Galvanic corrosion

[Hymodyne]

I had a tremendous amount of corrosion at the propeller last year. I bought a propeller nut and zinc, but they will not fit on the shaft (no room with tiller shaft). I have virtually no room between the back of the propeller and the beginning of the cutless tube, maybe 3/8", too narrow for a shaft collar zinc.

I know that much of my issue last year was due to my using the shore power at the marina where I worked on the boat. I'll be at an electric capable slip in Bivalve, and I don't want to repeat last year's nearly 40% of propeller loss.
Are there other places where an anode would help?

Pearson Triton.

James

[JOHN COOKSON]

I've heard of guppy zincs. They attach to the shaft with a wire inside the boat(?) then are thrown over the side. I think anyway.
Never have had any experience with them, know how they work, or know if they even do work.
Might be something to check out.

TRUE GRIT

[Wrsteinesq]

Possible solution

I've got a similar issue. Here's how I'm planning on dealing with it: please bear in mind I'm still in reconstruction, on the hard, so this is an idea, not a proven solution. Use a prop shaft brush (around $30) to make electrical contact with the prop shaft. Connect that inside the boat to two separate pieces of stainless all-thread, which run through the hull. Inside the hull it gets a wire from the prop shaft brush; outside the hull it gets one of two rudder zincs. (They're generally round, and sold in pairs, so I figured I'd use both.) So the path of conductivity is from the prop shaft, through the wire to the SS all-thread stud; through the hull, to the zinc, in the near vicinity of the prop. I can't see a reason why that wouldn't work, AND it would make swapping out the zincs easier for the diver, who's probably me. All sort of criticism welcome!

[edwardc]

If your galvanic corrosion is due to leakage from other boats in the marina, you should install a Galvanic Isolator.

In its simplest form, it is a device that installs between the ground lead on the shore power connector and the ground system in your boat. Since most galvanic corrosion is driven by electrochemical cells that have about a 1 volt potential, the way it works is by blocking conduction until the potential is above about 1 1/2 volts. This way , stray galvanic currents are blocked, but the safety ground still serves it purpose by conducting in the event of a short to ground of the full line potential.

There are more complicated ones that have monitors and indicators, but the simplest ones are a "brick" with two leads. Here's a link describing the installation of a simple one:

http://www.chessie.com/boat/projects.shtml#GalvanicIsolator

[ndutton]

You've received three good and direct responses to your question James so before I dig in, how deep do you want to go on the topic? If we get into it we'll describe the actual cause of last year's problem - BTW, your boat's wiring played a supporting role - and probably a few thousand words on how to be sure it never happens again.

Said another way, you can choose to manage the problem or address the cause directly and SOLVE the problem.

[Hymodyne]

lets solve it.

[ndutton]

lets solve it.

I could not agree more.

Last year's problem was not galvanic corrosion but rather stray current corrosion. Applying a galvanic corrosion remedy (sacrificial zinc) will not be effective unless you replace it as needed, likely once a week or even more frequently. Stray current corrosion can be exponentially more aggressive than simple galvanic corrosion so the zinc will be consumed much more rapidly.

Here is a discussion on the difference between the two.


Now that we know the problem, we can address it. There are three ways to solve it:

1. Disconnect from shore power
2. Install a galvanic isolator as advised earlier (complies with the ABYC)
3. Remove the AC ground - DC ground bond wire and simultaneously install a fault current interrupter (NOT ABYC compliant)

We discussed this in detail five years ago:

http://www.moyermarine.com/forums/showthread.php?t=6417

You still need a sacrificial zinc to independently protect against galvanic corrosion.

[Hymodyne]

at the moment I do not plan to add ac power to this boat. how would an isolator be installed w/o AC?

[ndutton]

how would an isolator be installed w/o AC?

You wouldn't. It isolates the AC ground and since you don't have one there's nothing to isolate. I'm missing something though. Did you have shore power when the prop was eaten up last year?

[Hymodyne]

I connected my extension cord to an adapter that fit the shore power outlet at the marina to complete my work.

[Marian Claire]

Was the extension cord ground wire attached in anyway to the boats DC ground? If not then I do not think it could be the cause if the issue.
Dan S/V Marian Claire

[Hymodyne]

It was not connected to the boat's dc ground.

[ndutton]

Please describe what was plugged into the extension cord: various tools, battery charger, anything else?

[azazzera]

Would like to ask a question. Wouldn't a "hot marina" with lots of stray current both D.C. And or AC cause his deteriorating prop? I'll go back to reading those articles you linked to Neil. Information good.

[ndutton]

James' boat had to be an active participant to complete the circuit. We are trying to ferret out how.