Reaction Ferries

Petr Hošek :"It's basically a kite. A water-kite, but the principle remains."

What a fabulous idea and so simple.
Probally 100's of years old.
Looked them up and found they have existed from between the 17th and 19th centuries, when they were quite common on the Rhine.
Today there are many across the world.

I love how the most elegant solutions seem soooo obvious after you see it done.