– Inboard Ski/Wake boat exhaust systems are wet exhaust systems. During normal operation there is water in the exhaust pipes and muffler. The water is pumped by the impeller first into your engine. The hot engine water is then injected, at the aft end of the header, into the exhaust system. In other words, the water is injected into the exhaust system where the rubber hose begins. Without this water, the rubber hose begins to smoke in about 30 seconds.
When a boat engine is not running, part of the boat’s exhaust is flooded with water (all the exhaust system that is below the waterline). On a Direct-Drive boat where the exhaust/muffler is in the floor, the entire muffler is typically flooded (that is why a leaking muffler can sink your boat). On a V-Drive less of the exhaust system is flooded. When you start the engine the water that is flooding the exhaust system is blown out.
When the engine is running the exhaust system is filled with a mix of exhaust gasses (including Carbon Monoxide) and water.

The reason for flappers is two-fold:

 If you are heading down the lake fast, and your engine should suddenly die, the following wave, when it hits your transom, can push water into your exhaust system, and theoretically push water up into your engine. If you back a boat on a trailer too fast into the water this could also occur. A flapper (or baffle) keeps this surge of water from pushing into the exhaust. If your boat has curved downward exhaust tips, they will also prevent water from surging into the engine. Some boats with curved downward exhaust outlets do not have flappers, such as the Nautique Surf Pipe. Fresh Air Exhaust (FAE) provides the same protection as other curved down exhaust outlets.

 Secondly, if there is a failure of the exhaust system inside the boat (like a hose connection comes loose), the flapper will slow the ingress of water which gives the bilge pump a chance to keep up. In this instance, a baffle, or turned down tubes will not prevent this.
We have thousands of FAE installed without a flapper with no known problems; but for the extra safety we recommend flappers for all boats; particularly new boats where the engine is still under warranty. Some boats have built in flappers or baffles that are not removed when FAE is installed. If you order flappers and don’t need them, we will let you know.