Care and feeding of rudders
A few years ago I got my shorts in a bind about my rudder after attending a safety at sea seminar during which Ralph Naranjo spoke of losing his E39 rudder in the Indian Ocean due to corrosion of the welds holding the tangs to the shaft. My rudder had exhibited splitting at all four seams, leaking around the shaft, horizontal cracks half way up on the starbord side combined with delamination of the skin in the same area. Nevertheless the rudder was structurally sound - the core is a very dense closed cell foam which cannot absorb water. Any water will be around the shaft coming from the top (bad) or under the skin. I had the rudder cored and no corrosion was detected. I then repaired the rudder by routing around the shaft and squeezing in 5200, taping all the open seams and then fairing them and injecting epoxy into the voids. I do not believe that it is necessary to apply fiberglass over the cracks, except at the seams. (For one thing, the bottom of the rudder is designed to break off in the case of a grounding so that you do not bend the shaft and either put a hole in the boat or lose all steering.) In any event, my rudder has been problem free for the last 5 years since the repairs.
With all due respect to Loren, I would be very carefully about drilling into the foam to check for saturation (which, in any event, is closed cell) because the resultant holes will continue to open up unless you then fiberglass over them and the holes will not be big enough allow you to inspect the welds. (My rudder was cored with a 1 1/2" hole and the yard was lucky enough to hit a the top weld.) I would first check for delamination (tap the surface with a coin or small hammer) and if you find delamination, then drill holes only through the skin and inject epoxy from the bottom hole until is comes out the top.
I have also started to paint my rudder white since, according to Foss Foam, heat from the sun (when the boat is in the yard) is a major cause of rudder delamination.