Course stability of a hydrofoil sailing boat, which is equipped with two surface-piercing dihedral foils and an inverted “T” foil, are examined as an extension of the previous works. For the analysis of the course stability in case of manual steering, linearized equations of motion are used for surge, sway, roll, yaw and rudder deflection. The analysis indicated that the boat was stable without rudder control, except for the cases of extremely higher boat speed and following wind condition. These unstable behavior, however, will be easily improved by helmsman's rudder control. Then the course stability with application of a windvane control system were also analyzed in terms of the same procedure as the next step. The windvane used here turns around a vertical shaft and operates an auxiliary rudder by gear linkage. Through the analysis, the optimum size of windvane was determined. The effect of the size of the windvane on the course stability are also examined through the frequency response analysis. Finally, using the non-linear equations of motion, the yaw and other motions of the boat were simulated numerically assuming a realistic variation of wind direction, which was regenerated from a spectrum of wind direction fluctuation. The numerical simulations, as the results, necessitated slightly larger size of windvane for stabilization compared with those of the stability analysis, especially for following wind condition.