In attempting to explain the aerodynamic principles of sails, many people refer to the principles of aerofoils derived from aircraft wing theory. Whilst the fundamental theory of aerofoils in undoubtedly relevant to both wings and sails, the conditions in which the two aerofoils operate are so very different that there have always been questions (doubts) in my mind about how appropriate the comparisons are.

My understanding of aerodynamics is not good enough to provide answers. Some of my questions (doubts) are given below. Any explanation  members might be able to offer as to why these seemingly huge differences do not render the comparison inappropriate would be much appreciated.

• Wings are designed to provide a force orthogonal to the direction of travel, to support the aircraft's weight - no propulsive force is expected. The purpose of a sail is to provide a propulsive force to the vessel.
• Wings and sails operate at enormously different speeds, and therefore in hugely different Reynolds conditions.
• An aeroplane generates nearly all of its apparent wind, which is nearly constant for most of the flight. As a result, the wing only needs to operate in a small range of angles-of-attack and airspeed (excluding take-off and landing, where extensible flaps are used). Sails are expected to operate in a wide range of wind speeds (~0 to 50 mph) and wind angles (~30 to 90 degrees).
  
• Unlike the wing on a fixed-wing aircraft, the windspeed at the top of the mast is different from that at the bottom, and not always by a consistent amount.