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

IMG_8287GR16Mykonos

Cultural Charters: Mykonos

In last month’s column, I covered the amazing mix of cultures that have called the Dalmatian Coast home over the centuries. Croatia cruising is like a smorgasbord of intertwined centuries, and the islands are a movie set. A little farther south, though, you’ve also got Greece, ...read more

cookinglead

Cruising: No Oven? No Worries

Many cruising boats, especially smaller ones, don’t have a conventional oven. But that doesn’t mean that you can’t have all the baked foods you want, from bread to brownies to breakfast rolls to casseroles and even a roast chicken. All it takes is the right bit of gear and a ...read more

TOTW_PromoSite

SAIL's Tip of the Week

Presented by Vetus-Maxwell. Got a tip? Send it to sailmail@sailmagazine.com Take no chances  This is my stern with the engine running slowly in gear against the lines. We all know that when we’re charging batteries this lets the engine warm up thoroughly. However, I have a ...read more

ZK-Seaboot-900

Gear: Zhik’s Seaboot 900

A Better Sea Boot Following up on its successful ZK Seaboot 800, Zhik’s Seaboot 900 was created in partnership with team AkzoNobel and Dongfeng Race Team, the latter the overall winner of the 2017-18 Volvo Ocean Race. Designed for serious, long-distance offshore racers and ...read more

01-LEAD-FP-Astrea-42-Gilles-martin-rajet---Navigation

Switching to Solar Offshore

No sensible bluewater sailor would consider setting off on a long cruise these days without some means of generating power other than by burning fossil fuels. The good news is that solar energy is becoming less expensive by the day, making it an obvious answer for providing the ...read more

190812-Tiwal-Video-600x

Video: Tiwal Cup 2019

Who says you need a superyacht to have fun? It would be hard to imagine having a better time on the water than these sailors recently did racing aboard a fleet of Tiwal inflatable sloops. ...read more

GEMINI-BATTERY-PASSAGE

Experience: No Batteries, Big Problems

After sailing on Puget Sound and in British Columbia for over 30 years, my wife, Jeri, and I retired to Florida, bought a house on Punta Gorda and worked hard to get it straight. The walls had been painted, furniture bought and assembled, belongings packed, transported and ...read more