Ask SAIL: Disappearing Zincs

Publish date:
Social count:

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



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

January 2016



The ICW North Bound Migration Begins

As the northbound migration begins, we are getting some early reports on conditions along the ICW. The overall impression this spring is that after the damages caused by the hurricanes, the winter storms have apparently not made too many additional changes. There is even some more


Charter: Historic Croatia

Heaps of history—that’s not usually what comes to mind when you plan a sailing charter, but if you like a bit of culture mixed with your cruising, Croatia is the place to go. Caught between two worlds, (the whitewashed laid back vibe of the Mediterranean and the brash demeanor of more


Gear: Pan-Pan man-overboard Locator

There He Goes!The Pan-Pan man-overboard locator won a Pittman award for 2017 as a great idea, and now it is in production as the Weems & Plath CrewWatcher. It’s a two-part system that employs a smartphone app to locate a small personal beacon that triggers automatically should more


SAIL 2018: Reader's Photographs

Are you out there sailing, cruising and living the sailing life? If so, we’d love to see it. Send your sailing photos to sailmail@sailmagazine.comAnd don’t forget to sign up for our free eNewsletter.Check back for updates!This was taken from half way across the 26 mile crossing more

Landing Page Lead

The Volvo Returns to the Southern Ocean

Since the Volvo Ocean Race’s inception, the Southern Ocean has made it what it is. And no part of the race says “Southern Ocean” like Leg 7 from Auckland, New Zealand, to Itajaí, Brazil. The 7,600-mile leg, which starts this Sunday, is not only the longest of the event, but far more


SAIL's Tip of the Week

Presented by Vetus-Maxwell.Got a tip? Send it to sailmail@sailmagazine.comTeak deck paradise  I had a call recently from the man who replaced the deck on my Mason 44 five years ago. He was worried about the way people are wrecking their teak decks trying to get the green off. more