I had that problem once. First look at the probable cause. Most production boats use rudders that the maker recommeded they be painted white, not blue, black or red. Why -- when the boat is out of the water the rudder collects heat from the sun and then falls apart. Once it goes back in the water it fills up with water. In any even, you get the idea. The fix is to drill holes in the rudder, fill it up with epoxy, put screws in the holes to reshape it until dry. Once dry, take the screws out, finish it off then paint it with white bottom paint or keep it covered with something white when it is out of the water. At least this is what I did to mine several years ago. Good luck. Paul
