A cheap and easy way to prevent costly prop damage.
The TravelLift had just rumbled off toward its home near the haul-out slip when I got my first up-close-and-personal look at Betty Jane’s underwater hull form, still dripping from the once-over the boatyard guys had just given it with scrapers and pressure-washer wands. Yikes! Close inspection revealed that the propeller blades were fringed with jagged mini-serrations, the prop-shaft zinc was simply gone, and adding insult to injury, the two brand-new zincs I’d installed on the transom only a month before were looking like chunks of moldy old blue cheese.
“Galvanic corrosion,” I grumbled with deep indignation, identifying the agency of the horror but not the underlying cause. For years, things had been chugging along smoothly with Betty, with no galvanic issues whatsoever. Now a virtual onslaught had occurred, with hordes of little electro-chemical beasts apparently chomping on her zincs for dinner and then nibbling the prop for dessert.
Luckily, the marine electrician I telephoned for help was a detective of sorts. He used a Fluke digital multimeter to vet Betty’s bonding system, a complicated array of copper-foil strips and green #8 jumper wires, all interconnected with screws and other fasteners and then ultimately tied into the aforementioned transom zincs. “The bonding system itself’s fine,” opined Scott Barnes while low-crawling Betty’s bilge. “I’d say there’s a problem in your slip—either you’re getting stray current from another boat or from the dock.”
“What’s next?” I asked. Barnes proposed some in-slip testing once Betty was back home at her marina and then added a critical point: Even if said testing revealed the source of the stray-current problem and a way to address it, the level of electrical conductivity between Betty’s bonding system and her propeller shaft, as measured by his meter, was virtually non-existent. The reason was pretty straightforward: the oil in Betty’s transmission was almost totally insulating her propeller shaft (and prop zinc) from her bonding system. So indeed the electro-chemical hordes had nailed her prop zinc, then her transom zincs, and then her propeller. “And fixing that prop is gonna cost you,” he concluded.
The guy was right. I soon had to shell out approximately $400 for propeller work, an onerous development if ever there was one. The friendlier cost of the shaft brush removed some of the sting, though. Barnes got a model from Professional Mariner, an outfit that also sells battery chargers, inverters, and other marine equipment, for just $30 and charged me an extra $50 for a less-than-an-hour-long install (see photos on previous page) that entailed merely securing one end to a nearby stringer (so the graphite brush on the opposite end would precisely rest against Betty’s propeller shaft) and then tying it into the bonding system via a green #8 wire.
The denouement? Things are chugging along smoothly again—Betty’s galvanic corrosion issue has seemingly disappeared although I can’t say for sure at present whether that’s due to the installation of the shaft brush or the movement of Betty to a different marina (occasioned by the approach of the catastrophic BP oil spill), or both. In any case, I’m worryin’ way less about the electro-chemical hordes now, although I’m continuing to watch those transom zincs like a hawk.
Professional Mariner (603) 433-4440. www.promariner.com.
(1) Note the ragged edges on the propeller blades.
(2) Attach #8 jumper wire.
(3) Secure and adjust shaft brush.
This article originally appeared in the September 2010 issue of Power & Motoryacht magazine.