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There are a variety of ways to improve the seal without relying on the o-ring around the tube. I don't trust just the o-ring.

My method requires a fair bit of work but its not difficult. I use the brass tubing as a mandrel and wrap a few layers of fiberglass cloth around it. Wrap the brass tubing with wax paper first so you can slide the fiberglass off. I let the fiberglass setup them remove it from the brass. This gives me a fiberglass tube. I then epoxy the fiberglass tube into the drain trimming and fairing the inside and outside flush with the hull. I then install the brass tube in the conventional manner. No matter what happens to the brass, water will have a tough time getting into the foam at the drain.

I'm sure you already know but one of the most important steps to getting the brass to roll over nicely is making sure the ends have been annealed good.