Depth sounder, transducers and fishfinder sonar
Navigation
A depth sounder times sound pulses bounced off the bottom to show the water under you; a fishfinder uses the same idea to draw the bottom and fish. The transducer, the sensor on or through the hull, makes or breaks it.
How it works
The transducer sends a pulse of sound and listens for the echo; the time taken gives the depth. Lower frequencies reach deeper, higher ones give a clearer picture. Transducers mount on the transom, through the hull, or inside the hull shooting through solid fiberglass. Air bubbles, turbulence, marine growth and a tilted transducer all weaken the echo. The display can add a keel offset so it shows depth under the keel or true depth from the waterline.
Using it
- Know what your display shows: depth below the transducer, below the keel, or from the waterline. Set a keel offset so it means what you think it means.
- Leave range on automatic so it tracks the bottom. One maker suggests more gain in deep water and at speed, less in shallow water and at low speed.
- If depth drops out at speed, slow down; air bubbles flowing over the transducer are the usual cause.
- Cross-check the sounder with the chart. It only knows what's directly under the transducer right now.
Looking after it
- At haul-out, clean the transducer face with a scouring pad and mild detergent; one maker says heavy growth can be lightly wet-sanded.
- Use only water-based antifouling made for transducers, reapplied each season. Never use ketone-based paint, which can attack the transducer's plastic.
- For an in-hull transducer in a fluid-filled reservoir, check the fluid (propylene glycol) level; flashing or missing depth can mean it's low.
- Check the cable for cuts or crimps; one maker says a damaged cable usually means replacing the transducer.
- Route transducer cables away from power cables, crossing them at right angles where they must meet.
Troubleshooting
Depth readings jumping around or blank
- Slow down to see whether the readings come back; aeration at speed is the commonest cause.
- Check the transducer face for growth or scratches and clean it.
- Check the cable and its connections.
- For an in-hull transducer, check the fluid reservoir.
- Look for interference from pumps, other electronics or another sounder, and try switching them off one at a time.
- A transom transducer must sit parallel to the water surface; any tilt gives wrong or weak readings.
- If it has always been poor, the transducer may be behind turbulence (strakes, intakes, the propeller) and may need moving.
Depth reads consistently deeper or shallower than the chart
- Check the keel offset setting: positive for depth below the keel, negative for true depth from the waterline, in one maker's system.
- Allow for the tide and for charted depths being measured from a chart datum.
Buying a boat that has one
- Find out what kind of transducer is fitted and where. Through-hull units are underwater fittings and should be inspected at every haul-out.
- Look at the transducer face and cable when the boat is hauled for survey.
- In-hull transducers only work through solid fiberglass, not cored, wooden or metal hulls.
- If you plan a new display, check whether the existing transducer and its connector will work with it; adapter cables exist for some pairings.
Sources disagree on installation: how far ahead of a fin keel to fit a through-hull transducer — left out rather than guessed. Check your local authority.
Sources (4)
support.garmin.com · support.garmin.com · www8.garmin.com · airmarweb.com
General guidance for recreational boaters, researched from the sources listed and written in our own words. Your equipment manuals, local knowledge, the Navigation Rules and a qualified instructor come first.