Performance prediction for interacting wing sails: A viscosity-corrected non-linear lifting line model compared to wind tunnel experiments at pre- and post-stall conditions

• Non-linear lifting line theory models sail-sail interaction effectively. • The ISILL code is validated against wind tunnel experiments. • The ISILL code remains stable beyond stall, with reduced accuracy. • 3D standalone/single-sail data calibration enhances model accuracy. • The sail-sail interaction significantly impacts the yaw moment. • Viscous corrections to potential flow theory can improve force predictions. As wind propulsion for ships emerges as a key measure to meet climate goals, there is an increasing need for effective performance prediction methods. This paper presents an efficient non-linear lifting line method (ISILL), enhanced with viscous corrections, specifically developed for interacting sails. The validation is conducted against wind tunnel experiments, covering a wide range of sheeting combinations, both pre- and post-stall conditions, at four different apparent wind angles. The results focus on predicting maximum driving force, corresponding sheeting angles, and yaw moment of the sail system. Up to and including the onset of stall, the overall agreement with wind tunnel experiments is satisfactory. The method remains computationally stable beyond the point of stall, although the accuracy decreases in the post-stall regime.