The harvesting of wind energy and its transformation into a thrust force for the ship propulsion is the basics of Wind-Assisted Ship Propulsion (WASP). The concept has been gaining in popularity in the last years due to the expected benefits in emission reduction. To exploit the benefits, a proper integration between the conventional diesel engine-screw propeller propulsion plant and the WASP is mandatory. This paper aims to study the integration of the Flettner rotor technology with a conventional ship propulsion plant with controllable pitch propellers. The method allows to evaluate the engine-propeller working points and, eventually the total ship propulsive power, considering the influence of the rotor and the wind conditions. The total ship power is modelled on the amount of power required to spin the rotor, providing a way to compare hybrid propulsive solutions in terms of fuel consumption and CO2 emissions. A 3000 ton Ro-Ro/Pax ferry has been selected as a case study. Results on the parametric analysis of rotor dimensions and wind conditions are presented. Assuming fixed wind conditions, the effect of the rotor at different ship speeds is shown.