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From: B Geres (no email)
Date: Mon Sep 29 2003 - 09:16:51 EDT
In the ancient times, the only way to measure ship
speed was to throw a wood log into the water and
observe how fast it moves away from the ship. This
approximate method of ship speed measurement was
called 'Heaving the Log' and was used until 1500-1600s
when the 'Chip Log' method was invented (both methods
probably invented by Dutch sailors.)
The 'Chip Log' apparatus consisted of a small weighted
wood panel that was attached to the reel of rope, and
a time measuring device: a half-minute sand glass (as
shown in image below to the right.) Rope had knots
tied at equal distances along the reel. Sailors would
throw the wood panel into the sea, behind the ship,
and the rope would start unwinding from the reel.
A sailor would count the knots as they ran out through
his fingers. Another man would time the process as
sand flowed through his sandglass. As the last grains
fell, he would cry “mark” and the line tender would
stop it running out.
The faster the ship was moving forward the faster the
rope would unwind. By counting the number of knots
that went overboard in a given time interval, measured
by the sand glass, they could tell the ship's speed.
That's the origin of the nautical speed unit: the
knot.
[Traverse Board - The traverse board was used to
record the course and speed of a vessel during a four
hour watch. Simple traverse boards were used in
northern Europe by the 16th century. They had a series
of holes along lines marking the thirty-two points of
the compass. Pegs were attached to the board by string
and placed in the correct hole for the course being
steered, normally one hole for each half hour of the
watch, as measured by a sand-glass. Later versions
also had a series of holes along the bottom for speed,
taken by the log and line. At the end of each watch
the records were written down, usually by the ship's
master, and the pegs pulled out ready for the next
watch.]
So, how fast is a 'knot'?
Well, first we should know that for distance sailors
used (and still use) the so called 'nautical mile'. If
you slice Earth into two equal halves right through
its center along equator for example, then divide the
perimeter (the circumference) into 360 degrees, then
each degree into 60 arc minutes, the length you get is
approximately 1 nautical mile. So, to recap, one
nautical mile is the arc distance of about 1 minute of
a degree (or 1/60th of a degree) of Earth. We say
approximate because if you choose to slice Earth along
the line that goes through the North and South poles
you would get a slightly different result due to the
fact that Earth is not a perfect sphere - it is
slightly flattened at the poles. Difference between
the polar and equatorial diameter being about 23.4
nautical miles out of 6880 nautical miles. Exact value
for the nautical mile is taken to be the average of
the two (polar and equatorial) and is:
1 nautical mile = 1.15 miles = 1852 meters = 6067 feet
Naturally, sailors wanted to have their ship's speed
in units of nautical miles per hour (just like
American car drivers like their car speed in miles per
hour - my apologies to the rest of the world. However,
don't fuss too much since the meter was also defined
quite arbitrary around 17th century as one part in 10
million of the distance from the North Pole to the
Equator along the meridian of Paris.)
To avoid ropes that were miles and miles in length,
they usually had ropes that had knots every 50 feet
and a sand glass that measured half a minute. If you
work out the math you will convince yourself that the
number of knots that went overboard in half a minute
is exactly the number of nautical miles per hour the
ship was cruising at. For example, if 10 knots went
overboard in half a minute, then the ship was moving
forward at the speed of 10 knots or 10 nautical miles
per hour (which would be about 11.5 standard miles per
hour.)
Records from 1917 (Bowditch) indicate that the
official U.S. Navy sand glass measured 28 seconds, and
that knots were spaced out exactly 48 feet (or 8
fathoms - a popular length unit of that time). With
this setup, ship's speed could be measured with an
error of about 1.5%. This is speed relative to the
water and assuming that the wood panel does not get
pulled significantly from the place where it was
initially dropped and that the rope does not stretch -
all of which actually does happen and should be
accounted as sources of error. Not to mention the
problem of sea water currents adding or subtracting
from the actual ship's speed relative to land.
So there you go.
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