Curious Zincs

"I’ve had my boat, which has a Volvo Penta saildrive engine, for eight years. For the first six years, when I hauled the boat for annual maintenance I found the zinc anode on the saildrive was slightly eroded but was still firmly attached. I changed the zinc annually.However, when I hauled the boat two years ago, I found the anode was loose and the erosion had occurred primarily around the
Author:
Updated:
Original:

"I’ve had my boat, which has a Volvo Penta saildrive engine, for eight years. For the first six years, when I hauled the boat for annual maintenance I found the zinc anode on the saildrive was slightly eroded but was still firmly attached. I changed the zinc annually.

However, when I hauled the boat two years ago, I found the anode was loose and the erosion had occurred primarily around the mounting screws. I’ve continued to see erosion around the screws, while the rest of the anode doesn’t seem any more eroded than it was during the first six years. Can you tell me why this might be happening and how to correct it?"

-- Donald Sorensen, Sequim, Washington

Nigel Calder replies : I talked to the folks at Volvo Penta’s headquarters in Sweden. They assume that original Volvo Penta anodes are being used, which means there has been no change in the composition of the anode material. They suspect that external galvanic influences may be causing the corrosion. As designed, all Volvo Penta saildrive packages are electrically isolated from the engine in order to minimize external influences. Of course, it is possible this isolation has somehow been compromised; this could happen if some dirt between the engine and flywheel housing gets moist. To test for this put the probes of a sensitive ohmmeter on the engine block and on the saildrive. The resistance should be very high or infinite. If there’s low resistance, there is an electrical connection of some sort that needs to be traced and broken.

Even if the drive is correctly isolated, in some circumstances stray current flowing through the water can cause corrosion by entering the saildrive at one point, passing through the unit, then exiting at another point. The exit points are where the corrosion occurs.

Here are some things to think about: Did your problem start after you moved the boat to a different dock space? Did it start when a different boat took up residence near you? That boat could have had an electrical problem. Or perhaps something changed in the marina itself. Were any new steel pilings or other structures installed? Tracking down these seeming unrelated causes calls for an electrician who is skilled in corrosion detection.

My gut feeling is that your problem involves galvanic rather than stray-current corrosion. I say this because stray current will corrode whatever metal discharges it back into the water, while galvanic corrosion will initially be concentrated at the zinc, which is what is happening in your case. That’s why I would first carefully check to see if the saildrive is electrically isolated. You can do this yourself with a good multimeter.

Related

01-LEAD-lagoon46-ncz4503-a3

10 Places to Cruise With a Catamaran

Navel gazing doesn’t get much better than from the deck of a sailboat anchored somewhere exotic. You can think great thoughts staring up at the stars from a South Seas anchorage. It’s also better doing so on a catamaran. Full confession: I’m a cat convert, a cat evangelist if ...read more

Radome

Ask SAIL: Some Random TLC

Q: I recently removed my radar’s white radome, which covers the internal rotating antenna. I gave the radome a light sandblasting to clear it of years of grime and discoloring. Should I paint it, too? — B. Anderson, Aberdeen, MD GORDON WEST REPLIES Stop! First, make sure the ...read more

L42-Sea-Trails-3728

Boat Review: Leopard 42

Sticking with its proven design formula, but also cherry-picking popular features from its recent models, Leopard Catamarans has launched a “best of” package with this new boat that sold nearly 30 units before hull #1 even touched water. Like a greatest hits album, the Leopard ...read more

01-LEAD-Cut8

Know how: Reinforcing Engine Stringers

If I were to ask, “What are the top five parts of the engine you want to be able to easily access?” How would you respond? Would it be the dipstick? The overflow coolant? I’d wager the raw water pump and its impeller would also make the list. Am I right? The reason we want to be ...read more

Sail-VOE-4-a

Experience: Under the Eyes of the Bar Bunch

Sitting quietly at the bar of a local yacht club, I gaze out over a rambunctious Lake Michigan on a sunny but blustery spring afternoon. I am enjoying watching a small sloop approaching the marina and recognize it as belonging to one of our newest members. “Pretty little thing. ...read more

01-LEAD-Bocas_Marina2

Cruising: Hurricane Heaven

As I write this, another hurricane season has passed. In hundreds of harbors and marinas, sailors are breathing a sigh of relief. I know the feeling since I rode out eight spinners aboard my sturdy 30-footer. I can recall the precise moment when I said, “No more!” It was in ...read more

J45-Podcast-vert-600x-02

Point of SAIL: J/Boats Inc. President Jeff Johnstone

In this episode of Point of SAIL, sponsored by West System Epoxy, Principal Editor Adam Cort talks with Jeff Johnstone, president of J/Boats Inc., the company that has brought the world such iconic designs as the J/24, the J/105 and the J/22, to name a few. In their ...read more

100719BTSC-9304

Boat Review: Catalina 545

Catalina has long been the largest All-American family cruiser company, building what sailors might call “standard” boats. Moving up from the popular 30ft to 45ft sizes puts the company into “yacht” territory, and the new Catalina 545, winner of the SAIL magazine 2020 Best Boats ...read more