Ebb, I suppose if using the outboard hole as a large scupper one could put a large flapper valve in the small access behind the rudder post to prevent back flow. Do you think back flow would be a problem in heavy following seas? I had a Cal 20 with the outboard well in the floor of the cockpit and always expected more slosh than I ever really got. Funny story about that Cal 20 if anybody is interested.