I have a 16' Tournament Cat and this was a problem. I put check valves in the line, plus inspect the rubber flappers on the hull penetration (they are supposed to act as check valves but wear out, wrong size, missing- the usual. Hyman Rickover "You get what you inspect, not what you expect". I understand the self bailing function when up to speed but always that it was a strange design. Like in the above pic a 90 degree pvc check valve would work well.