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Prop shaft zinc on keel cable?

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HudsonM1
1st Mate

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USA
41 Posts

Initially Posted - 04/18/2006 : 19:51:07

Hi all. I’ve been using the old trusty zinc fish to ward off any galvanic corrosion (I have a swing keel), but I’d like to install a zinc on or near the keel so I don’t have to deal with the fish anymore. The problem is that my keel is encapsulated in epoxy and holding up extremely well so I don’t want to breech it to install zincs directly to the keel. I read on an older post the concept of attaching a prop shaft zinc to the keel cable swag as close as possible to where it attaches to the keel. (I never have to raise the keel all the way up so the zinc not passing through the thru-hull isn’t a concern.) Unfortunately, I can’t find a prop shaft zinc with a hole small enough to tighten to the swag.

What if I wrap the swag with copper or stainless wire to increase its diameter, then tighten the zinc around it? Any thoughts on whether this would be an effective alternative to my not-so-beloved fish? Would it introduce any problems you can think of?

Thanks.
Hudson

Hudson
'81 SK/SR #2531

Edited by - on

Leon Sisson
Master Marine Consultant

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USA
1893 Posts

Response Posted - 04/18/2006 : 22:11:06

Hudson,

When installing sacrificial anodes (zincs) it is very important to have the lowest possible resistance (best possible electrical conductivity) between the anode and the part or parts it is intended to protect. Have you noticed the spots on a shaft zinc you're instructed to whack with a hammer after tightening the mounting screws? And the little yellow metal grommet in the center of the pancake zincs? Those are there to improve the electrical connection between the zinc and the protected part. Each interface where the electrical current has to cross over from one part to another, particularly parts of dissimilar metals in a corrosive environment, is an invitation to corrosion, introduces additional resistance, and so reduces the flow of current, which reduces the effectiveness of the sacrificial anode.

In the sort of installation you're proposing, the layers of wire you would add between the zinc and the cable, keel, and related parts would start out with a much smaller than optimum contact area, and would quickly corrode, thus providing a much less than desirable path for electrical current.

I also have an iron swing keel which is encapsulated in fiberglass. I installed two pancake zincs, one on each side of the keel, near the cable attachment point. Each pancake is mechanically attached and electrically connected to the keel by a stainless steel bolt tightened firmly in a deep threaded hole tapped into the iron. Where the bolt exits the zinc and passes through the fiberglass, I actually added a small amount of polysulfide bedding compound to discourage saltwater from getting to the electrical interface between the various metals. As hard as I crank down on those mounting bolts, I'm not too concerned about the sealant acting as an insulator preventing good contact. The effect is more like those wiring connectors which tear right through the insulation to get at the copper.

At the pivot end of the keel, I have two teardrop zincs thru-bolted to the hull, and electrically bonded in the bilges to an all silicon bronze reinforcement which is in very hard contact with the pivot hanger bearings. The bonding wire terminals are crimped and soldered. Even so, each time I service the zincs, those connectors need corrosion cleaned off them where they contact the bronze fasteners.

-- Leon Sisson