Why a sailboat can move faster than the wind

OUTRACING THE WIND: Lots of boats can sail faster than the wind... ask a physicist, and she'll say that it's just a question of vectors and relative velocities.

Downwind is easy. If the wind is 10 knots, and the boat makes 6 knots in the same direction, then the crew feels a wind of 4 knots coming over the stern of the boat. The true wind equals the speed of the boat plus the relative wind. As the boat speed approaches the wind speed, the relative wind drops towards zero and so there is no force on the sail. So you can't go faster than the wind.

When the wind is at an angle, we have to add vectors representing these velocities. The faster that the boat goes, the greater the relative wind, the more force there is on the sails, so the greater the force dragging the boat forwards. So the boat accelerates until the drag from the water balances the forward component of the force from the sails. In a fast boat, there's no point going straight downwind: you can never go faster than the wind. So you travel at an angle.