Before designing a ship, the requirements for the ship are required. For innovative or complex technical ships, this is not instantly clear. In many cases, the expected operations of the ship need to be investigated first. Currently, this is often done on a ship-by-ship basis. However, a ship seldom operates alone, especially for technically complex ships. The ship will often be part of a fleet executing e.g. complex operations such as windmill installation or complex missions in case of navy ships. It is therefore important to investigate the role of the ship in this fleet and establish the contributions to the expected missions or tasks. Of course, the fleet will change over time and each stepwise change of the fleet could be investigated independently but should be investigated in cohesion. To enable this, an approach and validation model was developed within the DES4Ops project over the past two years. This paper will discuss the development of an optimal fleet design model to support decision making and requirement elucidation. The impact of existing ships on the choice for a new ship (brownfield development) and of a complete fleet redesign (greenfield development) are supported. A simple example case for an arbitrary set of navy missions will be discussed to highlight the potential of this approach. In the end, we conclude that the model allows the user to check the performance of a fleet in different scenario’s, but more importantly, it will lead to fleet considerations that lead to better performances overall and over time.