Ask SAIL: Disappearing Zincs

Author:
Updated:
Original:

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 sailmail@sailmagazine.com

NIGEL CALDER REPLIES:

Calder-head

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 sailmail@sailmagazine.com

January 2016

Related

2048x

Know-How: Helm Stations

Walk around any boat show, and you’ll see a number of differences in the way designers and builders have decided to locate the steering stations aboard their cruising cats. Each position has its good points and bad, among them visibility, protection from the elements, ...read more

Jerome

Point of SAIL: Jerome Rand

In the first episode of Point of SAIL, the SAIL magazine podcast, Principal Editor Adam Cort talks with circumnavigator Jerome Rand about his adventures, past and future. For more information, visit Jerome's YouTube channel July 2020 ...read more

01-NEW-shutterstock_727520281

Cruising: Belize on a Multihull

In my experience, every charter has a kind of a theme to it, often encapsulated in a single moment. For me, during a recent weeklong charter off the coast of Belize that moment came toward the end of our first day out. We’d left the Sunsail base (sunsail.com), located part way ...read more

01-LEAD-View-of-the-Bow

Know-How: Marlinspike Seamanship in the Arctic

I was crewing aboard a boat named Breskell, a 51ft cutter-rigged, cold-molded, mahogany sloop. We were voyaging from St. John’s, Newfoundland, to Port Townsend, Washington, via the Northwest Passage. A few days before setting sail, the captain, Olivier Huin, asked me to secure ...read more

Prop-Coat-Barnacle-Barrier-Quart-No-Background

Gear: Prop Coat Barnacle Barrier

Prop Coat Barnacle Barrier 1792 is now available in a quart-size can and, as always, can be used on all underwater metals, including saildrives, shafts, strainers and folding and non-folding props. Two or three coats are recommended, after which the coating will purportedly ...read more

DY_171021_6877

Boat Review: Seawind 1600

Seawind Catamarans introduced its 52ft 1600 model in Europe last year, where the boat promptly started winning awards. The more jaded among us may look askance at such things, especially when it comes to a bluewater-rated catamaran billed as a providing a combination of ...read more