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Ask Sail: Swing Keel Corrosion

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Klaus Brauer of Seattle, Washington asks:

I have a 1984 Vandestadt & McGruer Siren. She’s in excellent shape but for the galvanized swing keel. After spending most of the past 25 years in fresh water, the keel is badly pitted. The boat still suits me after all these years, and I don’t find myself wanting to own much else—aside from that keel.

I’m looking to replace the keel, which is a flat plate measuring 1/2in x 11 1/2in x 54in. I’ve got a lathe and milling machine, and if necessary I can fabricate a new hinge pin and related bits to avoid galvanic issues. What metal would you recommend the keel be made of to eliminate future corrosion and to minimize or eliminate freshwater marine growth? Bronze or stainless steel? Can I isolate the keel from its associated hardware when they’re in close proximity under water? Or is that not an issue with a bronze keel and a stainless steel cable and shackle?

 
Nigel Calder replies:

How old are you and how much longer do you intend to sail the boat? If the first galvanized keel has lasted 25 years, do you really need a replacement that will last even longer? If not, a new galvanized keel will be cheaper than stainless steel and far cheaper than bronze. It will also reduce galvanic conflicts, as the galvanized keel will be a large anodic mass in relation to the hinge pin and retracting mechanism, whereas a stainless steel keel may prove cathodic to the hinge pin and hardware. Bronze will certainly be cathodic (unless you use a bronze pin and other hardware). If you do use stainless steel, try to get 316 alloy for the plate, and be sure to use 316 or an even more corrosion-resistant alloy (such as Aquamet 22) for the hinge pin. Even so, in the somewhat anaerobic environment inside the keel trunk and around the hinge pin, you may still get corrosion. As to marine growth, you will need to use an appropriate antifouling paint, regardless of which metal you use.

 

Top Photo by Peter Nielsen

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