Q: The prop shaft zincs on my Cal 33, which I keep in a marina on Long Island Sound, have been disappearing rapidly for a number of years. Prior to 2013, when the boat was hauled the lead keel exhibited numerous round “pops” where epoxy and bottom paint had lifted off and whitish corrosion appeared. Repeated reapplication of epoxy did not help. The zincs were usually nearly gone by October. In the spring of 2013 I broke the connection between the keel/mast/rigging bonding and the bonding for the engine/prop shaft. That fall, the keel was fine (no more “popping”), but the zincs had disappeared. In 2014, I hauled the boat in late August, and the zincs were already gone. I relaunched with new zincs and by October those were 40 percent gone. The keel was perfect. Coincidentally, three other sailboats in the marina (two adjacent to me and one on a different dock) also had no zincs left when hauled in October. I have a galvanic isolator and seldom use shore power. What are the probable causes?
Elizabeth Wagner, via firstname.lastname@example.org
NIGEL CALDER REPLIES:
The blistering paint can be symptomatic of what is called “cathodic disbondment,” which can result from too much galvanic protection (i.e., too much sacrificial anode relative to the metal area being protected), but I doubt this in your case. Stray current corrosion can generate much higher currents in the water than any galvanic current, and stray current is agnostic with respect to metal composition. Whatever metal is discharging the current into the water gets corroded regardless of its composition. Stray current can emanate from your boat or from outside it. It’s possible the zincs and keel have been in such a circuit, and when you disconnected the bonding wire to the keel you took the keel out of the circuit.
The fact that the neighboring boats are having a similar problem suggests that the stray current is not coming from your boat, but from some other source, and your boat, and the other boats, happen to be in the path to ground. If you have bonded your underwater metal, your bonding wire is the lowest resistance path in this part of the circuit. The stray current will enter through one underwater fitting, which will not corrode, pass through the wire, and exit through another underwater fitting, which will corrode—for instance, your keel, before it was taken out of the circuit, and your zincs.
Depending on the age and construction of your galvanic isolator, it may not be providing much protection when you are plugged in. Many older galvanic isolators do not incorporate a capacitor (a round cylinder, typically about 4in or 5in long), and as a result are easily “biased” into conduction, at which point they are effectively useless. You should check the isolator to see that it complies with recent ABYC standards. Even if it doesn’t, given that you rarely plug in I doubt this is your problem. I would go on a stray current hunt.
Do you have a question for our experts? Submit it to email@example.com