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.
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)
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)
At 30° to the apparent wind, the wing’s 10° angle of attack generates low drag and plenty of lift / Driving force.
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.
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.
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.
The conventional sail's angle of attack is 90° so instead of working as airfoils, all their drive is created from drag.
Lift & drag
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).
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).