Tatolazo,
More than likely, a single line to the sail will work fine. It will make the sail travel the same distance as the line is pulled by the arm on the servo. By using the single attachment point at the end of the sail line, then using the servo arm to pull the ring over that line, the same amount of travel of the servo arm will cause the line to the sail to be pulled twice as far since the ring pulls in the 'center' of that line (not really the center, but that doesn't really matter). The servo will have to exert more force by pulling that ring, but the sail will travel much further than without doing it that way. [Not a very clear what of explaining it, is it? Sorry. It's been much too long since my last physics class to try explaining this kind of thing - lol.]
- 'Doc