A boat absolutely can sail downwind faster than the wind speed. The crazy cutting edge boats like you see in America's Cup or the giant trimarans that vie for v

A boat absolutely can sail downwind faster than the wind speed. The crazy cutting edge boats like you see in America's Cup or the giant trimarans that vie for various world records do this somewhat routinely. The key to understanding this is: sails are devices that work like wings, via lift, not via drag (exception, spinnakers and such). A sailboat has two wings: the sail, and the keel (or centerboard). The boat is extracting energy via how the lift vector of the sail projects onto the vector the keel keeps the boat tracking along. As you sail downwind the apparent direction of the wind begins to rotate forward in response to the boat's forward momentum. With extreme performance boats, this process can continue to the point where the boat is experiencing a forward apparent wind while sailing downwind. This is all pretty counter intuitive but valid physics. Speaking for myself, the most easy way to get an intuition for it all is to rent a windsurfer for an afternoon. It'll all make sense after that. People have tried sailboats with propeller rigs like this cart, but they end up being more trouble than they're worth. The current best anyone has figured out is solid wingsails, which have anywhere from 3x to 7x the lift drag ratio of traditional cloth sails. Hence their use in the crazy billionaire bragging contest races.