Direct to Hull Mounting Stainless Transducer Bracket - Corrosion?

[Tin Man]

I am preparing to install a Lowrance 3 in 1 AI transducer on my new Tracker Hull which is painted aluminum.
Lowrance transducer mounting bracket is made of stainless steel.

Reading install instructions from Lowrance, it suggests using a plastic isolating material such as "King Starboard" to prevent corrosion between Alum hull and SS mounting bracket (dissimilar metals).

I was planning on installing SS bracket directly to hull, as I want to use the existing two holes from previous transducer.

However, after reading install instructions, perhaps I should use a piece of King Starboard (HDPE) and screw it into hull using the two existing holes and attach transducer bracket to HDPE.

Will direct mounting bring about corrosion?

Thoughts?

 

[Stand Up]

I would mount a piece of composite decking to the hull using some marine epoxy and then mount the bracket to the composite decking piece. Holes from the old transducer could be filled with epoxy and covered with the decking piece. Just a thought.

 

[LDUBS]

The reason I would use an HDPE pad like starboard is to avoid having to put holes in the transom. Doing it now will at least cover/seal the holes that are already there. If you do use the plastic pad, drill some shallow holes in the back side at an angle to give the epoxy more to bite into.

My boat maker had the foresight to weld some brackets on the transom for mounting a transducer. Because my H-Bird transducer bracket is plastic I can't share any experience about corrosion. The SS bolts holding it to the bracket look fine. I'm 100% fresh water so far.

If you want to mount directly to the hull using the existing holes, the paint will provide some barrier. It might be good to further isolate the SS bracket from the hull with a plastic gasket. Of course you already know to seal the heck out of those holes.

 

[thill]

The biggest question is, are you strictly freshwater, or do you also venture into saltwater?

In my last boat, I mounted the stainless transducer mount directly, with a square of duct tape as a barrier and a little goop sealing the fasteners for maybe 6 years with no issues.

I would take that boat in fresh and saltwater on occasion, but I always rinsed it carefully afterwards.

That being said, if you’re in saltwater often, crevice corrosion is a big thing. Once it starts eating, it eats quickly.

I don’t recommend decking material as it often has sawdust mixed in which holds moisture and tends to swell. I like 3/4”-1” thick cutting board material the best. Through-bolt the plate and seal it carefully.

When installing transducers, use 3/4”, #12 screws. Pre-drill and take the sharp tip off the screw with a grinder or Dremel or whatever. This is the method I’ve used for many years with 100% success, mainly on fiberglass boats to prevent transom rot, but it applies to corrosion, too.

The biggest thing I like about it is that you can add, remove and change transducers without issue, and without punching a bunch of holes in your boat.

I installed a transducer plate on my current tin boat.

 

[Tin Man]

Thanks for the suggestions. Since the two original transom holes that were for the transducer that came with boat matched my new bracket, I decided not to add a HDPE sacrificial mounting plate. Instead, I used a 1/8"x2"x3" piece of ABS plastic between ss mounting bracket and hull. Used Boat Life Polysulfide as sealant.

If transducer height is not correct after a water test, I may have to add a piece of HDPE for increased height/drop adjustment. If I do, I'll make a piece small enough where I can still use the two holes already in transom and screw HDPE using those two holes.

 

[airshot]

Pure SS would not be an issue in your lifetime in fresh water, but not many brackets or screws are true SS, mostly an alloy. When ever I want something SS, I take a magnet, if it attracts to the SS item, it isn't pure SS !!! Depending on the alloy, it might corrode. Learned that lesson about 60 yrs ago when my running light screws let loose from corrosion and came back and hit me in the face !! Bought true SS screws and washers and recounted to the aluminum deck after a thorough cleaning. No corrosion since even after 60 years. Carry a magnet when ever buying SS and test the items.

 

[thill]

Good point that Airshot makes. You want 316 grade, marine stainless, not 304 or 308 if possible, especially if you are near salt or brackish water.

 

[Tin Man]

Not sure what grade Lowrance uses on their brackets, but I checked it with a magnet, and no attraction at all.
Always fishing in fresh water reservoirs or the CO River!

 

[moecarama]

I just mounted a transducer as "thill" recommended using a cutting board on my Topper 12; it works great!

 

[jethro]

I bought a transducer mounting plate from BPS a long time ago. Glad I did, there are a whole bunch of holes in there now.