The BBC has a great video of the Sailrocket’s world record beating run where it went over 50 knots. Interestingly the wind was only 27 knots, so how was it done? The physicist in me was intrigued.
First, let us do a thought experiment. The boat is in a vacuum on a frictionless surface. Although in our vacuum we don’t have any wind we can apply a force to the boat and keep that force steady.
We know from Newton’s laws that
acceleration = Force/mass, (from F=ma)
Therefore for as long as we apply a force the boat will accelerate, that it, it will go faster and faster. In our simple thought experiment there is no limit to its speed but in the real world the drag on the boat will increase with speed until it is equal to the force provided by the wind and the boat reaches its top speed.
So how can the Sailrocket sail faster than the wind? Well, the answer is that it does not sail with the wind behind it like a medevial galleon, rather it sails at 90 degrees to the direction of the wind and uses an areofoil to provide the force.
When the wind is behind a sail, the faster the boat goes, the slower the wind appears to be to the boat, eventually when the boat is going at the wind speed there does not seem to be any wind at all! (slight simplification). We all know this instinctively and that is why it seems strange that a boat can sail faster than the wind.
But the Sailrocket does not sail in the direction of the wind, it sails across it. For it, the wind speed does not drop as it goes faster, it is like our boat in the vacuum with a constant force applied. It will go faster and faster until the applied force equals the drag.