Forces on sails result from movement of air that interacts with sails and gives them motive power for sailing craft, including sailing ships, sailboats, windsurfers, ice boats, and sail-powered land vehicles. Similar principles in a rotating frame of reference apply to windmill sails and wind turbine blades, which are also wind-driven. They are differentiated from forces on wings, and propeller blades, the actions of which are not adjusted to the wind. Kites also power certain sailing craft, but do not employ a mast to support the airfoil and are beyond the scope of this article.
Aerodynamic force components for two points of sail. Left-hand boat: Down wind with stalled airfow— predominant drag component propels the boat with little heeling moment. Right-hand boat: Up wind (close-hauled) with attached airflow—predominant lift component both propels the boat and contributes to heel.
Points of sail (and predominant sail force component for a displacement sailboat). A. Luffing (no propulsive force) — 0-30° B. Close-Hauled (lift)— 30-50° C. Beam Reach (lift)— 90° D. Broad Reach (lift–drag)— ~135° E. Running (drag)— 180° True wind (VT) is the same everywhere in the diagram, whereas boat velocity (VB) and apparent wind (VA) vary with point of sail.
Decomposition of forces (in horizontal cross-section) acting on a sail, generating lift. FT is the Total Force acting on the sail for the Apparent Wind (VA), shown. This resolves into forces felt by the sail, Lift (L) and Drag (D), with vectors shown in red and angle of attack noted as α.
Wind forces acting on a sailboat sail (L and D) and being transmitted to the boat (FR—propelling the boat forward—and FLAT—pushing the boat sideways), while close-hauled, are both components of total aerodynamic force (FT).
An iceboat is a recreational or competition sailing craft supported on metal runners for traveling over ice. One of the runners is steerable. Originally, such craft were boats with a support structure, riding on the runners and steered with a rear blade, as with a conventional rudder. As iceboats evolved, the structure became a frame with a seat or cockpit for the iceboat sailor, resting on runners. Steering was shifted to the front.
David Vinckboons: Landscape with skaters (cca. 1615), 17th century boer type iceboats
Ice boat on Saint Lawrence River, Quebec City, c. 1858–1860
Classic iceboats on the Hudson River at Barrytown, NY
Ice Boating in Toledo, Ohio
