6.4 Sail Coefficients during Tacking Operation

　

 In the case of the tacking operation of a square-rigger such as Higaki-kaisen, when the ship heads to the wind, we have to turn around the yard about 180 degrees. When the ship changes the wind over the bow, the sail momentarily fall into the "caught aback" situation as shown in Figure 14. At this moment the measured coefficients became as follows: Xs' was almost -0.9, Ys' was -0.2 to 0.2 according to the yard angle and Ns'was almost 0.0.

　

 Therefore we can assume that the aerodynamic coefficients during tacking operation varies with the apparent wind angle γA as shown in Figure 15. In the Figure 15 , the arrows indicate the varying direction in the case of the tacking from port to starboard tack. When the apparent wind angle yA is 45 degrees in port tack, Xs' is 0.34 and Ys' is 1.0. Then these values decrease with decreasing of apparent wind angle. In the range of -15 deg < γA < 15 deg, we will turn around the yard, hence Xs' becomes -0.9 constant and Ys' varies from -0.2 to 0.2. Then at the range of yA < -15 deg, the sail coefficients indicate the values for the starboard tack.