Re: Another missing transom plug
I was thinking more along the lines of in conjuction with a regular compression plug. You'd lift a spring-loaded rubber-gasketed flap that shuts tight from the outside, then remove the plug. You'd have to stand there and hold open the flap while yer bilge drains, but that's a small price to pay if you ask me.
In conjuction with the spring, as long as there is pressure on the flap from the water column outside, it will be forced closed...sort of like what you have on the fill valve on an air mattress, but heavier duty...and in reverse of course.