A
chip log is a
navigational tool used by
mariners to determine
speed through the water - a combination of speed over ground and the effect of water current. A chip log, literally consisted of a piece of
wood. The rope tied to the log had a number of measured
knots tied in it. The speed of the ship was indicated by the number of knots passing over the stern during a certain period of time. The
unit,
knot, for
nautical mile per
hour, was derived from the knots tied in the rope of a log.
A chip log consists of a piece of
wood, most often cut in the shape of a quarter
circle, or "quadrangle", and a length of
rope or line with
knots tied every 47
feet and 3 inches (14.4
meters). To determine a
ship's speed, the chip log would be placed into the water
aft of the ship, line paid out, and the number of knots paid out in 28
seconds counted, giving the speed in
knots. A number of considerations had to be taken into account -- the amount of following sea, stretch of the line, and inaccuracies in the measurement of 30 elapsed seconds. Time passage was most often marked with a "30 second glass", a small sand filled glass, which would often run fast or slow based on ambient
temperature,
humidity, and
sea state. Frequent measurements helped in mitigating some of these inaccuracies by averaging out individual errors, and experienced navigators could determine their speed through the water with a fair degree of accuracy. Mechanical chip logs, operating on physical principles in a manner similar to a car's
odometer, were eventually developed and replaced the traditional chip log.