Electromagnetic Speed Logs

Thursday, August 26, 1999
IMO-compliant EML500 Series Electromagnetic Speed Logs, distributed by Offshore Systems International, Inc., features a variety of hull configurations to satisfy any type of application, including high-speed dual-axis systems. They also feature a wide range of external digital and analog interfaces to allow communication with any piece of navigational equipment requiring speed information. In addition, in the event of damage to the sensor, the EML500 will accept the NMEA speed information from a DGPS and will feed the repeaters and navigational equipment normally. A recent order for the high-speed version of the EML500 system was secured by Marine Electronic Solutions, Inc. in Jacksonville, Fla., to the USCG and Bollinger Shipyards for the Coastal Patrol Boat Program. The Marine Protector Class will replace the aging 82-ft. Point Class Cutters in use, currently. The principle of an electromagnetic speed log is simple. It is based on Faraday's law of electromagnetic induction or Electromagnetic Flux (EMF). With speed logs, a magnetic field moves along with the ship, while the seawater - the conductor - passes underneath it. A coil in the sensor of the EML500 creates this magnetic field, and as the ship moves, a pair of sensors detects the EMF and converts it to a corresponding voltage. The EML500's computer then detects this voltage and converts it to actual "speed through the water." Electromagnetic speed logs (unlike Doppler speed logs, which utilize sound waves) are fundamentally not affected by the salinity, silt or aeration in the surrounding water, thus improving the system's accuracy and overall performance in all conditions, says the company.

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