Navigation and Seamanship

3.2.6. Induction log or Electromagnetic speed log or Em log

    The electromagnetic speed log also called as induction log or EM log measures the electromagnetic induction generated in sea water by its movement relative to magnetic field by the electrodes fitted in the hull of a vessel. The sea water acts as the conductor and generates an emf.. Electromagnetic Speed Log is highly accurate in indication of speed though the water even at low speeds. Two electrodes are mounted athwart ships and the emf (50-400 micro volts) induced in them is proportional to the speed of the vessel. This method of measuring a ship’s speed has been found to be accurate, economic and simple.

Principle

The flow of a conducting seawater past a sensor causes electromagnetic induction in the sensor which can be measured and which is proportional to the speed of the flow of the water.

A typical EM log consists the following parts

1. Master Indicator

2. Preamplifier

3. Sensor