i have a 1974 catalina traditional layout and have installed the holding tank under the quarter berth on the starboard side. (a 20 gallon todd) my question is about the vent and it's outside location. i've noticed that all the vents come out on the upper part of the topsides and wonder how when you are healed over with the rail in the water, that water isn't forced down the vent opening into the holding tank? btw, i've been sailing 25 years and have never thought about this until now.