Thanks Gunni but we've proven in over 50 years the exhaust location is 100% sure,not to backflood. If you look at the original double exhaust pipe drawing, it's evident that if any water goes over the high loop, it goes directly into the engine(no muffler or low hoses).Tom;
You might consider an exhaust thru-hull that includes a built-in back-flow preventer. Here is what the Vetus model looks like:
https://www.vetus.com/en/exhaust-sy...som-exhaust-connection-check-valve-60-mm.html
Plus it took a while to locate a new outlet that wouldn't stain the transom as it dribbles exhaust/raw water. This was the original outlet and our son 12 or so years earlier. The new exhaust outlet, larger diameter (I increased hose size), has a long enough spout that it eliminates the previous problem.
BTW, I'd go with one of these preventers you posted if I had concerns about back flooding, before I'd consider putting in a shut off. Thanks.