A chip log consists of a wooden board attached to a line (the log-line). The log-line has a number of knots tied in it at uniform spacings. The log-line is wound on a reel to allow it to be pay_out easily in use.
Over time, the log was standardized in construction. The shape is a quarter circle, or quadrant and the log-line is attached to the board with a bridle of three lines connected to the vertex and to the two ends of the quadrant's arc. In order to ensure that the log submerges and is oriented correctly, the bottom of the log is weighted with lead. This provides for more resistance in the water and a more accurate and repeatable reading of speed. The bridle is attached in such a way that a strong tug on the log-line results in one or two of the bridle's lines releasing, allowing the log to be retrieved with relative ease.
When the navigator wished to determine the speed of his vessel, a sailor dropped the log over the stern of the ship. The log would act as a drogue and remain roughly in place while the vessel moved away. The log-line was allowed to run out for a fixed period of time. The speed of the ship was indicated by the length of log-line passing over the stern during that time.
The first known device to measure speed is often claimed to be 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. However, the first reference to a Dutchman's log is in 1623, after the ship log. The Dutchman's log could be used with a brass tobacco box, rectangular with rounded ends. This box had tables on it to convert log timing to speed.
Initially, the log-line was not knotted and the length was measured directly on the line. With the introduction of the nautical mile as a standard unit of measure at sea in the 15th century, the line began to be marked at equal intervals proportional to the nautical mile and to the time interval used for measurement. Initially, the markings were in the form of knots in the line. Later, knotted cords were worked into the log-line.
Originally, the distance between marks was 7 fathoms or 42 feet used with a sandglass with a 30 second running time. Later refinements in the length of the nautical mile caused the distance between knots to be changed. Eventually, the distance was set to 47 feet, 3 inches (14.4 meters) for a standard glass of 28 seconds.
The use of a log did not give an exact measure of speed. A number of considerations had to be taken into account, for example:
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.
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.