foam is the filler in the clam shell, it isn't structural. There's very little of it. The rudder is rather narrow, right? The rudder is as bouyant as a piece of lead, or bronze. As I intimated above, this is not a good design, A good design is the original rudder. The 6 or 7 feet of 1" bronze shaft-rod at about 3# a foot doesn't make the original a floater, either. If the original rudder had been made totally out of wood, it would have been a floater. I have not heard that a lite foamfilled neutral rudder, like those that stand alone behind a fin, is desirable on the end of a keel. Would like to hear a discussion on this!!!
A rudder with two or three narrow triangles of flat bronze bar welded to the sides of the rod instead of allthread drilled thru the rod - the rest made up of divinycell, glass and epoxy would make a good substitute - come in at about the same weight - easier to maintain - ten times stronger - than the original. IMCO.
A two part socketing rudder like the one above could be designed at the cost of more weight. A full size working model out of wood would be made first.
Will make a valient attempt at finishing the present project. I advise everyone against it. Unless you make a compleat working model first! If the fiberglassing is successful there hopefully will be some photos.