Galvanic corrosion on a mooring

[KennyBene]

We have a Beneteau OC352 on a mooring on the North Shore of Long Island. The diver who cleans our bottom every 3 weeks is telling me the prop zinc is deteriorating far to rapidly and the prop itself and the shaft log have galvanic corrosion- I even have a zinc "guppy" in the water.

I turn off all systems as well as the batteries when leaving the boat and I've looked everywhere for a stray or chafed wire laying in the bilge.

Has anyone experienced excessive galvanic action like this and more importantly, how do I stop it?

Thank

 

[Charles Erwin]

If you are on a mooring then it is likely your boat is the culprit - and (although I do not know what your diver considers rapid) the problem is probably not a galvanic event - that is a value of less than 1.2 vdc.

If it is stray current (and it probably is) then just turning things off will not eliminate the threat because there are more than one 24 hour circuit in play on your boat - bilge pumps, radio memory, stereo memory, etc. Also many boats have continuous power to the starter motor, the cable from battery to the alternator, windlass cabling.

Anyway measure your hull potential - another name for the shaft propeller assembly potential. Then measure with the batteries fully disconnected - and if you have more than one disconnect all of them. The arithmetic is a bit confusing depending on whether your overboard probe is zinc or a silver/silver chloride half cell (which is not so reliable salt water anyway.)

Oh - two more things - do insist on sea shield, electro guard, or some other of the few tip top anode brands, and absolutely no magnesium anodes in salt water.

Here is a how to do it diagram.

Charles

 

[Gunni]

Charles, nice bit of easy to understand info there. His Beneteau has a shaft drive and the prop would have connectivity with the shaft. Would clipping the meter pos to the shaft coupler provide a good connection or does the pos lead need to actually attach to the 30mm shaft?

 

[Charles Erwin]

A very momentary sampling is all you need. So just 'rub' the VOM positive lead to the shaft surface and read the result. Do not favor sample at the coupler - likely there will be resistance in the shaft-to-coupler joint which could fake you out. So - try both and see if there is a difference

Charles

 

[Ken13559]

Get your diver to take some photos of the prop shaft.

 

[KennyBene]

Get your diver to take some photos of the prop shaft.

There is only about an inch of shaft exposed on this boat. What will the photo tell me? Before launching, I saw the prop corrosion myself.
BTW, in our harbor, there's only about 1 foot of visibility so photography is a challenge.