I live in B.C. where we tend to keep our boats in the water year round (and sail year round
) but in the east where boats are hauled out for the winter I believe there are more reasons for this. Most rudders are built with a stainless frame in foam and glass on the outside. Where the shaft joins the rudder blade or where hardware is attached to the rudder water will eventually get in. Most rudders will eventually develop cracks and however small, they can let water in. This starts the corrosion to the underlying structure of the rudder. After many freeze/thaw cycles damage will increase and corrosion continues. Eventually, after a long time, and many of our boats are over 30 years old, the metal structure is compromised and weakened. When sailing in a breeze there is a lot of sideways force on the rudder and when weakened enough it will break.