2.3 Wind force and moment coefficients

　

 Fig.7 shows the coordinate system of the model and definitions of forces and moment. The forces in the horizontal plane with respect to the wind is the drag force D, positive in the wind direction, and the cross force L, positive to the right when facing into the wind. The moment M for the vertical axis in the X-Y plane is the yawing moment. The α is the angle of the apparent wind based on the rigid wing sail. The β and the γ are the slat and the boom angle respectively.

　

 The lift, drag forces and the moment coefficients are defined in non-dimensional forms as follows:

　

　

 Here, ρA; density of air, U; velocity of wind. S; lateral projected area, C ; chord length.

　

 S is defined as the sum of the lateral projected area of the wing sail with the slat on the basis of X-axis at each setting and the area of the soft sail, that is, S slightly depends on the slat angle β. C is obtained from S/H, which H is height of the sail as in Fig.4 and Fig.5. The aspect ratio (AR) is defined as H²/S.

　

 The relationship between course direction of a ship and each wind force coefficient is shown in Fig.8. The thrust and side force coefficients Cx, CY, which have influence on the voyage of a ship, are defined as follows:

　

　

　

　

2.4 Experimental results and discussions

　

 All experiments were carried out in uniform steady flow. The wind velocity is 8m/s. In this case, the Reynolds number used the average chord of the model is about 2 × 10⁵. It is confirmed that CL and CD are almost constant under the region of the Reynolds number. The list of the experiments is shown in Table 1. The results of each subject are shown in the figure from next sections.