[nestorpr]

I don't know if this will help but back when my dad owned a Bertram 25 we installed these black rubber flapper valves that would let water out but would bend and seal at any other times. The boat was always in the water (saltwater) and they lasted for a looong time unless the crabs destroyed them! Then it would be just a small matter to replace them for a few bucks and they were small and unobtrusive, sometimes the simplest design is the best.