Chip log
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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.
The first known device to measure speed is the Dutchman's Log. An object that would float was thrown overboard and the time required to pass between two points on deck was measured with a sandglass.
Today, the most accurate means of maritime speed measurement comes from Doppler measurement, either derived acoustically by use of Doppler Sonar or radio interferometrically by Doppler measurement of satellite signals such as those from Global Positioning System (GPS). Most commercial GPS systems are not configured to operate in this mode, however.
[edit] External links
- Chip Log pattern on the webpage of the Navy & Marine Living History Association, Inc. note: the distance given on this pattern is 33 1/3 feet, the correct distance should be 47 1/4 feet. (accessed Sept. 2006)
- The ‘Dutchman’s Log’ or the Seaman’s Tobacco Box of Pieter Holm
