In the past I have boated on some very uncomfortable cool late nights. I ran across an article that advertised a heater for wake board boaters (etc) that ran off of the engine hot water. So I checked into it. I liked the idea; however the heater box they offered would not disperse the air in the way that would be efficient for my needs. They sold me the hardware without the box, and I designed my own. I put 2 car heater cores in it, which gave my more volume then theirs, and an automotive squirrel cage type circulating fan. One vent is aimed up at the helm seat, and the other 2 to the port side. So there is a wall of warm air coming up in the front of the cock pit area, just enough to take the chill off, kind of like riding in a convertible with the top down and the heater on. With the bimini and front curtains up it is real toastie.