I don't know that rudder per se but many of that vintage are foam core
and glass over a metal armature. A finished skin is what you see. Water intrusion into the matrix weakens the structure and the skin delaminates or, at least, water gets into the foam. Freeze-thaw (I assume winter storage) further wrecks the foam. You will get a little water out if you drill a "drain" but the foam is saturated and will never dry. I have a few rudder photos here.
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there are other photos of our repairs
on Yahoo here. http://groups.yahoo.com/group/camper...58390/pic/list
My method was to take measurements and make templates. I dismantled most of the structure, leaving only the top and bottoem ends as markers to work towards. I built up the stock and metal ribs with glass and epoxy
. I used plastic film as a release material and thin plywood
backed by loose foam rubber and other materials to form the lower side of the rudder. The plywood
was elevated so that the upper side of the rudder was level. I filled the cavity with a slurry of 3 gallons of US Composites #635 resin mixed with 10 gallons of their microballoons. The cured matrixhas a 3000 psi compressive strength and can be sanded. I faired the structure and had to add some troweled on slurry to fill low areas. My rudder is 3 feet X 6 feet and 6 inches thick at the stock. 30 gallons of slurry. When I was satisfied with the fairing, I paid a local marina service
dept. to vacuum bag the rudder with carbon fiber skin.
THe epoxy and microballoon slurry is totally water-tight pretty much forever. I will never use foam or balsa fillers again, especially below water.