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Low speed transportation is more energy efficient than high speed transportation in general. Ships with lower design speeds provide possibilities for using auxiliary wind propulsion devices to further reduce energy consumption. The kite is one type of auxiliary wind propulsion device that can be used for this purpose. The aim of this study is to investigate the influence of auxiliary kite propulsion on the performance of a panamax tanker, specifically on engine power reduction, course-keeping ability in different headings and on pitch and roll motion of the ship. The study is carried out by computer based simulations. Scenario combinations are established for various ship speeds, headings and weather conditions. For the purpose of simulation, a simplified auxiliary kite propulsion model is built and verified against full scale data. The SSPA in-house time domain maneuvering and seakeeping software – SEAMAN II is used for the study. The kite model is created as a subroutine to the SEAMAN software. The kite flies on the spherical surface along a predefined track. It performs span-wise rotation to achieve an optimum angle of attack. A NACA 4415 wing profile is adopted as the cross section of the kite. Aerodynamic forces are calculated based on the effective relative wind velocity and the optimum angle of attack. The radial component of the aerodynamic forces provides a traction force in the connection line. The traction force contributes to forces and moments acting on the simulated tanker. The numerical kite model was verified against full scale data. The deviations of traction force and thrust force between simulation and full scale could be controlled within 7%. Throughout the simulations, forces, moments, and motions of the tanker are recorded, as well as rudder angle and propeller efficiency. These output variables are plotted and analyzed to describe the influence of the auxiliary kite propulsion to ship performance. The result shows that auxiliary kite propulsion plays a significant role in reducing engine power in beam and following sea conditions. According to the SEAMAN simulations, a reduction of at least 40% could be obtained in many cases. The course keeping ability of the simulated panamax tanker is not affected by using the kite. The present study is a first step towards a fully dynamic kite model, where a control mechanism will be introduced to find the most favorable flying trajectory.