A sailboat can’t sail straight into the wind, but it can sail at about 45 degrees to it. The sails work like wings, producing lift, and the keel stops the boat sliding sideways, so she moves forward at an angle to the wind and zigzags to get directly upwind.
No sailing boat can sail straight into the wind. For roughly 45 degrees either side of the wind, an area called the no-go zone, the sails just flap and the boat stops. Outside that zone, though, a boat can make good progress towards the wind, and two things make it possible.
The first is that the sails work like wings. Set at a small angle to the wind, a sail takes up a curved shape. Air flowing round the curved leeward side speeds up and its pressure drops, while on the windward side it slows and its pressure rises. The difference produces lift, a force that pulls the sail towards its leeward side and slightly forwards. The sail is pulled by the low pressure on its leeward side more than it is pushed by the wind.
The second is the keel. Most of the sails’ force acts sideways, and on its own it would push the boat across the water. The keel, a deep fin under the hull, is very hard to push sideways but slips forwards easily. It resists the sideways force, and what is left, the forward part called the drive, moves the boat ahead. The keel works as a wing in the water, producing its own sideways lift to balance the sails. A dinghy’s centreboard does the same job.
To reach a place directly upwind, the boat sails close-hauled at about 45 degrees to the wind on one side, then turns through the wind (a tack) and sails at 45 degrees on the other side, in a zigzag. This is called beating, or beating to windward. Each tack turns the boat through about 90 degrees.
A common mistake when beating is to steer a little too close to the wind to save distance. The sails start to flutter, the boat slows and slips sideways more, and she ends up no closer to her goal. Sail at full speed just outside the no-go zone instead.
More in the book: Chapter 3, How sailing works