Theis,

About four years ago I replaced a rudder on a Cal 25 three boats down from my existing slip. The stainless steel rudder shaft was completely eaten through near the water line right where the shaft passed through the fiberglass hull. The rudder was fiberglass, and the tiller was wood. The shaft and tiller head fitting were metal, but they were completely isolated from any metal on the rest of the boat. We discovered the problem when we lost control of the boat at sea on a light wind day. We discoverd that the rudder had failed when we notice the rudder floating off the lee side with a sea gull standing on it.

That ain't a harbor. It's a giant battery.