In this paper an original modeling approach is proposed. This approach is based on power exchanges and highlights the most influential parameters of a complex system, without having to choose particular technological solutions. This is thus a perfect solution for a first and global design or for the validation of a concept. This approach has been efficiently employed for the optimal sizing and control of a marine hybrid propulsion which comprises a thermal engine and two electric motors in series configuration associated with sails. The control strategy is organized in two levels: a global one, which defines power objectives, and a local one which maximizes the efficiency and guarantees the component security.