Ask Sail: Keel Attachments

Sail News

Fiberglass alone is not sufficient to keep a fin keel securely attached

Q: I have an early ‘70s Catalina 27. The keel bolts look pretty good. My question is, why not glass over the keel to bond to the hull rather than changing the bolts if, or when the bolts are too far gone? I haven’t seen anything on this, so could you discuss? Full-keels are permanent. It seems the fin keel could be made the same.

— Jon C. Anderson, Deer Island, OR

DON CASEY REPLIES

A little clarification here: it is not just fin keels that are bolted in place. Full-keel boats often have the keel bolted to the hull as well. When a keel is “permanent,” to use your description, the hull of the boat is molded with a keel cavity that is subsequently filled with a ballast material, generally lead.

In the fiberglass layup process, resins are applied on a schedule that allows each layer to bond to the previous one at the molecular level, creating essentially a homogeneous solid. However, once the resin has fully cured any additional layers will become attached almost entirely through mechanical adhesion, a far weaker bond.

In the case of a hull molded with a keel cavity, the keel and hull are one and the same, with the stresses widely distributed. When a keel is added externally, however, the stresses are concentrated at the joint, which means any attachment method must be up to resisting some pretty major stresses. It is not uncommon for the joint between the hull and an external keel to be “taped’’ over with fiberglass. However, the purpose here is not to support the keel, but rather to seal or fair the joint. Even then, the bond between the plastic resin and metal keel, often fails. Simply stated, as a method for attaching a metal keel to a hull, fiberglass is woefully inadequate for resisting the prying forces at work. External keels require a robust mechanical attachment, i.e., a set of stainless steel bolts.

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