Fish Finder and Depth Sounder Transducers: Mounting, Fixing Bad Readings and Cleaning

Following Airmar's guidance as described by Boating Magazine:

• Single engine: mount on the side where the propeller blades are moving down, which on most single outboards and sterndrives is the starboard side.
• Place it as close to the centerline as you can, but at least 3 inches outside the propeller's swing so the prop's turbulence doesn't hit it.
• Twin engines: mount between the drives.
• Avoid spots directly behind through-hull fittings, water pickups, strakes or anything else on the bottom that churns up bubbles.

Also look at how the boat sits on its trailer: the transducer must not hit a bunk, roller or the trailer frame when you load. On aluminum boats, check that the area behind it is a smooth part of the hull, not a rivet line or keel.

Pontoon boats: there's no flat transom, so the usual spot is a bracket at the back end of the pontoon tube on the console side, which keeps the cable run short. J.D. Power's pontoon guide points to the brackets already on the back of many tubes. Drill only the bracket, never the tube itself, and use stainless hardware. A cable that barely reaches is common on long pontoons; plan the route before mounting rather than cutting and splicing. Keep it clear of livewell pumps and where it would hit trailer bunks.

Before drilling, look inside at the spot you picked: no fuel lines, wiring, livewell plumbing or a hollow area you'd drill into. If your transom is soft or you already suspect rot (see the transom rot guide), fix that first. If you'd rather not drill the transom at all, Boating Magazine suggests a plastic mounting block glued to the hull that the bracket screws into, which is especially handy on aluminum boats.

Most transducers come with a paper template. Tape it to the transom with its bottom reference aligned with the bottom edge of the transom and the template parallel to the waterline (not to a tilted transom edge). Use the hole positions marked for your transom angle; many templates offer two sets.

• Wrap masking tape around the bit to mark the hole depth your instructions give, so you don't drill through.
• Drill square to the transom surface, not level with the ground, so the screw heads sit flat.
• On fiberglass, Boating Magazine suggests running the drill in reverse until you're through the gelcoat, then chamfering each hole lightly with a countersink so the gelcoat doesn't crack.

Fill each hole with below-waterline marine sealant and coat the screws too. Screw the bracket on, but leave the screws snug, not tight, so you can slide it in its slots to set the height. Wipe off excess sealant.

Your transducer's instructions give the exact starting height and angle; follow them. The general rules:

• The face of the transducer should sit slightly below the bottom of the hull so it stays in solid water at speed.
• The bottom of the transducer should run parallel to the water when the boat is at running trim, not just sitting at the dock. Many instructions call for the front to sit slightly higher than the back.
• Too high and it sees bubbles; too low and it throws a "rooster tail" of spray and drag.

Tighten the screws once it's set, then snap the transducer into its bracket.

• Run the cable up and over the transom if you can, so there's no new hole.
• If it must go through the transom, drill the hole well above the waterline and use the cable cover or grommet supplied, bedded in sealant.
• Leave a small drip loop and enough slack for the bracket to kick up.
• Clamp the cable every so often along its run. Keep it away from engine wiring, VHF coax and power cables where you can, and don't cut and splice it unless the maker allows it.
• Coil extra cable neatly near the display; don't shorten it.

If it has a speed paddlewheel, Boating Magazine suggests spinning it after connecting to confirm the speed reads, and checking that the temperature reading is close to the air temperature. On the water, check the depth reading at idle, then at increasing speeds. Note the speed at which you lose bottom.

Losing the bottom or seeing a screen full of "snow" as you speed up almost always means air bubbles are passing under the transducer.

• Settings first: try manual depth range and gain instead of auto. If the reading comes back, the transducer may be fine and the auto settings were confused. If your unit has more than one frequency or a CHIRP option, try each one.
• Connection: an intermittent plug at the display gives the same dropouts as bubbles. Check it is fully seated with the locking ring tight, and test at idle: dropouts at rest point to wiring, dropouts only at speed point to bubbles.
• Growth or damage: a dirty, scratched or cracked face reads poorly at any speed. Clean it (step 11).
• Height: lower the transducer in small steps (an eighth of an inch or so at a time) and retest. Stay within the range your instructions allow.
• Angle: adjust the running angle slightly so the face meets the water flat at cruising trim.
• Location: if nothing works, something upstream (a strake, through-hull or the prop) may be throwing bubbles. Moving the transducer, adding an in-hull unit (solid fiberglass only), or a thru-hull installed by a pro may be the real fix.

Some boats will lose bottom at high speed no matter what; a transom mount that reads well at fishing and cruising speeds is often as good as it gets.

• No reading at all: check the cable connection at the display (pins clean and dry, plug fully seated and locked), the display's power and fuse, and the cable for cuts or crushing along its run.
• Display flashing zeros or dashes, often with the alarm beeping nonstop: owners often blame the depth alarm settings, but this usually means the unit isn't receiving a usable signal at all. Work through the connector, the cable and the display's 12-volt power and ground first. On an in-hull (shoot-through) transducer, check that it hasn't shifted or lost its bond to the hull.
• Reads fine going forward, garbage in reverse: normal. Prop wash in reverse sends bubbles straight past a transom transducer.
• Side-imaging dark on one side: a hull strake, keel or chine can shadow one side of a side-imaging transducer. Moving it is a trade-off (more prop wash, risk of disturbing flow to the prop if mounted ahead of the lower unit), so read the transducer maker's mounting guide before you drill new holes.
• Interference or lines on the screen when the engine runs: route the transducer cable away from engine and ignition wiring, check the display's power and ground connections (see the electrical corrosion guide), and ask whether two sonars on the same boat need different frequencies.
• Shallow or false readings: confirm the depth offset or keel offset setting in the menu; a wrong offset shifts every reading.
• Speed reads zero: the paddlewheel is fouled with weed or growth; clean it and check it spins freely.

Raymarine notes marine growth can build up on a transducer face and hurt performance within weeks. To clean:

• Start with a soft cloth and mild household detergent.
• For heavier growth, use a non-scratch or green scrub pad with detergent; for severe buildup, fine wet-and-dry sandpaper used gently with detergent.
• Never use harsh solvents such as acetone, and don't scratch the face; both damage the transducer.

On a boat that stays in salt water, Raymarine recommends a thin coat of water-based antifouling paint made for transducers, renewed every six months or each season. Raymarine also warns:

• Never use ketone-based paint, which can attack the plastic.
• Never spray-paint a transducer: spraying traps tiny air bubbles that block the sound.
• Transducers with a built-in temperature sensor may not read correctly if painted, and some "smart" transducers limit where paint can go. Check yours.

Don't use ordinary bottom paint on it, and don't bother painting a transducer on a trailer boat that's rinsed after every trip.

Installing, replacing or removing a thru-hull transducer means a large hole below the waterline, usually a fairing block to aim it straight down, proper sealing of any hull core, and a correct backing and nut. A mistake can sink the boat. Have it done by a qualified installer, usually at haul-out. In between, you can still:

• Inspect the inside nut and fitting for weeping, cracks or corrosion every time you're in the bilge (see the seacocks and through-hulls guide).
• Clean and paint the face during haul-out as in steps 11 and 12.