Hi Charles,
Your rudder looks similar to our 372 and they may be made the same way. We stopped at Tartan last Spring where we studied diagrams of our rudder. Its basically a SS tube/pipe welded closed at the bottom. About 4 thick SS horizontal straps/ribs are welded to the rudder tube like a giant rake. This structure is sandwiched by the two fiberglass shells and filled with foam. I asked if water penetration is an issue and the answer was 'not really' because the water generally won't permeate through the foam. But like you discovered, it can be an issue. I have a couple caulked drain ports on the bottom of my rudder but haven't seen any drips.
One path to consider is to drain the rudder for a couple months then rebuild the crack with a fiberglass patch. You should be able to do this while the rudder stays attached. Water may still get in because the foam core may have lost integrity. Plan to drain it every year with weep holes.
Another path is to contact Tartan and buy a new rudder assembly. They will make a replacement for you.
Hope this helps.
Edgar