How to Replace Transducers: Don’t be Intimidated by the Challenge

through-hull transducer, boat transducer

The original transducer prior to being removed.

A transducer is a device installed below the waterline that provides data to a display. This information is usually in the form of depth and speed and in more advanced systems can provide sonar, fish-finding and side or forward views.

An installation, upgrade or replacement can trigger the need to make an install — but a replacement (especially to a larger or more sophisticated unit) tends to be the most daunting.

Upgrade Job

Recently, I worked with a client who was upgrading to a new system on a 1980’s era Catalina sailboat and was replacing a transducer with a 2” (5.08 cm) diameter through-hull to a 3” (7.62 cm) diameter through-hull. To compound the problem, the transducer had been leaking lightly throughout the 2020 season. The job involved removing the existing 2” (5.08 cm) transducer, cleaning up the mounting location and determining the cause of the leak (and repairing, as necessary), enlarging the hole to accommodate the new 3” (7.62 cm) diameter transducer and installing it.

cutting out an old transducer, removing a transducer

Cutting out the old unit with a saw.

The existing transducer was a removable type — the data sending unit could be removed and replaced with a plug. It fit in a permanently installed sleeve. Access from inside the boat was restricted and none of the existing components were going to be replaced. In this case, in the interest of ease and time, I removed the through-hull sleeve by cutting in with a reciprocating saw and using a mallet to slide it out.

Next, I cleaned the area: removing sealants, dirt, grime and excess material to allow the surface to be fresh and ideal for installing the new unit.

old transducer inside the boat, yacht transducer

Access to the transducer from the inside was limited at best.

Check Twice Cut Once

Because the diameter of the new transducer was larger than the original, the mounting hole had to be enlarged. I used a hole-saw to create a 3” (7.62 cm) diameter template out of white oak. I transferred the dimensions to the hull and used a Dremmel tool to cut out to the correct size. I checked the fit many times because the hole was being enlarged to ensure that just enough material was removed to accommodate the new unit.

A dry fit is important. Before using any sealants or making a mess, I fit the unit, and ensure that the backing plate was the correct thickness and that the backing nut fits and tightens appropriately. Finally, the orientation is important (which side faces the bow, or points to the keel) and can affect proper operation.

old transducer removed, damaged yacht transducer

The old transducer was definitely ready to be replaced.

During the dry-fit, I applied tape to the outside of the hull, so that any sealant that is squeezed out while mounting the transducer won’t make a mess. The ideal finished product will be watertight, sealed and will have clean lines.

Application of sealant comes next and it’s main purpose is to ensure that the tightened transducer will be watertight. To that end, a sealant designed for below-the-waterline applications should be used and applied it in the right quantity in the right location. I used 3M 5200, applied a generous bead of sealant around the inner lip of the transducer and fitted it into the mounting hole. I confirmed the orientation and ensured that the tape guide was placed currently. I then moved inside the boat and completed the work from there.

More sealant was added to the area between the threads and the mounting hole, filling any gaps. I applied a layer of sealant along the underside of the white oak backing plate and fitted it, screwing on the tightening nut. I did not apply any sealant to the threads of the transducer.

transducer hole, hole cut out of a boat bottom

It’s important to clean the surface around the hole before applying sealant.

I tightened the nut by hand as far as I could and then used channel-lock pliers to finish the job. As I was tightening, I ensured that the transducer stayed straight, that threads were lined up correctly and that sealant was squeezing out of the tightened areas indicating that the sealant is filling the area and able doing its job.

I used a dry cloth to clean of excess sealant. It’s easier to use products like acetone to clean away excess, but I shy away from solvents near transducers and plastic for fear of causing damage. I returned to the outside of the boat and checked to make sure that the sealant had squeezed out well. I removed the tape and cleaned up the lines.

new transducer installed, finished transducer installation

The finished product looks clean and the sealant will prevent leaks.

The owner asked to run the wires and make connections to the multi-function display on his own — so my portion of the job ended here.

Removal, installation and re-sealing can be daunting, especially with some of the unknowns of what may be lurking beneath a tightening nut or a backing plate. The great news is that most of the issues associated with through-hulls and transducers can be fixed in a straightforward way. If you’re unsure, contact a marine tech to consult or complete the work.

Andrew McDonald, Canadian Yachting, driving a sailboat

Andrew McDonald is the owner of Lakeside Marine Services — a boat repair/maintenance firm based in Toronto. Andrew has worked in the marine industry for 12 years and is a graduate of the Georgian College ‘Mechanical Techniques — Marine Engine Mechanic’ program.