This paper introduces the tacking control of a low-cost mini-sized unmanned catamaran sailboat. Compared with traditional researches in sailboats based on acquiring wind speed and direction by a giant wind sensor and controlling the system by velocity polar diagram, to further increase the speed and motion agility, we propose an alternative way based on Force Polar Diagram (FPD). Thereafter, we propose a novel control algorithm to compute the sail angle that produces the maximum forward force along the boat`s heading. We employ a 3-D force sensor to obtain the propelling force on the mast. FPD is created offline given the data collected from different apparent wind angles. In the experimental platform including a water pool, two electric fans generating a quasi-controllable time-invariant wind field and a camera for boat Iocalization, we conducted experiments and discovered that the proposed method increased the maximum speed by 56%, shortened the time by 44% and improved the motion agility in an autonomous tacking for upper wind motion.