Generally a rudder is built of foam and glass sandwiching the rudder shaft and associated metal structure. If I am reading your post correctly, it sounds like the shell of the rudder has been forced up toward the hull
relative to the post and frame it is built around?
Pics would help to diagnose.
If so then it needs to be cut open , the core
repaired, and reglassed back together.
Something like this
Rudder rebuilds that just involve glass and core
are not terrible to DIY
, just cutting glassing and grinding.
Once a rudder is not watertight, it will.get heavy and also risk corrosion
and failure of the metal structure/shaft.
Hope that helps!