The basic idea

When air flows over a wing, there are changes in air velocity and pressure. These changes determine the aerodynamic force on the wing . Lift is a component of this force and is defined as the force that is perpendicular to the air flow.

Lift is the force that holds aeroplans up in the air. When air flows over a wing that is vertically positioned on a boat, the same lift force drives the boat forward.

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Air flow

Wings

The Angle Of Attack as well as the aerodynamic efficiency, are kept in all wind directions.

(AOA: a line drawn between the leading and trailing edges of the wing and the apparent wind direction)

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Sails

The greater the Angle Of Attack, the smaller the aerodynamic efficiency is.

(AOA; Angle of Attack is the angle between the apparent wind and the chord line of the sail) 

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Driving force

Wing Sails

At 30° to the apparent wind, the wing’s 10° angle of attack generates low drag and plenty of lift / Driving force.

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Up wind
Sails

With their greater angles of attack the genoa and mainsail create more drag and less lift. Heeling force is increased at the expense of driving force.

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On a beam reach, the lift and therefore the driving force, is mostly directed forward and there is little drag or heeling force.

The conventional sailplan suffers because the well-eased genoa does not work as an airfoil, and  generates lots of drag and heeling force. The main is still relatively efficient.

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Beam Reach
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Downwind, the well-eased wing, still with a 10° angle of attack, retains its airfoil shape and creates lift to windward. Drag is minimal, and most of the heeling force is directed to windward.

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Down wind

The conventional sail's angle of attack is 90° so instead of working as airfoils, all their drive is created from drag.

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Lift & drag

Lift

At 10° Angle of Attack (wing's working angle), the lift generated by a wing is greater than the lift generated by a sail at 20° AOA (sail's working angle).

Drag

At 10° Angle of Attack (wing’s working angle), the drag generated by a wing is smaller than the drag generated by a sail at 20° AOA (sail’s working angle).

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