Hi, I am trying to understand the origin of the 28 second interval used in
measuring the speed of ships. If a certain length of rope paid out in 28
seconds the ship was moving at 1 knot - if two paid out the ship sspeed was
2 knots etc. Why use 28 seconds? What is the origin of the 28 second
interval?

Peter Thejll
Denmark

"Peter Thejll" wrote in
k:

Hi, I am trying to understand the origin of the 28 second interval
used in measuring the speed of ships. If a certain length of rope paid
out in 28 seconds the ship was moving at 1 knot - if two paid out the
ship sspeed was 2 knots etc. Why use 28 seconds? What is the origin of
the 28 second interval?

Peter Thejll
Denmark




A good question one could ask now is: What did the sea navigators use for
speed measurement before the GPS?

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. 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. In fact that is the origin of the nautical speed unit: the
knot.


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.