Stuart Goldman of Shelter Island, New York, asks:
"For the last two years the zincs on the shaft and MaxProp on my Hinckley SW 42 have started to really dissolve in early July—after the boat has been in the water for about a month. The boat is on a mooring that’s at least 500 yards away from any other boat except a 50ft powerboat, which is also on a mooring about 50 yards away, and a 40 footer that is moored at about the same distance at a dock. I’ve asked both these owners whether they are having similar problems and they say that they are fine. This tells me, although I could be wrong, that stray electrical current in the water is not bombarding the boat.
When I leave the boat to go ashore I turn off both the house and starting batteries at the main breakers and the only device that remains activated is the emergency bilge pump, and it operates with a float switch. Am I missing anything?"
Nigel Calder replies:
Although your problem could be caused by either galvanic or stray current corrosion, because stray current corrosion will attack whatever metal is feeding the current into the water— and often that is not the zinc— I suspect that you have galvanic corrosion. Because you are not plugged into any shore power, the chances are that it is being generated on board.
The first question you need to answer is whether you have any substantial underwater metal masses that are tied into the bonding systems. These could include such things as bronze through–hulls, a bronze rudder, or an exposed metal keel.
But you should also test whether there is stray current in the water and to do this put two long leads into a millivolt meter and then hang both over the side in the water at different points around the boat. If you get any reading at all, in either an AC or DC mode, there is stray current in the water.
The best way to test for galvanic current is to put a silver/silver chloride half cell on a long lead and use it with your multi-meter (boatzincs.com/corrosion-reference-electrode-specs). There are some other tests that will help you determine whether you have galvanic corrosion and they are explained in more detail in my Boatowner’s Mechanical and Electrical Manual.