A surveyor once told me that when he surveyed a boat and found no problems,
he knew that he could "always" find water (moisture meter) in the rudder.
He felt the problem was the penetration of the rudder shaft into the
fiberglass rudder regardless of whether it was a solid layup, foam covered,
wood reinforced etc. The joint is bound to leak due to the loads imposed
and the problematic adhesion of resin to stainless steel. On a different
tack, I once owned a foam core boat with a hollow rudder. It had weep holes
that had to be cleared every fall to allow the accumulated water to drain.
Dave