Lee Andrews of East Hampton, New York, asks:
"I’ve had electrolysis issues on my Tartan 3800, so I’ve installed a galvanic isolator, I deploy a zinc grouper whenever I’m in a slip, and I use shore power only when I have to. When I hauled out last fall I could see my shaft zincs were far less corroded than in previous years, but paint continues to pop off my lead keel in 6in diameter swirls. I should mention that whenever I’ve taken the keel down to bare metal and then applied an epoxy barrier to the metal before repainting it with bottom paint, the problem has almost disappeared. What can I do to permanently solve this problem?"
Don Casey replies:
A bronze prop mounted on a stainless shaft creates an electrical connection, and when the two are submerged in seawater, the inherent difference in electrical potential will start current flowing from the more noble steel shaft to the less noble bronze prop.
Unfortunately, the flow of electrons destroys the bronze, which is why it has to be protected by zinc, which is a less noble metal. If a typical zinc shaft collar or prop cone corrodes too quickly, there must be an electrical connection to more underwater metal, perhaps bonding on your boat or a connection to the submerged metal of nearby boats through your AC grounding wire. A galvanic isolator should interrupt the DC flow between your boat and others that are plugged into the same circuit. (For a step-by-step installation project, see page 62.) However, isolators sometimes fail, which is why unplugging from shore power is the best solution. Typically anodes are sized to lose about 50 percent of their mass annually, so if your zincs are not disappearing significantly faster, your corrosion problem isn’t getting worse.
As for the paint flaking off your keel, you’ve already discovered the solution, which is to break the electrical connection between the copper and lead by encapsulating the lead in epoxy before applying bottom paint.
However, the circular swirls suggest that stray DC current may be leaking to ground through the metal keel. So make sure all your bilge pump circuits and any other wiring near the keel attachment is sound. In fact, I suggest that you test the entire DC system for possible leakage to ground.