Why must a sail be stiff
In the early days, all windsurfing sails were made entirely of fabric material except for a small window to enable the sailor to see through. They were so-called "soft" sails, much like the sails used on the club beginners' boards. There were two or three short battens held in pockets in the leech area. When the sail filled up with wind, a curve would be formed in the sail and the sail would be cut in such a way that the curve would form near the mast in the luff area. However, the shape of this curve was not very precise and it would change depending on the wind conditions. In strong winds, the position of the deep part of the curve would move towards the leech. This caused the position of the pull of the sail to move towards the end part of the boom. The end result is that the sail became very unstable and difficult to control in rough conditions. Harness lines in those days were spaced very far apart to take this into consideration and even then you would end up with very sore arms after a day of sailing in strong winds!
Then some sail designer decided to fit the sail with full-length battens which extended all the way from the leech to the mast. To ensure that the sail formed a curve in the luff area, the batten was made stiff for about two-thirds of its length and then gently tapered towards the mast. This helped the sail to keep its shape from mast to leech and greatly improved its stability. Without such battens, it will be almost impossible to control a "soft" sail of 10 sqm or bigger in anything but the lightest winds.
