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Abstract: In this paper, a method to calculate the force coefficients of a wind propulsion system consisting of single wind propulsion elements is presented. With this method it is possible to calculate the forces of the whole system using measured or calculated force coefficients of single elements, without knowledge of the section shape of the wind propulsion elements. The single elements are modelled as horseshoe vortices to calculate the interaction inbetween the elements. This method is based on potential theory in order to quickly obtain sufficiently accurate force coefficients, which are necessary for velocity respectively power prediction programs. These programs balances the forces acting on the ship to evaluate the performance of the wind ship or wind assisted ship in an early design stage. Calculations of wind propulsion systems consisting of five and six curved rectangular plates are presented and compared with wind tunnel measurements. These calculations demonstrate adequate accuracy, particularly in the domain of small angles of attack, which is of interest for wind-assisted ships.