Echo Sounder
An Echo Sounder measures water depth by sending a pulse of sound from a transducer in the ship bottom down to the seabed and timing how long the echo takes to return. Knowing the speed of sound in water, it converts that time into a depth, displayed and recorded on the bridge.
Depth under the keel is a primary safety input: it warns of shoaling water, confirms the ship is in the expected depth for its charted position, and is watched closely in pilotage and confined waters. The echo sounder complements the chart and GPS as an independent check that the water is as deep as expected.
Grounding is one of the most common and costly casualties, and the echo sounder is the instrument that gives direct, real-time warning of shoaling water under the ship. In pilotage and poorly charted areas it is often the deciding safety input, independent of GPS and the chart.
What does under-keel clearance mean?
The depth of water between the bottom of the keel and the seabed, monitored using the echo sounder and the ship draught to ensure the ship does not touch bottom, especially in shallow ports.