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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Smyth (whose dictionary was published in 1867, though much was out of
date by then) said that the common practice was to have 42ft between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


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Trevor J. Kenchington PhD                         Gadus@iStar.ca
Gadus Associates,                                 Office(902) 889-9250
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