EM Log

 Principles of Electromagnetic Speed Log:-

• The electromagnetic log is based is upon the induction law, which states that if a conductor moves across a magnetic field, an electro motive force (e.m.f.) is set up in the conductor.
• Alternatively, the e.m.f. will also be induced if the conductor remains stationary and the magnetic field is moved with respect to it.
• The induced e.m.f. is directly proportional to the velocity.
• Velocity when integrated with time gives distance
• The induced e.m.f. "E" is given by the following:

E = F X L X V

• Where F = magnetic field
• L = the length of the conductor
• V = the velocity of the conductor through the magnetic field.

Errors / Limitations:

Siting of the probe is critical. This is so since if too close to the hull then due to the non-linearity of the hull form the speed of the water flow may give a wrong representation of the vessels speed. This is minimized by careful siting of the sensor as well as by calibrating the instrument while installation.

Pitching and Rolling also give rise to errors however these are reduced by having an electrical time constant that is longer than a period of vessel motion. A welladjusted log can have an accuracy of better than 0.1 percent of the speed range.

Sign of Speed, it can show astern speed as well, but without sign if AC current is used, if DC current is used to create the magnetic field it will show sign of speed range. This type of log can give only speed through water and is greatly affected by the current flowing under the ship.

While navigating in area with greater current, one must exercise precautions.

Advantages

• No moving parts
• Less affected by sea growth than Pit sword

Disadvantages

• Salinity and temperature of water affect calibration.
• Measurements affected by boundary layer, (water speed slowed down close to the hull by friction).
• Provides boat/ship speed relative to water not ground. Current affects accuracy.