In some spots I could insert the scraper over an inch until I hit the interior fiberglass bonding the hull to deck. I sealed the gap with thickened epoxy, cleaned up the rub rail and reinstalled it. ...The interior fiberglass bonding between the hull and deck looked fine but was not completely bonded so it was not watertight. Water would ooze out of the fiberglass like a spring when it rained.
I think Mark's observation solves a puzzle: if Ericson bonded the hull and deck with inside roving, in addition to the sealed flanges, how could it possibly leak?
In replacing aluminum rub rail I noticed those occasional gaps in the flanges. (Ericson hull and deck each had a large flange; they were mated and bolted together to cure. Then the big fabrication flanges were sawed off, leaving the 3/8th stub around the boat. The stub requires our specific rub rail sections, which must cover the stub).
It so happens that the rub rail screws go directly into the mated flanges. Ericson kept the screws short--1 1/4"-- so they wouldn't penetrate the roving inside. That's not much holding power, which is OK because the rub rail is only decorative.
Longer screws, likely used by owners as replacements, probably do penetrate the roving. That would be OK if liberally coated with sealant. But we have had reports of screws leaking if they do penetrate.
It isn't hard to pull a section of rub rail and epoxy any gaps, although removing the old caulk can be tedious.
Any rub rail screw that turns without holding suggests the join of the flanges is questionable there.
Let me add that "gaps" don't necessarily mean the boat leaks. I have replaced two aluminum rub rails, noticed and filled flange gaps, but neither boat leaked a drop.
It is also not a hull deck integrity issue, since the join is accomplished by the roving.