Energy storage devices integrated in diesel-electric power systems is believed to have impact also in marine applications, when it comes to emissions and fuel efficiency. For certain load conditions energy storage can act as load buffers which will decrease the load variations on the generator sets, hence optimizing the operation when is comes to emissions and fuel efficiency.In this context this thesis is aimed at development of simulation tools for hybrid marine electric propulsion systems. Assessment of two energy storage concepts is included, focusing on how their different attributes is utilized in the best manner. The bond graph language is chosen for modeling the physical components, because of its ability to represent the power interaction between a large selection of energy domains.Component models of the essential electrical and mechanical components on theconsumer side of a conventional diesel-electric propulsion system is modeled and connected. Additionally component models of a rechargeable battery and a supercapacitor is implemented to the model. A simple power control method is used to control the power flow of the energy storage devices.Hybrid power plants are complex systems and the simulation tool developedin this thesis is considered far from complete, especially when it comes to control systems. To determine the actual reduction in fuel efficiency and emissions, dynamic models of diesel engines and generators should be added. However the model serves as a promising basis for further development. It visualizes the peak shaving effect from the power input point of view as expected and the principles of utilizing energy storage under varying load conditions.