Centek industries makes a through-hull with a flapper made of fiberglass - which is fine for the wet exhaust from your genset. Give them a google. If you install one of their water lift mufflers in the exhaust circuit (about $125 off the internet for a 1.5" inlet/outlet) I think you wouldn't need a flapper. Depending on the vertical distance between the genset's exhaust and your water line, you should consider including a vertical loop when routing your exhaust from the engine (or muffler) to the through-hull to prevent siphoning of water when heeling. The instructions accompanying the muffler I bought recommended an 18" loop above the waterline.