Abstract
A directional stability has been analyzed for a hydrofoil sailing boat with two surface-piercing dihedral front foils and an inverted ‘T’ rear foil. For the analysis, the interactions of the motions in sway, yaw, roll and surge were accounted. The stability discriminant was calculated for three rudder conditions: fixed, manual controlled and windvane controlled. Even in the rudder fixed condition, the stability in directional motion of the boat was achieved in relatively wide range of true wind directions. Although the range of stable sailing was narrowed by increasing the boat velocity, the stability was considerably improved by the manual rudder control. Using the windvane control system, the directional stability was also improved, where the area of windvane could be decreased by shortening the chord length of the rudder and by lowering the gear ratio of the system. Frequency response analysis, which was also conducted to detect the response of heading angle to the change in true wind direction, confirmed the validity of the results of directional stability analysis in conjunction with resulting in the quantitative relation between the windvane area and the directional motion.
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