ELECTROMAGNETIC SPEED LOGS
Electromagnetic speed logs are an older method of determining velocity of a vessel through the water. This system utilizes a flow probe that extends out beyond the ship's hull. The probe contains a coil through which an electrical current is passed producing a magnetic field around the probe. As water moves past the probe, the magnetic field becomes altered indicating the vessel's speed through the water. Unfortunately, when measuring speed through the water, the unit cannot compensate for current.
The probes normally retract into the hull where they can be protected when not in use and serviced as necessary. Generally the probe requires regular routine maintenance. This system has become less common since the introduction of the Doppler speed log system.
