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True wind (V T) is the same everywhere in the diagram, whereas boat velocity (V B) and apparent wind (V A) vary with point of sail. 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.
An airfoil (American English) or aerofoil (British English) is a streamlined body that is capable of generating significantly more lift than drag. [1] Wings, sails and propeller blades are examples of airfoils. Foils of similar function designed with water as the working fluid are called hydrofoils.
A sail batten is a flexible insert in a fore-and-aft sail that provides added stiffness and definition to the sail's airfoil cross-section. [1] The most common use of sail battens is in the roach of a mainsail. The batten extends the leech past the line that runs from the head and the clew of the sail to create a wider sail towards the top.
To act like an airfoil, the sail on a sailboat is sheeted further out as the course is further off the wind. [46] As an iceboat sails further from the wind, the apparent wind increases slightly and the boat speed is highest on the broad reach. In order to act like an airfoil, the sail on an iceboat is sheeted in for all three points of sail. [45]
An airfoil with camber compared to a symmetrical airfoil. The maximum lift force that can be generated by an airfoil at a given airspeed depends on the shape of the airfoil, especially the amount of camber (curvature such that the upper surface is more convex than the lower surface, as illustrated at right). Increasing the camber generally ...
The distribution of forces on a wing in flight are both complex and varying. This image shows the forces for two typical airfoils, a symmetrical design on the left, and an asymmetrical design more typical of low-speed designs on the right. This diagram shows only the lift components; the similar drag considerations are not illustrated.
Points of sail and approximate apparent wind for a conventional sailboat on starboard tack. A point of sail is a sailing craft's direction of travel under sail in relation to the true wind direction over the surface. The principal points of sail roughly correspond to 45° segments of a circle, starting with 0° directly into the wind.
On points of sail where it is possible to align the leading edge of the sail with the apparent wind, the sail may act as an airfoil, generating propulsive force as air passes along its surface, just as an airplane wing generates lift, which predominates over aerodynamic drag retarding forward motion. The more that the angle of attack diverges ...