A large fleet of commercial sea-going vessels is currently operating under conditions different from those assumed during design. With a continuously growing number of greening technologies and regulations for environment protection measures, a strong need evolves for ship operators and engineering companies to determine optimal solutions for retrofits of vessels already in operation. An optimized combination of technologies is an essential but difficult challenge. Within the R&D project RETROFIT55, a web-based decision support system is being developed that is designed to provide functions to model an existing vessel and its operational profile in a straightforward, simplified way, to identify and configure a collection of greening technologies and to assess the expected effects regarding vessel performance. By executing a guided design process with substantially reduced processing time investigation of many options is facilitated. Using data from different levels of analysis – from estimates based on historical data to specific simulations – this allows first checks as well as more sophisticated studies, enriching data quality successively. The architecture of this approach is discussed, explaining the key features to facilitate the evaluation of engineering tasks and to determine the net effects of combinations of greening technologies regarding vessel performance, cost and life-cycle assessment. The aim is to offer high-level decision support for retrofitting so as to identify potential combinations before engaging in detailed design work.