Zinc went bad in 15 days

[hooligan6a]

I hauled out last month and installed a new zinc in my shaft. had to haul 15 days later and the new zinc is half gone in 15 days. I was at sea and not in a marina. Any idea what I should look for?

[sailorboy1]

you have a negative grounding problem, check all your battery cables back to the battery

[djmarchand]

More specifically you have a hot DC wire that is sitting in bilge water. This is typically a bilge pump connection- the hot lead to the float switch, that is sitting in water. This causes rapid electrolysis of the zinc.

David

[fstbttms]

Bare wires bleeding 12vdc into a wet bilge can destroy big metal parts in a hurry.

This ocurred in a matter of weeks. Despite resolving the problem (broken insulation on a bilge pump wire) the prop eventually fell off.

[Andina Marie]

Hmmm?

I can see how exposed positive wires in a bilge could cause havoc INSIDE a boat but I fail to see how that would erode a zinc OUTSIDE the boat.

Thinking of scenarios, it could be that the shaft is insulated from the engine so positive leakage inside the bilge could put positive potential on the shaft. But outside the boat, there would still have to be something else of significant size in the water that is connected to battery negative to complete the circuit.

At least the first line of attack would be to make sure the shaft is grounded to the engine (battery negative). If you have a non-metallic coupling you may need to mount a small jumper across it.

On a steel boat it is not uncommon to see a grounding brush on the prop shaft since a flexible coupling plus the rubber lined cutless bearings can insulate the shaft.

But in general if the shaft is grounded, electrolysis problems inside a bilge are independent from electrolysis problems external to the hull.