Autonomous sailboat velocity prediction program considering the sea-surface wind velocity gradient

Abstract

A wingsail converts wind energy into a driving force, and the angle-of-attack polar curve of a wingsail must be obtained from a velocity prediction program (VPP) for the automatic sailing control of autonomous sailboats. Thus, a VPP is required to predict and evaluate the performance of autonomous sailboats. The wind under natural sea conditions has an atmospheric boundary layer, and its speed varies with altitude. Although wingsails are generally 1–2 m high, they are located in the bottom boundary layer of the atmosphere, where the wind speed changes rapidly. Thus, the effect of the wind velocity gradient on wingsail aerodynamic performance cannot be ignored. In this study, an autonomous sailboat VPP that considers the sea-surface wind velocity gradient was developed. The autonomous sailboat Seagull was considered as the research object. Moreover, the sailboat was modeled as a rigid body undergoing surge, roll, and yaw, and a three-degree-of-freedom mechanical equilibrium model of the autonomous sailboat was developed. The relationships between navigation resistance, heeling angle, drift angle, and velocity were calculated by computational fluid dynamics simulations using Star CCM+. The aerodynamic coefficients of the three-dimensional wingsail were calculated. The rigid wingsail was discretized along the height direction, and the aerodynamics of each unit were calculated by combining the wind velocity gradient model. Furthermore, the driving and lateral forces generated by the wingsail were obtained via integral summation. Optimization was performed to maximize the velocity of the sailboat, and the speed and wingsail angle-of-attack polar curves of the Seagull autonomous sailboat were obtained. Based on the calculation results, a comparative analysis was conducted regarding the changes in the maximum speed and optimal angle-of-attack of the autonomous sailboat, considering wind velocity gradients. Finally, the proposed VPP was verified by a comparative sea trial evaluating the sailing performance of two types of autonomous sailboat VPPs with and without considering wind velocity gradients. When the wind velocity gradient was considered, the upwind sailing ability and maximum speed of the autonomous sailboat outperformed those assuming a uniform constant-flow field.