A phat phat boat is powered by a very simple heat engine. A pop pop boat is a toy with a very simple steam engine without moving parts, typically powered by a candle or vegetable oil burner. The name comes from the noise some versions of the boats make. Why: the steam pushes the steamboat, and this causes the boat to move. A phat phat boat is powered by a very simple heat engine. This engine consists of a small boiler, which is connected to an exhaust tube. When heat is applied to the boiler, water in the boiler flashes into steam. The expanding steam pushes some of the water in the exhaust tube, propelling the boat forward. The steam bubble then condenses, creating a vacuum which draws water back in through the exhaust tube. The cooled water that is brought back into the boiler is then heated and flashed into steam, and the cycle repeats. This constant flashing and cooling cycle of the engine creates the distinctive "phat phat" noise for which the boat is named.