Speed Log
Speed logs measure the speed of a ship and are also known as ship logs. There are two ways of measuring the speed. The first one is with reference to water which flows by the hull, called water reference speed. The second one is in reference to the seabed and is called ground reference speed. In the age of sailing, they already used logs to measure the speed. Thus, they would tie a rope around a log, put several knots in and throw it overboard. The speed was measured by the total number of knots passing by during a specific time span. Today's versions are electromagnetic logs (EM logs), doppler logs or ultra speed sensors.
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The EM Log creates an electromagnetic field and a voltage is induced in the water. The magnitude of the voltage varies depending on the speed of the water which flows past the sensor. This voltage is measured and translated into the vessel's speed through water. Doppler logs create a very accurate maritime speed measurement, either derived acoustically with Doppler-Sonar — or with radio interferometrically by Doppler measurement of satellite signals, such as those from the Global Positioning System (GPS).
Furthermore, the ultrasonic speed sensors use two ultrasonic transducers that send ultrasonic pulses through the water flowing past the hull. By calculating the time difference in pulse propagation from one sensor to the other, the device calculates the speed of the hull through the water. You can use various devices to determine your ship’s rate of motion. Whether you choose a GPS system, an electromagnetic technology or the speed log depends on your preferences.