods of Metric on 1852 metres. By 1800, the
    diameter of the Earth was known with some precision and even the
    difference between Polar and Equatorial diameters had, I think, been
    estimated, so Nelson's contemporaries presumably knew and used the
    (modern) length of a mile -- or as close to that as the precision of
    their navigation would allow.
    Trevor Kenchington
    Trevor J. Kenchington PhD                         Gadus@iStar.ca
    Gadus Associates,                                 Office(902) 889-9250
    R.R.#1, Musquodoboit Harbour,                     Fax   (902) 889-9251
    Nova Scotia  B0J 2L0, CANADA                      Home  (902) 889-3555
                         Science Serving the Fisheries

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