Updated 11 Oct 2019
Radar may give ships the ability to ‘see’ other ships and fixed and floating objects at and above sea level but it is equally important for vessels to stay in water deep enough to keep afloat and avoid running aground.
All passenger vessels and other ships above 300gt are required to be fitted with an echo sounder which uses sonar to measure the depth beneath the ship. SONAR (SOund NAvigation Ranging) is a technique that uses sound propagation to detect objects on or under the surface of the water. The term SONAR is more usually associated with military applications used for detecting submarines but in commercial applications it is used for measuring under keel clearance and occasionally for object detection, mostly rocks or wrecks.
The IMO performance standards require a range from 2m-200m and the ability to use two different scales; one for shallow waters to 20m and a second for deeper waters.
The echo sounder must be capable of giving an alarm when a pre-determined minimum depth is encountered. Data must be recorded and at least the previous 12 hours of information must be stored and be available. Older echo sounders recorded their information on paper rolls and this is still a permitted method. Newer more advanced models make use of electronic recording and also take positional input from a GPS.
The display on a newer model will generally be in colour on a small LCD screen. In common with other modern equipment, echo sounders often have features that exceed IMO requirements.
There is a potential conflict between sonar and ECDIS where the data included in ENCs covers depth information. Using ECDIS for passage planning involves entering a ship’s draught and setting alarm parameters. Just as with paper charts there is potential for the depths recorded in ENCs to be inaccurate and it may be necessary to deviate from the passage plan if the sonar reports a difference in the ENC data and the actual under keel depth.
Water depths measured in real time by buoy-mounted sensors may be a complement to sonar at some time in the future.