A depth sounder is an instrument used to measure the distance between the vessel's keel and the seabed.
By measuring water depth, the depth sounder indicates risk of grounding when sailing in shallow waters, canals and ports. Most vessels use a single beam depth sounder. Hydrographic and research vessels use multibeam sounders.
The depth sounder comprises a transmitter/receiver unit attached to the boat's hull and a screen on the bridge. The sounder measures the time required for an emitted signal to return to the vessel and calculates the depth of the water. Particularly long vessels have sending/receiving units at both bow and stern for more complete information. Most depth sounders have frequency adjustment controls.
Choice will depend on specific needs. Single beam sounders give the depth, while multibeam units furnish a much more complete image of bottom configuration. The aperture characteristics of a multibeam sounder constitute an important consideration. Vessel length will also determine installation of one or more sounders.
