Just a few options.... a picture says a thousand words
1. the hull has a flat bottomed hole with axis parallel to the rudder post
2. a close fitting brass washer is soldered on axis to the rudder post...the OD of the washer is a tight clearance eon the flat bottomed hole
3. an o-ring cavity is drilled in the base of the nipple [on axis to the rudder shaft]
4. a o-ring & waterproof grease is smeared on the rudder shaft
5. a Dubro locking collar locks all components so they cannot fall or rise to damage the underside of the hull
Option 6.......
6. you could always increase the thickness of the hull by epoxy resining a fibreglass pad to the inside of the hull...[say 40 mm square x 10 mm high]
7. this would then allow a deeper flat bottomed hole to be drilled & then the this would allow the rudder post o-ring to be installed from outside the hull & hence a replaceable item
Option 8 ......
8. drill the cavity for the rudder shaft o-ring from the top....this wold the be pressure energised by the Dubro locking collar and again is replaceable