Global warming is a topic of major concern the last decade and for reason the IMO adopted a strategy to reduce the emissions of greenhouse gasses of vessels. In order to meet the IMO requirements ships propulsion systems need to become more sustainable. An option to reduce emission is the implementation of fuel cells running on hydrogen together with batteries. Since hydrogen cannot be stored into conventional fuel tanks different fuel tank options are investigated, which led to the use of cryogenic liquid hydrogen storage tanks. During the research a case study was preformed for the Pioneering Spirit. The first step in the designing process is investigating the operational profile to get a better understanding of the projects done and the corresponding power demand and to see how the operational profile influences the power demand of the ship. After investigating the operational profile and its influence on the power demand of the ship a design methodology was written to help the user find an optimal solution for the new hybrid propulsion system. This methodology contains three parts: data importing, solution generation and new system specifications. In the first part data will be imported and processed to find the power demand during a project. Subsequently, a range of fuel cell capacities will be determine over which solutions will be generated in part two. The second part of the methodology contains three steps. During the first step solutions will be generated over the range of fuel cell capacities determined in the first part. This first step will be iterated for different battery capacities. After generating solutions in the first step in the second step suitable solutions will be defined. These solutions will be compared in the last step to find an optimal solution. For the optimal solution, the third part of the methodology will calculate the new system specifications.