A performance prediction program dedicated to merchant ships was developed to assess fuel saving abilities of a kite. The solving of the parameterization presented led to kite velocities and tethers tensions prediction continuously along a flight path within the wind window, including especially wind gradient and ship velocity. Both static and dynamic flight cases were considered regarding optimization strategy for kite tow efficiency. For dynamic flight case azimuth, elevation and orientation of the trajectory are continuously optimized in the present algorithm. Magnitude orders of towing forces induced by the kite were compared to those obtained in the literature. Especially in upwind conditions, which are the most frequent point of sail for fast vessels, results are dramatically improved. Finally, using a 320 m(2) kite on a 50,000 dwt tanker, the fuel saving predicted is about 10% for a wind velocity of 9.77 m s(-1) (Beaufort 5) and reaches more than 50% for a wind velocity of 15.68 m s(-1) (Beaufort 7). (C) 2015 Elsevier Ltd. All rights reserved.