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Solved problems · Electronics · Crew answer

Why is my depth sounder giving erratic readings?

Erratic depth usually comes from a fouled transducer face, air bubbles and turbulence over the transducer, or a wiring fault. Soft bottoms and thermoclines can confuse it too.

Symptom

The depth display jumps around, shows random shallow readings, goes blank at speed, or doesn't match the chart.

Likely cause

The transducer needs clean, bubble-free water over its face. Growth, the wrong antifouling paint or a scratched face weakens the signal. Bubbles from a nearby speed paddlewheel, through-hulls, strakes or the propeller cause dropouts, especially at speed. A loose or corroded connector, a damaged cable, or electrical noise from other equipment causes random readings. Very soft mud, heavy weed, deep water beyond the unit's range, or fish and thermoclines can also give false readings.

Stop now if

The readings don't make sense near hazards. Slow down, use the chart, a lead line if you have one, and visual cues. Never rely on a doubtful sounder in shoal water.

The fix, in short

  1. Slow down and compare. Note whether readings settle at low speed. Trouble only at speed points to turbulence and bubbles over the transducer.

  2. Clean the transducer face. When hauled or with a diver, clean growth from the face gently. Airmar's paddlewheel instructions call for mild household detergent and warn against solvents, which attack the plastic. Use only antifouling paint the transducer maker approves.

  3. Check the location. Garmin's installation guide says transducers shouldn't sit behind strakes, struts, fittings, water intakes or discharges, or in the propeller's path. A speed paddlewheel directly ahead of the depth transducer can also disturb it.

  4. Inspect cable and connections. Look for corroded or loose connectors and damaged cable, and keep the cable away from VHF coax and high-current wiring.

  5. Adjust settings. Check the display's depth offset, gain and range settings. Auto gain sometimes struggles in soft mud or very deep water.

  6. Replace a failed transducer. If cleaning and wiring don't help, the transducer may have failed internally.

Written by Sailor Cruiser crew. General guidance, not an inspection of your boat.

Equipment involved

  • Navigation electronics: Depth sounder or fishfinder, through-hull or in-hull transducer, cable and display

Related guides

Sources

Background reading we checked while writing this answer. The wording above is our own.

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