I hauled my boat out on Monday after it had been in the water for three years. I was told by one of the men at the marina where the boat was kept when I bought it (in 2011) that they had worked on the rudder. When I hauled it, there were quarter-sized blisters all up and down the rudder, on both sides. I sliced into them and gouged them out, and it seems there's a skin consisting of several layers sheathing the rudder that has gotten water under it. This looks to me like layers of vari-colored bondo or something. Is this perhaps the original (Whitby) material, or something the local boatyard used to patch things a few years ago? Here's a photo or two: