The Offshore Voyaging Reference Site

Q&A: Can We Install A Bronze Transducer In Our Aluminum Boat?

Question: We want to replace the current inoperable depth sounder on our aluminum boat with a forward looking type (EchoPilot or Interphase), but we have run into the dilemma that the transducers suitable for our hull have bronze stems.We have been advised that bronze stems can be installed in aluminum hulls if the stem is well isolated from the hull and an aluminum donut is installed as well. We would appreciate any information you could provide us and what your experience has been, especially if you did install a bronze stemmed transducer on Morgan’s Cloud.

Answer: I would not recommend installing a bronze through hull on an aluminum boat, even if it is isolated from the aluminum; there is just too much risk of electrolysis. Also, if there is any dampness, and there will be, copper salts will form on the bronze and drip into the bilge…not good.

When we bought our EchoPilot FLS II, EchoPilot provided a custom aluminum through-hull at additional cost. This is not a cheap solution, but definitely the way to go. Once we had the aluminum through-hull we had an aluminum donut machined that compensated for the angle of the hull since it is vital that the transducer be installed absolutely vertical.

The next challenge is to get all this installed and safely watertight. You could weld the donut in, but this is messy and it is very difficult to get everything lined up perfectly when welding, due to heat distortion of the surrounding plate. To get around this we used Plexus adhesive to glue the donut down before drilling the hole. The cool thing is that Plexus sticks to aluminum brilliantly, particularly if you use the Plexus aluminum primer, and it will gap fill up to ½” so you can use it to make any small adjustments to the leveling of the donut. Once the Plexus is dry, drill the hole through the donut and the outside plate making sure it is absolutely vertical.

Before starting this project make sure the boat is level on the ground, both fore and aft and side to side. Remember, you are trying to level the actual water line (scum line) since you want the beam from the sounder to be parallel to it. If you want to get really anal retentive about it you could rent a self leveling laser that surveyors use to line up buildings. We rented one for a recent project for about US$100 and it was great, although it is probably overkill for this. This process will drive your travel lift operator crazy, but is the only way to get things right.

Your next issue is to fair-in the transducer through-hull on the outside, since it won’t lie flush. The answer here is to cover it in mold release wax and then install it onto a pile of epoxy putty on the outside of the hull (we make our own putty from WEST SYSTEM resin and micro-balloons). Tool the putty to a fair shape while wet. Once dry, remove the transducer through-hull, clean off the mold release wax with acetone, sand the epoxy putty to a final smooth and fair shape and make the final installation with a good underwater sealant.

It’s a lot of work, but well worth it; once you have a forward scan sounder you won’t be able to imagine how you got along with out it.

For the high latitudes, I would recommend the new EchoPilot Platinum model which replaced the FLS II that we have and, as I understand it, fixes most of our criticisms of the older model. I would also recommend having a spare transducer aboard since they seem to lose their sensitivity after a few years and can die suddenly.

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John Lundin

Hi John,
Thanks for posting such a thorough install procedure. I haven’t sailed yet with a forward-looking sounder, but I’m wondering if this can totally replace a traditional depth sounder? For instance, in a new construction is it feasible to invest in an Echopilot and forego the other transducer (plus thru-hull, circuit, gear, etc…) Or do you prefer to also have the standard depth reading for coastal approaches?
Best, John L

peter bateman

We as a boatyard have to work on all manner of Hulls. We have just welded the aluminium transducer thro’ hull tube supplied by Echopilot into an aluminium hull using TIG argon process using a 5356 filler rod. This was we felt a stronger and more reliable than the backing nut option. We welded outside the hull, holding the tube in situ with the backing nut and then we removed the backing nut and welded inside the hull. The Hull plate is 8 mm 5083. The skin fitting provided by Echopilot was very well machined. We now have a well faired skin fitting because of the Tig build up. Post the welding the hull was sanded back and then re primed with a two pack epoxy etch primer and an aluminum Jotun antifoul. Not a cheap way to do the job but in a boat that is a nice as the one we worked on it was the only way to do it!


Hi John

We are also planning to install the Echopilot FLS on our aluminium hulled Allures 45, having had one on our previous GRP yacht. I wonder do you have the bronze locking nut on the transducer or aluminium there too?

Its obviously a question of potential galvanic corrosion vs aluminium thread galling. Would Tefgel be effective in preventing galling?

Your wisdom and experience on this would be appreciated!


De Veyrac

Hi John,

I am looking into fitting a B&G forward scan ( forward looking sonar ) on my aluminum hull. The transducer is plastic, but the through hull fitting ( skin fitting / housing) is made of stainless. I am concerned with the galvanic corrosion risk between the aluminum hull and SS fitting under water. The alternative is a plastic through hull fitting ( such as Airmar) but again I do not trust plastic in case of impact … What do you think ?

Many thanks for you input , and for sharing your wealth of knowledge though this web site. I am a regular reader and subscriber.

Best regards


Colin Speedie

Hi Chris

I’m looking at this for two new build boats at present, and it may be the case that if the stainless fitting is carefully installed and isolated it’s OK – but personally I don’t like the idea. Echopilot make a first class product with an optional (if expensive) aluminium transducer housing, and I think that’s the way to go.

But, if your boat is not like ours (an Ovni with no keel box) but has some form of keel structure when drying out, then maybe the Airmar plastic unit would be fine. My concern with the plastic ones is simply the risk of it sitting on a rock or similar when dried out.

Given that these transducer housings are far from complicated, it really shouldn’t be difficult to get a decent machinist to make one in the right grade of aluminium – that’s what I’d do.

Best wishes