A local dynamic collision avoidance algorithm based on an improved fast-marching square (IFMS) method is proposed to solve the multi-vessel collision avoidance problem of unmanned sailboats in open water.

Considering the uncontrollable speed of a sailboat, its collision avoidance behavior is transformed into a heading control problem, with the direction of the total potential field gradient decreasing as the desired collision avoidance heading. When sailing without risk of collision or downwind, a temporal potential field is constructed to avoid all stationary irregular obstacles and reach the target point. When the sailboat has sailing constraints, Gaussian likelihood functions are designed for specific encounter situations to dynamically construct obstacle potential fields according to the convention on the international regulations for preventing collisions at sea (COLREGS), while the local wind potential field is introduced to consider the dead zone of a sailboat and realize the application requirements of combining dynamic collision avoidance with a zigzag sailing strategy.

The results show that the proposed algorithm can enable an unmanned sailboat to successfully achieve collision avoidance operations with other sailboats and restricted motor vessels in various encounter situations, while complying with collision avoidance rules and avoiding navigational dead zones. Compared with the original fast-marching square (FMS) algorithm, the upwind sailing time is significantly reduced and the planned collision avoidance trajectory is safer and more reasonable.

The proposed method complies with the characteristics of sailboat motion and collision avoidance rules, has high safety and robustness in complex environments, and possesses scientific value regarding the development of autonomous obstacle avoidance technology for unmanned sailboats.