Method for determining an excitation force exerted by swell on a moving part of a wave-energy converter, said wave-energy converter converting the energy of the swell into energy via interaction of said moving part with a conversion machine, said moving part making a movement with respect to said conversion machine under the action of the swell, characterized in that the following steps are carried out: a) the position and speed of said moving part are measured; b) the force u exerted by said conversion machine on said moving part is measured; c) a model of the radiation force τ rad exerted on said moving part is constructed, said model of the radiation force relating said radiation force τ rad to the speed of said moving part; d) a non-linear model of the drag force τ d exerted on said moving part is constructed, said model of the drag force relating said drag force τ d to the speed of said moving part; e) a dynamic model of said wave-energy converter is constructed, said dynamic model relating said excitation force τ w exerted by the swell incident on said moving part, to said position of said moving part, to said speed of the moving part, to said force u exerted by said conversion machine on said moving part, to said radiation force τ rad exerted on said moving part and to said drag force τ d exerted on said moving part; and f) said excitation force τ w exerted by the swell incident on said moving part is determined by means of said dynamic model, of said model of the radiation force, of said model of the drag force t d, of said measured position and speed and of said measured force u exerted by said conversion machine on said moving part and by means of an unscented Kalman filter constructed on the basis of a random-walk model of said excitation force exerted by the swell incident on said moving part. a) the position and speed of said moving part are measured; b) the force u exerted by said conversion machine on said moving part is measured; c) a model of the radiation force τ rad exerted on said moving part is constructed, said model of the radiation force relating said radiation force τ rad to the speed of said moving part; d) a non-linear model of the drag force τ d exerted on said moving part is constructed, said model of the drag force relating said drag force τ d to the speed of said moving part; e) a dynamic model of said wave-energy converter is constructed, said dynamic model relating said excitation force τ w exerted by the swell incident on said moving part, to said position of said moving part, to said speed of the moving part, to said force u exerted by said conversion machine on said moving part, to said radiation force τ rad exerted on said moving part and to said drag force τ d exerted on said moving part; and f) said excitation force τ w exerted by the swell incident on said moving part is determined by means of said dynamic model, of said model of the radiation force, of said model of the drag force t d, of said measured position and speed and of said measured force u exerted by said conversion machine on said moving part and by means of an unscented Kalman filter constructed on the basis of a random-walk model of said excitation force exerted by the swell incident on said moving part.