Electrolysis around engine sea water intakes

RRIPP1 · 07-13-2011, 01:17 PM

I have a 38ft sport fisher, with a fiberglass over wood hull. I;m having a problem around the engine sea water intakes. The bottom paint is in fairly good shape except around the intakes, where the paint seems to be flaking off. I have been told that this is due to electrolysis and bad grounds in the elect. system. Can any one shed any light on this? Does the electrolysis usually occur in the AC or DC power supply or both? What is the best way to trouble shoot?

Thanks for the help
Jim

m2m · 07-13-2011, 01:39 PM

the paint could be flaking off due to poor surface preperation. Are the thru hulls bronze and if so are they displaying signs of corrosion or a pinkish color? If the answer to question 2 seems correct hire a amrine surveyor and have him test for stray current.

Marmot · 07-13-2011, 01:57 PM

Quote:

Originally Posted by RRIPP1

fiberglass over wood hull. I;m having a problem around the engine sea water intakes. The bottom paint is in fairly good shape except around the intakes, where the paint seems to be flaking off.

Is that intake electrically bonded?

What does the wood around the through hull look like inside the boat? Is it showing fuzzy fibers and/or white crystals? Does it appear swollen at all?

RRIPP1 · 07-13-2011, 02:17 PM

There are ground wires on the shut offs on the intakes. and the inside of the boat is fine, no visible sign of anything.

Marmot · 07-13-2011, 02:24 PM

I would remove those throughulls, inspect the penetrations very closely, dry and treat the wood in that area with vinegar, and if in good shape, epoxy seal them then replace the throughhull without bonding. Apply the final coat of anti-fouling and look at them next haulout.

m2m · 07-13-2011, 02:33 PM

Marmot, I never heard of vinegar treatment what does that do? I know it is a preatreatment prior to painting galvanize but wood that's new. You learn something new every day.

Marmot · 07-13-2011, 02:46 PM

Quote:

Originally Posted by m2m

Marmot, I never heard of vinegar treatment what does that do?

Wood is subject to a form of electrolysis that creates sodium hydroxide, a strong alkaline chemical (lye - Drano) that dissolves the lignin that binds the wood fibers together and destroys the structure. When it gets really bad you can see white crystals all around the metal bits that appear to be sinking into a bundle of loose wood fibers.

Vinegar is acidic and it neutralizes the hydroxide and stops or at least seriously inhibits the process. If you have such a process going on, the vinegar (white apple cider vinegar - nothing fancy) will fizz and bubble when it soaks into the wood. If it doesn't fizz, you are OK and only stunk up the place for a bit.

Unbonding the throughull opens the circuit that is responsible for the electrochemical process that produces the hydroxide.

Go talk to some of the old timers at the wooden boat school in Newport, they can probably tell you some horror stories.