How to Place and Adjust a Side-Imaging and Down-Imaging Transducer

Side imaging builds its picture from hundreds of thin slices as the boat moves past things, a bit like a medical scanner. Humminbird explains that the boat must be moving for SI to work, and recommends about 2 to 8 mph for the best detail. It notes SI generally works best under 10 mph; larger SI transducers are strongly affected by turbulence at higher speeds.

So you are tuning for a clean picture at trolling and slow cruising speed. If you also want depth readings at planing speed, that may take a separate, smaller transducer (see step 12).

Humminbird lists three ways to mount a side-imaging transducer:

• Transom mount (comes with most units): the usual choice on outboard and sterndrive boats.
• Trolling-motor mount (adapter kit): puts the transducer on the trolling motor's lower unit.
• Thru-hull (optional): for boats where the transom water is too turbulent, such as larger boats and inboards.

Side imaging can't be glued inside the hull. Humminbird says side beams can't shoot through the hull properly, and it doesn't recommend in-hull mounting for down imaging either.

Look at your propeller. Boating Magazine's guidance is to mount on the side where the prop blades move downward, which is usually the starboard side on a single outboard or sterndrive with a standard (right-hand) prop. Prop wash is rougher on the other side.

Keep the transducer clear of the prop's swing and its wash, using the distance your instructions give. On twin engines, the usual spot is between the drives. Get it as close to the centerline as those limits allow; that helps the hull stay out of the way of the side beams (step 5).

The transducer needs solid water without air bubbles flowing across its face. Air bubbles come from:

• Lifting strakes and steps on the hull bottom (turbulence trails behind each one).
• Rows of rivets on aluminum hulls.
• Through-hull fittings and water pickups ahead of the spot (Boating Magazine warns against mounting behind them).

Lie under the trailer and look at the hull bottom forward of your planned spot. You want a smooth, flat run of hull with nothing sticking out ahead of it. Also check the trailer: the transducer must not land on a bunk or roller when you load.

Humminbird's troubleshooting guide says not to mount an SI transducer next to other transducers, speed or temperature sensors, or the motor, because they block the signal on that side. Picture a flat fan of sound going straight out left and right from the transducer, slightly downward. Ask:

• Will the hull block it? On a deep-V hull, a transducer far from the centerline can "see" the hull on the inboard side. Moving it closer to the centerline or a bit lower usually fixes this.
• Will the motor block it? On some boats the outboard's lower unit sits right in the line of one beam. Humminbird's fix is a second matching SI transducer on the other side, joined with a splitter cable.
• Will another transducer block it? Keep the SI transducer away from the old 2D transducer, speed paddle wheel and live-sonar units.

On a pontoon, the tubes themselves can block the side beams. The transducer has to sit low enough, and in a spot where neither tube is in its line of sight.

Most transom brackets let you slide the transducer up and down. Too high, and air sneaks under the face at speed, so the picture breaks up or goes blank. Too low, and it throws spray (a "rooster tail"), adds drag and is more likely to hit things.

Start at the height your instructions give; many start with the face just slightly below the bottom of the hull at the transom. Mark the bracket so you can see later how far you've moved it.

For SI to show both sides evenly, the transducer must point straight down when the boat sits in the water. Humminbird's instructions start by confirming the boat is level, then setting the transducer body parallel to the hull before tightening the pivot.

• Set it parallel to the waterline fore and aft (use a level or angle finder, with the trailer adjusted so the boat sits as it floats).
• Make sure it isn't tilted side to side. A transducer leaning toward one side makes one side's picture reach farther than the other.

Some installers add a very slight nose-down tilt to keep water flowing smoothly; follow your instructions here.

• Tape the template to the transom, check it's level, and drill to the depth your instructions give (wrap tape around the bit as a depth stop).
• On fiberglass, Boating Magazine suggests running the drill in reverse until you're through the gelcoat and chamfering each hole to prevent cracks.
• Fill the holes and coat the screws with below-waterline marine sealant before driving them.
• If you'd rather not keep drilling the transom, a mounting plate (a polymer board block bonded to the hull) takes the screws instead. This is popular on aluminum boats, and you can re-drill the plate if you need to move the transducer. Mercury's version needs just two small screws and sealant in the transom. BoatUS describes a taller board that reaches above the waterline and is through-bolted there, so no hole in the transom is below the waterline at all.

If a hole goes into a wet or soft transom core, stop and read transom and stringer rot check before going further.

A trolling-motor mount keeps the transducer in clean water at fishing speed and points wherever the motor points. Two cautions from Humminbird:

• Depending on which way the trolling motor is turned, the side beams may be looking at your own hull. Check the picture with the motor pointed straight ahead.
• On trolling motors that deploy, stow or trim themselves, leave enough cable slack for the full motion.

Secure the cable to the shaft with the clamps in the kit, not tight zip ties that can cut the jacket.

Run the cable to the display along its own path, away from engine wiring, trolling-motor power leads and VHF coax where you can. Don't cut and splice the transducer cable; use an extension cable made for it if it's short.

If you see random lines or speckles, Humminbird suggests testing with the boat sitting still and everything electrical and the motors off, then turning things back on one at a time until the noise comes back. If you have more than one transducer pinging at once, Garmin notes they can create cross talk, which its units reduce with the Interference setting. BoatUS adds that another boat's sounder nearby on a similar frequency can do the same, and that transom transducer cables are easily damaged on the trailer, so check the cable along its whole run. See electronics power dropouts for supply-voltage problems.

Pick calm water with known structure (a dock, a channel edge, a bridge piling):

1. At rest: confirm the 2D depth reading is steady.
2. At 2 to 8 mph in a straight line: watch both sides of the SI screen. They should look similar in brightness and range. Run past the same structure with it first on your left, then on your right.
3. At higher speed: note where the down and 2D picture breaks up.

Clean the transducer face first and rub off any air bubbles on it when the boat goes in. Humminbird notes some antifouling paints can't be used on transducers, or only in one thin coat.

Change one thing at a time and re-test:

• One side dark or short: check for blocking (step 5); move the transducer slightly lower or closer to the centerline, or check that it isn't tilted.
• Picture breaks up at speed, fine when slow: move it down slightly, check for a strake or fitting ahead, or accept that SI is a slow-speed tool.
• Spray, drag, or the transducer kicking up: move it up slightly.
• Port and starboard swapped: many units have an SI orientation setting (Humminbird calls it Reverse) instead of remounting.

If you need good depth readings at planing speed, Humminbird suggests pairing the SI transducer with a separate high-speed 2D transducer through a splitter cable. Note the final height and angle in BoatMend so you can reset it after a trailer bump.

A well-placed transducer can still give a poor picture on auto settings. BoatUS's electronics editor suggests:

• Range: you rarely get useful side returns beyond about 20 times the water depth, so in 4 feet of water a range near 80 feet uses your screen better than 100. Don't leave blank screen past the returns.
• Frequency (if your unit lets you choose): higher frequencies give more detail but less range.
• Contrast and sensitivity together: more contrast over hard bottoms, less over soft mud; and usually less sensitivity over a hard bottom, more over soft. Mercury's pro angler adds that in shallow water (20 feet or less) you rarely need to add 2D sensitivity at all.

Then set the depth offset. Mercury explains a fishfinder measures from the transducer face, so with the boat on the trailer measure from the waterline stain to the bottom of the transducer and enter it in the unit's keel or waterline offset menu. Check it against a tape measure dropped to a firm bottom in calm, shallow water.