What Is a Transducer?

A transducer is the underwater sensor for your depth sounder: it sends out an ultrasonic pulse and works out the depth from the time it takes for the echo to return from the seabed. In shallow water it is your only real eye, but what the number means depends on how it has been installed.

Offset setting tells the instrument whether it is measuring from the waterline or from below the keel. Get the offset wrong, and that is the hidden cause behind the classic “the instrument showed 2 metres” grounding story.

Maintenance and calibration

• Fouling: once weed and mussels start to build up on the transducer surface, readings become erratic and then fail altogether; cleaning it is a routine diver job.

• Air-bubble blindness: at speed or in a strong following current, bubbles against the hull can swamp the signal, and the display will show “no depth”.

• Start-of-season check: compare the instrument with a known depth, such as alongside a marina pontoon, and confirm the offset.

When applying antifouling, do not paint the transducer face with standard paint; use a special transducer paint or leave it bare, depending on the maker’s instructions. On combined units, the tiny speed impeller can seize with slime, and speed then reads 0.