Forces on sails apart, which generally prevents heeling. Each sailing craft is a system that mobilizes wind force through its sails—supported by spars and rigging—which provide motive power and reactive force from the underbody of a sailboat—including the keel, centerboard, rudder or other underwater foils—or the running gear of an ice boat or land craft, which allows it to be kept on a course. Without the ability to mobilize reactive forces in directions different from the wind direction, a craft would simply be adrift before the wind. Accordingly, motive and heeling forces on sailing craft are either "components of" or "reactions to" the
Forces on sails sail and the apparent wind ("α" ) exceeds the point of maximum lift on the "C"–"C" polar diagram, separation of flow occurs. The separation becomes more pronounced until at "α" = 90° lift becomes small and drag predominates. In addition to the sails used upwind, spinnakers provide area and curvature appropriate for sailing with separated flow on downwind points of sail. Again, in these diagrams the direction of travel changes with respect to the apparent wind (V), which is constant for the sake of illustration, but would in reality vary with point of sail for a constant true wind. In