Next message: Louis & Mariella: "[world-cruising] Where are all the messages?"
One method that could be adapted to day was to through over the stern a
wooden board arranged like a parachute. this log was attached to a line with
equally spaced knots tied along the line.
The length of line dragged over the stern by the log was measured by the
number of knots passing a reference point.
Hence the speed was measured in knots.
Barry
----- Original Message -----
From: "B Geres" <>
To: <>
Sent: Monday, September 29, 2003 11:16 PM
Subject: [world-cruising] Re: chip logs
> 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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