Fuel consumption is viewed as a fundamental problem in boats which are to operate on sea for a long time without need to refueling. In this paper, hybridization of a conventional medium-sized boat to reduce fuel consumption is considered. For this purpose, the optimal sizes for the main components, electric motor, internal combustion engine, and battery and generator are calculated via a constrained particle swarm optimization algorithm which considers accelerating and speeding expectations as the constraints. Furthermore, a new fuzzy-thermostat controller whose parameters are tuned via optimization is proposed to manage the energy flow between the main components. To estimate the fuel consumption, a backward-forward simulation approach based on the quasi-static maps of the diesel engine and electric motor is employed. Moreover, the boat resistance forces are calculated for a typical driving cycle at both displacement and planning modes. Simulation results reveal that hybridization of the conventional boat leads to a 40 % reduction in fuel consumption without violating the performance constraints.