Commentary by Dr. Whitesel: As an avid sailor since a teen, I couldn’t resist sharing this.
By David Hambling, The UK Guardian Newspaper, 11/16/16.
… For modern high-performance yachts, boat design can be more important than wind speed for going fast.
The secret is that (the sailboat called) Sailrocket was not travelling in the same direction as the wind. If the wind is blowing due north at 30mph, then the fastest you can sail north is 30mph. By aiming north-west, you can maintain the same northwards motion and add some westwards motion to achieve a higher net speed.
… by sailing at 80 degrees to the wind, a boat could, in theory, travel almost six times as fast as the wind.
This technique relies on having enough resistance to stop the boat being pushed sideways by the wind. Traditional boats have a heavy keel for stability, but this makes them slow. Instead, yachts like Sailrocket have a hydrofoil resembling an underwater wing to keep them upright. The grip and the drag from this hydrofoil determine the top speed, not the strength of the wind.
Yachts that sail on ice or sand have a solid surface to push against, and so can go even faster than boats on water. The sand yacht Greenbird holds the record for a wind-powered craft at 126mph, achieved with winds blowing at just 30-50mph.