Among all aquatic species, mantas and rays swim by oscillating their pectoral fins; this motion is similar to other fishes in term of efficiency, but it gives better agility in turning with respect to fishes moving their caudal fin. The fin motion is featured by a travelling wave going opposite to the forward motion, producing a force thanks to momentum conservation. Another contribution to the generation of thrust is given by the generation of a vortex in correspondence of the leading edge of the fin, which pulls the fish forward thanks to the lower pressure in its centre. In literature these contributions have been highlighted, but it remains to understand which one of these two mechanisms is prevailing according to different conditions of swimming, how they affect each other and what is the influence of the two on energetical efficiency. The object of this activity is to investigate how thrust generation is influenced by geometrical characteristics of the fin, such as size, geometry and flexibility and by parameters of motion, such as speed, amplitude and frequency of fin oscillation and velocity of the travelling wave. A CFD model of the fish has been implemented in OpenFOAM, not only confirming that both upstroke and downstroke contribute positively to the forward movement according to the momentum conservation principle, but also highlighting the formation of a leading-edge vortex enhancing thrust generation. The description of how thrust generation is linked to motion parameters is simulated also coupling the CFD with a multibody to simulate the whole motion in its complexity.