I've been involved in a number of science forum threads dealing with the physics behind a craft powered only by the wind being able to go directly downwind faster than the wind.
The level of emotion and animosity these discussions can bring is truly awe inspiring.
A couple of the chief protagonists have recently set (waiting to be ratified) the inaugural World Record, administered by National American Land Sailing Association.
There blog is here:-
Ride Like the Wind (only faster)
They travel DIRECTLY downwind (not tacking or gibing, but in a straight line) at a sustained speed 3.2X the speed of the wind.
It is a counter intuitive concept, but interesting from an aerodynamic point of view.