Echo sounder

An echo sounder is the acoustic depth instrument that measures water depth below the keel or transducer by timing a sound pulse from a hull transducer to the seabed and back, halving the elapsed time and multiplying by the speed of sound in seawater, about 1,500 metres per second. SOLAS Chapter V Regulation 19 requires an echo sounder on ships of 300 gross tonnage and upward. The display reads depth in real time and logs a trace, supporting under-keel-clearance checks, position cross-checks against charted soundings, and grounding warnings. Calibration of the assumed sound speed sets the depth accuracy.