Traditional sailing systems, such as used on most sailboats, generate not only a driving force to propel the water craft, but also a downward component as the sail is heeled leewardly. This downward force increases the drag of the hull of the sailboat in the water. U.S. Pat. No. 4,677,928 discloses a "non-traditional" circular sail structure which also causes the watercraft to heel leewardly. The sail structure includes a circular band and an array of parallel, spaced-apart air foils extending across the circular band. The sail structure is mounted on an upright, rotatable mast capable of rotating the sail structure in the yaw direction. The sail structure may also be rotated relative to the top of the mast in the roll direction.
Other types of sailing systems are characterized by imposing a non-heeling straight-line force on the water craft. Such non-heeling sailing systems are characterized by a sail, a wing or a kite that develops the force acting along a line perpendicular to the span of the sail, wing or kite. Examples of such non-heeling systems are disclosed by U.S. Pat. Nos. 3,800,724; 3,987,982; and 4,708,078. One drawback of non-heeling sailing systems is that typically the entire sail or wing must be rolled to alter the amount of lifting force generated. The significant mass of most sails and wings does not permit this rolling operation to be carried out quickly enough to adjust to rapidly changing wave and wind conditions.
In a third type of sailing system, the sail or wing is heeled to windward to obtain an upward component of lift. Typically in this negative heeling mode, the aerodynamic lift generated is not sufficient to support the full weight of the water craft and sailor, but rather some of this weight must be supported by hydrodynamic means. A typical type of negative heeling water craft is a sailboard, for instance disclosed by U.S. Pat. No. 3,487,800; 4,501,216; and, 4,682,557. As in non-heeling systems, in negative heeling systems, including sailboards, the sail or wing for propelling the sailboard must be pivoted about its roll axis to alter the level of lifting force generated. This lifting force is beneficial in that it does produce an unweighting of the sailboard. As a result, the hull drag decreases permitting higher sailing speeds. One drawback of increasing the upward lift, however, is that the forward drive force is reduced thereby resulting in a decrease of the potential craft speed. Moreover, the time required to roll the sail or wing to vary the upward lift is significant even on a sailboard. The time response required for the rolling maneuver often is too long to adjust to changes in wind and water conditions encountered by the sailor.