I think it depends on the construction of the rudder. Foss Foam uses foam inside their rudders and so it would take forever to drain one. Other rudders are made with solid fiberglass matrix of micro balloons, etc. If that gets saturated with water, it will take a long time to dry it out and essentially the rudder has to be ground open to expose the interior so that it will dry out before rebuilding. Seidelman had a port of their rudders hollow and the manufacturer drilled holes in the rudder so that it drained when the boat was pulled. Apparently they didn't care if it held water when the boat was in the water. It seems that boat rudders crack on one side more than the other and this has something to do with which side is more exposed to the sun. Up north, people either remove their rudders for winter (hard to do if you have wheel steering), or they cover them in foil or a canvas custom made cover.