Ask SAIL: Disappearing Zincs - Sail Magazine

Ask SAIL: Disappearing Zincs

Author:
Publish date:

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

WaterLinesNov

Waterlines: Tangled Up in Pots

I learned to sail on the Maine coast as a boy, and one of the things my elders taught me was to respect fishing gear. If you got caught up with a lobster pot, you did everything you could to get clear without cutting the pot warp. It represented a family’s livelihood and thus was ...read more

7353

Harken’s Reflex 3 top-down Furler

Furl PowerAre you afraid of flying—spinnakers, that is? Harken’s new Reflex 3 top-down furler will tame A-sails on monohulls from 44-58ft and multis from 39-55ft, and Code 0’s on 39-54ft monos and 36-50ft multis. All you do is heave on the furling line and the sail will roll up ...read more

TOTW_PromoSite

SAIL's Tip of the Week

Presented by Vetus-Maxwell.Got a tip? Send it to sailmail@sailmagazine.comDitch the stress Owners of high-freeboard yachts best boarded via the stern sugar-scoop like to back them into a slip, but the process can be fraught on a windy day or when there’s a current running, ...read more

Sun-Odyssey-490-Bertrand_DUQUENNE-aft

Boat Review: Jeanneau Sun Odyssey 490

True innovation in monohull sailboat design can be a bit elusive these days. That’s not to say that there are no more new ideas, but it does seem that many new tweaks and introductions are a bit incremental: let’s say evolutionary rather than revolutionary. Just when it seems ...read more

X3M-family

Gear: X3M Flight blocks

Block PartyThe elegance of these new X3M Flight blocks from Ubi Maior conceals the fact that they can handle loads of up to 15 tons. Designed to be used with a variety of textile loops, as fixed or snatch blocks, the X3M blocks have resin frames to carry the loops and anodized ...read more

03-BAVARIA-C34_Interior-2k_2

Ask Sail: The Right Cabin Sole Finish

Q: I am working on refinishing my cabin floorboards. I have brought them home and sanded the old finish off and would appreciate comments on using varnish or polyurethane for the sole.— Danny Love, Grand Rivers, KYDON CASEY REPLIES Polyurethane is the better choice for a cabin ...read more