Delaminated Rudder Chapter II
what you found is open cell structural foam. strong but sucks up water like a sponge. It has its uses but I don''t like it. in a rudder you have two things working against you. you have a structural foam and FRP that have a differant coefficient of expansion than the stainless rudder stock. this always creates a crack where the stock enters the rudder and provides a path for water to get inside the rudder. I wish i had a definative answer for all but i don''t. obviously if the rudder fills with water its subject to freezing and bursting if it doesn''t drain. it can also cause constant blistering because it never drys out. On mine I finally bite the bullet, removed the rudder, split it and removed all the open cell foam. I then used FRP to make a much larger load area for the foam. glassed it back together and filled with closed cell foam. its not as strong as the open cell but I don''t have a problem with water in the rudder now. this is a large project but if your handy doable.
the key is to make sure that you have increased the load area enough to assure that you won''t crush the closed cell foam inside. I did this 8 years ago and havn''t had any problems since.