Is it the rudder shaft that's the problem, or the rudder itself? Ariels have a split shaft and often the upper portion gets corrosion damage, often from hot marinas. Replacing the upper portion of the shaft may be all that's needed. But, use marine bronze as SS will create a current between itself and all the other bronze underwater fittings on the boat, causing future failure. Search for rudder shaft using the Google method described in this forum for more information.