Transducer Troubleshooting: Replacing Your Boat's Sonar Sensor

Replacing a transducer on a boat can be a daunting task, but it is a necessary one to ensure the proper functioning of your vessel. A transducer is a device that sits below the waterline and provides critical data such as depth and speed information, as well as advanced features like sonar and fish-finding. In this guide, we will walk you through the process of replacing a transducer, covering topics such as choosing the right type of transducer, installation techniques, and tips for optimizing performance. By the end, you should feel confident in your ability to tackle this project and improve the functionality of your boat's systems.

Choosing the right transducer for your boat

• Boat Type: The installation location of the transducer depends on the type of boat you have. For sailboats, the transducer should be installed in front of the keel and as close as possible to the midship line. If you have a full-keel sailboat, opt for the end of the first third of the hull. Motorboat owners should install the transducer at the beginning of the last third of the hull, preferably near the midship line and in the area of the first stringer before the engine compartment.
• Transducer Housing Material: Choose a transducer housing material that is compatible with your boat's hull material. Plastic transducer housings are suitable for GRP (glass-reinforced plastic) or metal hulls but should not be used with wooden hulls as wood expands and contracts, which can damage the plastic. Steel or corrosion-resistant transducer housings are ideal for aluminium steel hulls to prevent electronic corrosion. Bronze transducers work well with wooden or GRP hulls but should be avoided for aluminium hulls.
• Desired Data Output: Consider what type of data you want from your transducer. Do you need depth information, speed details, or temperature readings? If you want a combination of these features, look for multifunction transducers.
• Performance: The strength of a transducer signal is measured in watts RMS (Root Mean Square). Higher-performance transducers have a higher watt RMS value, resulting in improved transmission and more detailed images, even in deep or murky waters.
• Installation Process: The most popular type of transducer is the transom transducer, which can be easily installed on wooden, fiberglass, aluminium, or steel hulls. They offer the advantage of adjustability and are less prone to damage during slipping or contact with driftwood. However, they may not be suitable for sailboats with sterns that lift out of the water during heeling. Through-hull transducers, on the other hand, provide good results even at high speeds but require a larger hole in the hull bottom and are more expensive. They are also more prone to damage from floating debris and trailer bunks.

How to replace a transducer without sinking

Firstly, it is important to note that if you are installing a thru-hull transducer, your boat must be on land. Never install a thru-hull transducer when the boat is in the water. Transom and in-hull transducers can be installed while the boat is in the water.

Choosing the right transducer

The type of transducer you need depends on the material of your boat. Plastic transducers are suitable for GRP or metal hulls but should never be used with wooden hulls. Steel or corrosion-resistant transducers are best for aluminium steel hulls, while bronze transducers are suitable for wooden or GRP hulls.

Choosing the right location

The right location depends on the type of boat. However, there are some general rules to follow:

• Never install the transducer near thru-hull fittings, drain holes, chine edges, outboard see valves, sacrificial anodes, speed sensors, keels, propellers or propeller shaft systems.
• For trailer boats, avoid installing near shoring, keel rollers or trailer components.
• Choose an area where the transducer won't be disturbed by interferences from other sources of power or radiation.
• Choose an area that is easily accessible, with sufficient space for the size of the transducer shaft and for fastening the nut.

Installation process

For thru-hull transducers:

• Drill a test hole of approximately 6mm in a vertical direction to the waterline.
• Drill a larger hole for the diameter of the transducer. Ensure the hole is vertical to the waterline.
• Remove any old antifouling and primer around the hole. Sand the area and clean the inside and outside of the hull.
• Stick the transducer through the hole to ensure everything fits. The transducer must be vertical to the waterline to send a signal at a straight angle.
• To avoid tilting, correct the angle using a skeg or fairing block.
• Cover the hull and transducer with sealing compound and insert the transducer from the outside. Tighten the screw but not too firmly to avoid water ingress.
• Check the area around the transducer for leaks when the boat is in the water.

For transom transducers:

• Mark and drill the drill holes for the transducer mount.
• Insert the screws and secure with sealing compound.
• Place the transducer into the mount and align it.
• Drill a hole above the waterline to feed the cables into the boat's interior.
• Feed the transducer cable and seal the hole with sealing compound.

For in-hull transducers:

• Clean the desired installation location and remove paint and dirt. Sand the hull and transducer.
• Mix the epoxy adhesive and apply it generously to the hull and transducer.
• Secure the transducer to the hull as vertically as possible.
• Spread the adhesive around the transducer and let it dry.
• Ensure the cables are running properly.

Tips

• Some transducers have a little flap that will slow the gush of water when you pull the transducer out.
• Have everything to hand and think through the process before pulling out the old transducer.
• Work fast and have someone help you—one person inside the boat and one in the water.
• Cover the hole with a dummy plug, a tarp, a plunger, or your hand.

Thru-hull transducer installation

A transducer is a device that is installed below the waterline to provide data to a display. This data can include depth, speed, and even sonar and fish-finding information. When replacing a transducer, it is important to consider the size of the new unit, as you may need to enlarge the mounting hole to accommodate it.

• Removal of the Old Transducer: If you are replacing an existing transducer, start by removing the old unit. This may involve cutting it out with a saw if it is not a removable type. Clean the mounting location and determine if there are any leaks that need to be repaired.
• Enlarge the Mounting Hole (if necessary): If the new transducer is larger than the original, you will need to enlarge the mounting hole. Use a hole saw to create a template of the correct size and transfer the dimensions to the hull. Cut out the hole with a tool like a Dremmel, ensuring that you frequently check the fit to avoid removing too much material.
• Dry Fit the New Transducer: Before applying any sealants, perform a dry fit to ensure that the new transducer fits properly. Check the thickness of the backing plate and the fit of the backing nut. Also, pay attention to the orientation of the transducer, as it can affect its proper operation.
• Apply Tape and Sealant: Apply tape to the outside of the hull to catch any excess sealant that may squeeze out during installation. Use a sealant designed for below-waterline applications, such as 3M 5200. Apply a generous bead of sealant around the inner lip of the transducer and fit it into the mounting hole.
• Secure the Transducer: Move inside the boat and add more sealant between the threads and the mounting hole, filling any gaps. Apply a layer of sealant to the underside of the backing plate and screw on the tightening nut. Do not apply sealant to the threads of the transducer. Tighten the nut by hand first, and then use pliers to finish tightening, ensuring that the transducer stays straight and the threads are aligned.
• Clean Up: Use a dry cloth to wipe away any excess sealant. Avoid using solvents near the transducer and plastic components to prevent damage. Remove the tape from the outside of the hull and clean up any remaining sealant residue.

It is important to note that working with thru-hull transducers can be challenging, and it is always recommended to consult or seek assistance from a marine technician if you are unsure about any part of the process.

Transom transducer installation

Transom transducers are usually mounted with a bracket on the transom. This is a convenient location that requires no large holes in the bottom of the boat and eliminates interference with trailer bunks. However, this area is also prone to excessive aeration as water streams out from under the hull and props agitate the water.

• Ensure the connector plug on the cable matches the transducer port for your particular brand of fish finder.
• On single-drive boats, locate the transducer on the side where the propeller blades are moving downward, usually the starboard side of a single outboard or sterndrive. It should be as close to the centreline as possible but at least 3 inches outside the swing radius of the prop blades.
• For twin drives, mount the transducer between the drives. Avoid areas abaft through-hull fittings or pickups that create turbulence.
• If you opted for a speed sensor, connect the cable to the fish finder and spin the paddle wheel to ensure it reads. Make sure there is an accurate temperature reading.
• An alternative to drilling holes in the transom, especially on aluminium boats, is to use a King Starboard (poly) block such as a Stern Saver to accept the retaining hardware. The block gets glued to the hull with a special adhesive, and hardware gets screwed into the block.

To install the transom transducer, follow these steps:

• Position the bracket: Cut out the supplied template and position it so the arrow at the bottom is aligned with the bottom edge of the transom. Ensure the template is parallel to the waterline, then tape it in place. Drill three holes 7/8-inch deep at the marked locations. To prevent drilling too deeply, wrap masking tape around the bit 7/8-inch from the point. To minimise gelcoat cracking, run the drill in reverse until the gelcoat is penetrated. After drilling, lightly chamfer each hole with a countersink bit to prevent further cracking. Drill holes 90 degrees to the angled transom surface to ensure that the screw heads sit flush with the bracket.
• Mount the bracket: Apply marine sealant to the three No. 10-by-13/4-inch self-tapping stainless-steel screws and the three mounting holes. Use a manual Phillips screwdriver to screw the bracket into the transom, but don’t completely tighten the screws. Adjust the bracket's height and ensure it is parallel with the waterline. Then, tighten the screws.
• Attach the transducer: Open the retaining cover on the top of the transducer bracket by depressing the latch and rotating the cover downward. Insert the transducer’s pivot posts into the slots on the top of the bracket. With the posts in position, push down on the transducer to click them in place. Rotate the transducer downward until it snaps into the bracket. Close the retaining cover. Install the paddle-wheel speed sensor, if you have one. Route the transducer cable to the fish finder. If necessary, drill through the transom to route the cable, using the supplied cable cap and sealant to prevent water intrusion. Use cable clamps to secure the cable along its length.

In-hull transducer installation

An in-hull transducer is installed without drilling or cutting holes in the boat. This type of transducer is mounted to the inside of the hull and transmits and receives sonar signals through the fiberglass. This eliminates the need to remove the boat from the water for installation and there will be no protrusions under the boat.

However, in-hull transducers have some downsides. They must be mounted on solid fiberglass; they will not function in cored fiberglass hulls without special modifications. In-hull models also exclude a sea-temp or speed sensor, so these will have to be added externally on the transom if desired.

• Choose a location: It is crucial that there are no external strakes, intakes, or other hull features that would cause aerated water to flow over the transducer's location. The location depends on the type of hull and propulsion. For inboard planing hulls, just ahead of the engines and prop shafts is the best spot. For displacement hulls, the transducer can be positioned further forward. On outboards and sterndrives, an aft location is optimal, except on step hulls, which necessitate placement forward of the steps. Locate the transducer as close to the center line as possible. If the mounting surface is not smooth, grind it flat with a disc sander.
• Testing: Establish a performance baseline by connecting the transducer to the fish finder and hanging the transducer overboard with the boat floating at rest in at least 25 feet of water and viewing the bottom return. At the same site, test the transducer inside the hull at the selected location. Place the transducer inside a thin plastic bag partially filled with water and cinched tightly with a cable tie. Wet the mounting surface and press the active face of the transducer against the hull. Compare the bottom return to the baseline, and if the reading is similar, the mounting location is good. If not, you will need to find another location and repeat the test.
• Measure and adjust for deadrise: Measure the deadrise, the angle of the boat's V, at the mounting location. A free app called iNstall enables your phone to determine the deadrise angle. Another way is to create a straight edge either inside or outside the hull that runs parallel to the waterline, then use a protractor to find the angle. Using a carpenter's square, draw a line on the inside of the hull perpendicular to the centerline through the center of the mounting location. Identify the number on the flange of the transducer base that corresponds to the deadrise, and find the matching number on the opposite side. These will align with the line you drew.
• Bond the housing: Ensure that the mounting surface is free of fiberglass wax, paint, oil, or moisture. If necessary, lightly sand and clean again with isopropyl alcohol, then redraw the line that positions the base. Now you're ready to secure the base to the inside of the hull. Remove the transducer from the base housing. Use a bonding agent such as Bondo 401, Marine-Tex Epoxy Putty, or 3M 5200 Marine Adhesive/Sealant. Apply the adhesive per the manufacturer's recommendation for optimal adhesion, and press the flange of the base firmly in place to form a liquid-tight seal. Allow the bonding material to cure per the manufacturer's instructions.
• Final steps: Once the bonding material has cured, pour 14 ounces of propylene glycol (non-toxic antifreeze) into the housing. Lubricate the O-ring from the top of the housing with petroleum jelly. Carefully place the transducer into the housing to avoid causing the propylene glycol to overflow. Turn the transducer until the number on the rim that corresponds with deadrise is directly over the alignment boss closest to the keel. The transducer should drop into place. Insert a rubber washer and screw into each of the two holes by the alignment bosses. Snug up each screw to 1.3 foot-pounds. Route the cable to the fish finder and plug it in.

Frequently asked questions