Sounders With Multiple HDMI Inputs, for measuring depth,are now virtually standard equipment on all but the smallest and simplest of boats. It works by transmitting pulses, or clicks, of ultrasonic sound from a transducer mounted on board, down to the seabed, and then receiving the returning echoes. Although the speed of sound in water varies slightly, it is always in the order of 1400 metres per second, so the time taken for each pulse to complete a down and back trip depends on the depth of water.

The most readily-understood timing system is that used in the ‘rotating neon’ type of sounder, in which the heart of the display unit is a fast-spinning rotor with a neon lamp or light-emitting diode at its end. Each time the rotor passes the upright position, the light flashes and the transducer is triggered to transmit its pulse. When the returning echo is detected by the transducer, the light flashes again, but by this time the rotor has moved on. How far it has moved depends on the time interval between transmission and reception, so the depth of water is indicated by the position of the second flash. It can be read directly off a scale marked on the face of the instrument around the window that covers the rotor.

For operation in deep water, the rotor speed can be slowed down, increasing the range of time intervals that can be measured and increasing the time between successive pulses, but reducing the accuracy and precision of the depth measurement.

With practice the appearance of the returning flash gives a clue to the nature of the seabed: a hard seabed such as rock produces a crisp echo which appears as a short flash; while a very soft bottom such as mud or weed gives a more drawn-out echo and produces a more diffuse or drawn-out flash. Sometimes, however, the echo sounder can be misleading.

Air bubbles are good reflectors of sound waves, so turbulence caused by the wash of passing ships can produce a mass of shallow flashes. The swim bladders of fish also contain air, so a single large fish can produce a brief flash, while a dense shoal of small fish produces a more consistent flash at a depth corresponding to the depth of the shoal. Fishermen find this useful and the echo sounder principle has been developed into fish finders, but for navigation purposes such echoes are simply a nuisance. Luckily, they are usually easy to identify because they are short-lived and erratic.