Fixing lead filled sheet metal keel with internal corrosion

After a massive ordeal, we got the 480KG swing keel out of my boat. It seems to be made of sheet metal with lead poured in the bottom half. The problem is that it appears as though water got in there somehow and there are pin holes of rust that made it through to the lead. It seems as though this has happened before and has been welded and mineral oil was poured in to prevent further corrosion. Well in didn't work, and the welding shop I brought it to complained about poison out oil fumes coming out with the heat and they reckon they cant weld it as the metal is too thin and molten lead keeps squirting out of the holes preventing them from welding it up.

Is there hope for this keel?

 

It can't be welded, it's made of metal that will rust and cause the same problem again, so consider making a new centerboard (what you're calling the keel) of fiberglass with the lead molded inside. This is one area I think 'glass really shines, that is making rudders and centerboards. They don't swell like wood, nor rust like metal and the shape can be accurately controlled, which is a good thing for these parts.

Take an angle grinder to the edge (the ugliest one will be the thinnest) until the metal is worn away and work your way around. At some point, brute force will speed things up, by tearing open the sheet metal skin and dumping out the lead, which you'll want to save, reasonably intact.

You could use the old centerboard as a male plug to help create a female mold, which you can then use to cast a copy of the centerboard in 'glass. I'd use plaster in a shallow tray, place the centerboard in the tray on some short blocks to keep it off the bottom of the tray and then pour in the plaster until it's up to the centerline seam. You'll need one for each side of the board. With the molds made, rip out the lead from the old 'board and toss away the worn out remains. It's probable you'll need to use a release agent on the 'board to get it out of the plaster without screwing it up. I'm not sure if peelply or clear plastic will work for this application, but worth checking into. If all else fails, plain old car paste wax on the 'board will do.

The lead will be much thinner then the 'board is thick (accounting for skin thickness) and some figuring of how thick this will be will be necessary. Once you know how thick your skins need to be you can start the casting process. Using one of the two mold halves, layup some polyester resin and cloth in the mold to about 3/4 or 7/8 of the required skin thickness in the areas where the lead must live. Let this dry overnight. The next day, lay one more wetted out layer of cloth over the area previously done, then place the lead in the mold, insuring goo oozes out all around. Then pack the rest of that and the other mold full of wetted out 'glass. Put the lighter one on top of the lead filled one and clamp the crap out of them. Let them sit until it cures, then break them apart. You'll have a fiberglass centerboard with a lead weight inside. Barrier coat with epoxy (like the hull) then paint.

 

Thanks again PAR

I always thought centerboards were light, and if it contained 400KG of lead, that made it a keel. You learn something every day!

I may as well design a whole new keel... ah, center board if I need to go to those extremes as the original seems to have been designed around ease of manufacture rather than good design. It has a really crap airfoil (hydrofoil?) shape. Its like they just bent 2 bits of steel then welded them together and pored in the lead. It would appear that the old method would be much cheaper and easier than your proposed fiberglass method. Although I am certain the glass way would be way better. I am still keen on bodgying up the keel somehow so it lasts another 5-10 years. Unfortunately this job has some monetary restraints