An echo sounder, or ‘sonar’ in American terminology, is a sound-detecting instrument used to measure the distance between the surface of the water and objects in the water or on the ocean floor. An essential item of navigational and safety equipment, an echo sounder detects objects underwater by emitting a sound pulse via a transducer mounted on the hull or lowered to the required depth and then measuring the reflected echo. By using a set formula – bearing in mind that sound travels more slowly in fresh water than in sea water – the time lapse between emission and echo gives an accurate measurement of sea depth and other marine topography. Modern echo sounders can analyse the pulse return in order to display information including the composition of the sea bed, the presence of obstructions, and the location and size of fish in the water.
The first echo sounders were used by the British Admiralty during the First World War and replaced the old wire and lead approach to measuring the depth of the sea. A modern echo sounder provides an accurate indication of the range of any submerged objects. Observation of the tone of the echo, called the Doppler effect, provides information about whether the object is moving – such as a submarine or fish – and if so in which direction.
A good echo sounder will show the user a host of detail about the bathymetric terrain and marine features, including the sea bed; rocks and coral reefs; shoals of fish; shipwrecks; submersible craft; and all sorts of submerged obstacles which may or may not be marked on the chart. They are vital for manoeuvring safely in areas of limited depth where there is a risk of a craft running aground.
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