an underwater exhaust system which acts to increase the speed of a boat , to reduce fuel consumption , to improve handling and to reduce the skin friction of the water in high seas . the exhaust system basically comprises an exhaust box built into the hull over the bottom of the boat and a diffuser positioned between the exhaust box and the boat bottom . the engine exhaust is directed through the exhaust box and out the exhaust holes of the diffuser into the water . a pump is used to pump seawater into the exhaust box for cooling of the exhaust . also , there is a step in the hull . the step is positioned approximately 25 % from the transom of the stern . the combination of the step and the directing of the exhaust under the hull enhances the speed of the boat and its handling .