For what it’s worth, I have a 1976 Amel Meltem and the generator and engine exhaust are combined into a fiberglass “Y” that is fiberglassed into the hull and exits on the port side just above the waterline. I am in the process of re-powering and the new turbo charged Diesel requires a 3 inch exhaust. Since I no longer have a generator, I plan to cut out the “Y” and glass in a new 3 inch tube. I like the idea of a flap on the exhaust to prevent backflushing sea water into the engine and may employ this. The former owner destroyed  two engines when sea water entered the engine from following seas, once across the Pacific and then again across the Indian Ocean before putting in a ball valve. I never liked the ball valve since it reduced a minimally sized exhaust by about 30%. Probably explains why I am on my third lift muffler due to excessive pressure caused by the restriction of the undersized exhaust.