A chip log — a simple navigation tool used to measure the speed of a ship — was designed around the end of the 15th century. It consisted of a wooden panel that was weighted with lead and attached to a line. The logline had several knots at uniform intervals (most often at 7 meters). To measure the speed of a vessel, sailors tossed the log overboard and used a sandglass to measure a certain time interval (14 seconds). During this time, sailors counted the number of knots that passed through their hands. This number was then used to convert log timing into nautical miles per hour. This method of measuring speed explains the origin of the term “knot”. A chip log is a device designed to record the speed of a ship and the distance traveled. Mariners use mechanical, electromagnetic, hydroacoustic, induction and radio Doppler logs.
Pitometer logs (also known as pit logs) are devices used to calculate a ship’s speed relative to the water. It measures the difference in pressure that depends on the ship’s speed. The main part of the log consists of two tubes that are immersed into the water below the vessel hull. The opening of one tube faces the direction of the ship’s forward movement, and the opening of the other tube faces sideways. The dynamic pressure is determined by the difference in water column heights, which is converted by the lag mechanisms into the speed units — knots. Apart from the speed, pitometer logs also show the distance traveled by the ship in miles.
The induction log (or electromagnetic speed log, sometimes called an EM log) is a relative log. It makes use of the physical properties by measuring the EMF induced in water moving through a magnetic field generated by the sensor. The magnetic field is created by the log’s sensor and sea water is an electrical conductor. When the ship moves, the magnetic field comes across static water, and an EMF is induced. The speed of the water flowing past the sensor is directly proportional to this EMF. The electrodes transmit the EMF to a special device that calculates the speed of the ship and the distance covered.
The acoustic correlation log measures the speed with respect to the seabed. The log derives the vessel’s speed by the use of signal acoustic correlation. There are Doppler and acoustic correlation logs.
The geomagnetic log determines true ground speed using the properties of the Earth’s magnetic field.
Doppler logs make use of the behavior of
propagating radio waves. In practice the measurements are made at the beginning
of every hour. The distance traveled (S) in miles and the speed (V) in knots
can be calculated based on the difference in numbers. Logs can be inaccurate,
and the errors are then corrected.
