Abstract: A transducer assembly for a speed measurement device for measuring the speed of a marine vehicle through the water is disclosed. The device comprises a transmitter circuit and a receiver circuit connected to two transducers which are positioned substantially in the direction of travel of a marine vehicle below the water line thereof. Pulse trains are transmitted in both directions between the transducers which are received by the receiver circuitry so that a signal is derived indicative of the time taken for the signals to travel both for and against the direction of travel of the ship, this signal being proportional to ship's speed. The assembly including the transducers includes outwardly projecting ribs for directing the aerated water away from the acoustic path between the transducers when the marine vehicle is moving through water.