Stricter regulations on emissions force ship owners and designers to come up with new solutions for ship propulsion systems. Hybrid drive systems consisting of prime movers and batteries, like in the Toyota Prius, show great potential, especially in special ships like harbour tugs. Economic and environmental performance of such systems strongly depends on the way it is controlled. Dynamic computer models of system components were built to compare different system lay-outs and different control strategies and thus support the ship system designer with design decisions. A combination of fuzzy rule-based and optimization-algorithm control offer the best results. The modeling method, the different simulations, the optimization methods and resulting consequences for the ship system design are the main topics of this paper.@en