THE DTMB MARK 2 KNOTMETER,

The DTMB Mark 2 Knotmeter is a portable system which measures the speed of a ship through the water at speeds from 3 to 15 knots. The system consists of a towed body, towcable, and associated electronic instrumentation. A transducer in the towed body senses the rotation of an impeller on the body and sends a signal up the towcable for display on an electronic counter, thus providing a measure of the ship speed. In contrast to many speed measuring systems, the Mark 2 Knotmeter is not influenced by the pressure field of the ship since the transducer can be located at a considerable distance from the ship. During steady towing conditions, the speed can be determined to an accuracy of about one percent. Unsteady towing conditions caused by ship motions in a seaway reduce the accuracy to between one and five percent depending on the severity of the ship motions. Author