Originally Posted by Tartilla (Post 4935192)

Just for clarity, I'm speaking to a metal dam on the bottom of the riser apex.

Assuming you have through transom exh?

Flappers on the tips are important...waves can drive water up the exhaust risers. Stern wake striking the transom will do it as well. Even prop exhaust systems have the internal flappers on the Y pipe system to arrest that...despite the prop being under water a ways.