Yacht encyclopedia
Iron keel
A bolt in a lead keel is cast in. On an iron keel it is drilled and tapped later. Fresh antifouling does not show which joint you are looking at here.
At MarsKeel, William Sauter writes about production boats: two ordinary materials, cast lead and cast iron. Bolts in a lead keel, in his account, are usually cast into the casting. On an iron keel they are drilled and tapped afterwards. The bolt metal is more often 304 or 316 stainless. Bronze, Monel and stronger grades occur. He took a welded fin of stainless, mild or hardened steel, and a milled bulb, out of the article. That is not the iron whose thread is in the casting.
What it is
An iron keel is ferrous ballast for weight and sometimes for lateral resistance. Fresh antifouling does not prove the bolts. These pages do not give a remaining thickness from a photograph.
Water at the joint, then frost. Sauter has seen the surface swell on lead: ice pushes from inside. On iron he has not seen that swelling. There it is a crack and a rust streak. One late sign is the same: the bolt turns when it is pulled. Sail Magazine writes that lead is too soft for a thread, so J-bolts are poured in, and they cannot be pulled out. In iron, stainless against the less noble iron enlarges the thread hole over time. The coating on the bolt is scraped off as it is screwed in. The worst place on both, according to the magazine, is just above the keel, inside the step, where the bolt cannot be seen. Light surface rust happens; deep pits and soft filler are what to look at. Epoxy on a sound surface slows it; cracks still hold moisture.
Open filler does not behave the same way for Sauter. He calls lead relatively stable. Once water gets in, iron rusts quickly, and the filler lifts further. His repair for iron: take the rust off by grinding or blasting, then an etching primer, then a barrier and filler. Lead dust and old antifouling are poisonous. He says plainly to ask a qualified person, and not to repair the bolt yourself. A new hole through the glass into the lead, without dropping the keel, he leaves only as a way to reach a yard. On a grounding both materials are damaged; what changes is the repair path, not a rule that iron is worse forever.
On many wooden boats, he also writes, the lead bolts were not cast in: the builder drilled them when the keel was already under the hull. Fresh paint does not close these forks. Nearby on the site: lead keel, keel types, keel bolts and survey checklist are other pages, not this thickness.