Hi Dennis,
Efficiency in the case of rigid-wing hang gliders pretty much boils down to less drag and more lift.
I wouldn't say that a wing sail has always more lift. A (wing) sail must work with wind from left and right, which is a significant limitation in profile design. A symetric profile without flaps has a lower maximum lift than a classic soft sail. Flaps increase the lift, but also drag. The ratio between lift and drag is closely related to vmg upwind, as long as there is enough wind. This is comparable with the best glide path of a glider, which is directly related to lift to drag ratio.
However you can significantly reduce drag by increasing span (thats way gliders have a lot of span). Since a soft sail is lighter, you can have a higher mast than a boat with wing sail and reduce aerodynamic drag or have the same mast height and a lighter boat and decreasing hydrodynamic drag.
Downwind you need maximum lift. A good genoa (or gennaker) - mainsail combination can provide a maximum lift close to that of a wing with flaps (however a genoa with wing sail...). Again you could have more sail area for same weight or less weight for same area.
As you see, it is not as simple as on glider planes, where aerodynamics rule everything. For a sail boat, you have to make a careful trade between weight and aerodynamics. As weight is more impoartant in light winds than in strong winds, soft sails are in general better in light winds.
The thing that gets me comparing wings to sails is that wings on gliders (and rigid-wing hang gliders) are not all that complicated but very efficient all the same. I don't see why sailboats should need so much more complexity to use a wing sail and be competitive, whereas all these gliders don't...
Efficient because of asymmetric profiles, simple because they have no flaps and because you don't store your glider outside on the beach for six months. Last but not least your glider operates at higher speeds and hence higher Reynoldsnumbers, where fixed wings are more efficient.
I seem to recall that having a separate upper and lower surface gets rid of lots of parasitic turbulence
If you have a free rotating aerodynamically shaped mast, you can minimise the turbulence zones by a fair amount. But on a fixed mast, the turbulence behind the mast is significant. So much that you can say that the rotating must is the key enabler for performance of cats (don't tell the skiff community...)
Remeber A/cat world championship 2008 in Florida: Ben Halls wing sail wasn't the superior in any wind speed. This is certainly related to Reynoldseffects, hence not transferable 1 to 1 to the large AC boats.
Cheers,
Klaus
