I don't think Catalina typically uses flapper valves at the exhaust exit. On our C30, the exit was low on the transom face. Under full power, the stern would sit lower and the exhaust exit would partially submerg as it did in following seas.  We had no flapper. On our C36MKII, the exhaust exit faces down (so any flapper would be useless unless it had spring load to close it) and is placed in the hull boot stripe facing down towards the water. It submerges in following seas. In both cases, there was enough height in the exhaust loop (as well as an antisiphone valve), so that we never siphoned water into the muffler or engine. That said, I think the manuals recomend shutting off the raw water intake valve to further reduce the chance of siphoning...which I've never done.

The exhaust goes very high after it leaves the water lift muffler so I doubt it can flow back to the engine. Mine exits pointing down on the port side. It is not on the transom like the cockpit drain. Is often under water when motoring or sailing on stb tack. Les