Climate change due to greenhouse gas emissions is prompting the IMO to reduce emissions by 80% by 2050. This thesis focuses on designing modular propulsion systems for mid-speed sailing vessels to meet these regulations. It designs a framework to generate and swiftly evaluate propulsion system designs on the basis of five performance indicators: Weight, Volume, Life Cycle Costing, Modularity and Emissions. Additionally, the framework is proven on the basis of a case study, which results in a modular concept.