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Author Topic:   Fishfinder Transducer Electrolysis
montaukmax posted 11-03-2002 10:01 PM ET (US)   Profile for montaukmax   Send Email to montaukmax  
When I tested my boat before I bought it (about a year ago) I noticed the depth finder didn't work when the boat was planing. The previous owner said that was the way it worked and he just used it for fishing and it didn't bother him.

I quickly realized why the depth readings disappeared when planing - the transducer was out of the water! After I bought a trailer and pulled the boat out I decided to move the xducer lower. I was surprised when the screws snapped off as I tried to back them out. The corrosion pattern seemed odd - the screws were basically hollow shells of a metal that appeared to be stainless. I figured they were plated steel or low grade stainless and replaced them with some decent 18-8 stainless screws.

I bet you already guessed the punch line - less than a year later my new screws did the same thing! I can send digital photos of the hollowed out screws if anyone is interested.

The only thing I can think of that would cause this type of corrosion is electrolysis. The xducer is plastic, but it is held by a stainless steel plate. I imagine the plate is a higher grade of stainless than my screws and they are becoming the sacrificial anode in a nasty galvanic cell. What is the solution? If I upgrade the screws to 316L stainless will the xducer mounting plate start to corrode? Should I attach a sacrificial zinc plate to the xducer mount? Could I run a stainless wire from the zinc plate on the motor bracket (15 inches away) to the xducer bracket?

As you can probably guess, this boat lives in the water most of the time.

whalerron posted 11-12-2002 11:29 PM ET (US)     Profile for whalerron  Send Email to whalerron     
I wonder if the ground on your depth finder is weak and somehow the unit is getting a ground from the transducer. Or, maybe the ground on the transducer cable is weak. To have that kind of accelerated electrolysis occurring, I always thought there had to be stray electric current finding its way through the metals. I have my transducer mounted like yours and the same stainless screws are intact and have been in place since 1981.
I think we have some EE's on the forum who could think this through more thoroughly. It is weird that the screws are being eaten from the inside out. That has got to mean something.
whalerron posted 11-13-2002 08:18 AM ET (US)     Profile for whalerron  Send Email to whalerron     
Are you seeing electrolysis damage on any other submerged metal parts? Take a look at this article:

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