Climate change and greenhouse gas emissions have been at the forefront of both public and academic debate for some years. Although the shipping industry has managed to remain relatively free of climate regulation this is changing with the IMO goals for 2050. The military sector has also escaped scrutiny on sustainability issues. But this is also changing as the Dutch Ministry of Defence expressed the ambition to reduce its carbon footprint, with the eventual goal to reduce the operational dependency on a scarce resource, and to comply with national and international regulation. The direct aim, therefore, is to reduce the fossil fuel consumption by 70%, and to comply with IMO regulations by 2050. In this thesis the possibilities for reducing fossil fuel consumption and greenhouse gas emissions will be examined. This will be done for the seagoing large surface vessels of the Royal Netherlands Navy by using alternative fuels. This question is answered through the execution of two case studies of vessels with different mission profiles: the Zeven Provinciën class Air Defence and Command Frigate and the Landing Platform Dock Johan de Witt. For both vessels, a design process is carried out in which more detail is progressively added whilst down selecting the most suitable technologies. The first step in the design process is an operational analysis. This operational analysis uses the perceived missions profiles and the RNLN maritime doctrine to make a prioritization in a set of technical properties or measures of effectiveness. In the second design step, a systematic design variation is used to estimate the effect that different energy carriers have on the main dimensions of the vessel. The parametric design tool developed for this is an adapted version of the SPEC tool developed by Marin. The model estimates the required power and the weight of different weight groups of the vessel. The final design step continues with a more detailed proposal for a power plant configuration for both case studies. With this detailed design a final assessment of the fuel consumption, exhaust gas emissions, and operational effectiveness is made. Throughout the design phases it was established that the displacement of both vessels would increase significantly due to the lower energy density of the selected fuels. In the case of the Air Defence and Command Frigate, the high top speed and relatively high fuel and system weight lead to a larger increase in required fuel and installed power due to the increasing resistance when using anything but the most energy dense fuels. For the Landing Platform Dock, which has a more modest power requirement and a relatively low system and fuel weight, the increase in displacement is smaller. In conclusion, it can be said that the goals stipulated by the Ministry of Defence are attainable. The effect on the operational effectiveness varies between vessels but the overall fuel consumption, cost, and displacement are sure to increase significantly.