An upwind sail, exactly like an aircraft wing, functions by developing a differential pressure between its windward and leeward surfaces. Obviously, the greater the pressure difference, the greater the created aerodynamic force. The useful component of this force is what keeps birds and all kinds of aircraft in the air, and is, therefore, termed "LIFT", even in sailing terminology. To obtain a significant differential pressure, airflow speeds over an airfoil's windward and leeward surfaces have to be significantly different.
Setting an airfoil spanwise under an angle to the wind will cause the airflow over the lee side of the foil to speed up, which, according to Bernoulli principle, results in a relative air pressure drop, creating a 'suction' zone. This is the most important part of the "lift" process. A slower airflow over the windward side will produce a positive pressure, so the pressure difference will exert a force in leeward (for wings upward) direction.
The airflow incidence upon a conventional single-sided sail must be at least tangential.
If the angle of attack is smaller, the sail will luff. The angle of attack for double sided airfoils is significantly smaller - by about 20 degrees (this is elementary geometry).
Increasing the angle of attack will lead to increase of the lift, but only to a certain point called the "stalling angle". Beyond this angle the flow will separate from the foil and the lift will die. Double-sided, asymmetrical, highly cambered foils (or simply wings) have very high coefficients of lift without stalling. At low speeds, the air bends over a highly cambered surfaces still retaining its laminar flow characteristics.
Determining the amount of camber, the airfoil thickness and the position of the maximum camber belongs to the science of aerodynamics and wing section design, which has reached its perfection long ago.
Aerodynamic properties of conventional thin sails are poor compared to the properties of wings. This fact, although unknown to most sailors, is recognized by speed sailors, to the extent that basically all records in speed sailing on water, land and ice have been achieved with wing sails, usually of rigid type. Rigid sails, on the other hand, are highly impractical, heavy and expensive. The construction of a soft wing sail with reversible camber has been the dream of many inventors and designers for a long time. Most constructions have been rib based and have included a lot of moving parts, sometimes even cog-wheels and hydraulic pumps.
The present patented design of a soft wing sail offers unprecedented lightness and simplicity of construction, yet maintains superb aerodynamic properties providing a much stronger propelling force and an astonishingly reduced heeling moment for the whole range of sailing craft, from canoes and sea kayaks to transoceanic multihulls.