Navigation and Seamanship

3.2.6.1. Operation of Electromagnetic Log

Operation

EM log consists of two electrodes placed beneath the hull. Magnetic field is produced by an electromagnet. In the electromagnetic log, the potential difference generated in water by its movement relative to the magnetic filed is sensed by the electrodes. The current induced in these electrodes by the water flowing past is a function of speed. The measurement of speed of a ships by this log has been found to be accurately economic and simple.

As the vessel moves, the sea water(conductor) relatively moving through the magnetic field induces a small electromagnetic field into it. This emf is detected and measured by two sensors on the hull region. The speed indicator signal is supplied to special generator which produces a number of pulses per minute which is proportional to the speed (say 200 pulses per nautical mile). This generator is connected to a distance run recorder which counts off the pulses and hence the number of miles travelled .Accuracy of indicated distance is about 1-2% of the distance travelled.

Advantages

i) The sensor device doesn’t project beyond the bottom of the ship.

ii) EM log operates with greater accuracy and can measure smaller ship speeds.

iii) Receiving device does not require regular maintenance.

vi) Small in size.

Disadvantage

· It cannot be used in freshwater.