How Can a Boat Sail Into the Wind? Lift, Keels and a Zigzag

A sail set for upwind work is not a flat sheet catching wind but a curved aerofoil held at a slight angle to the airflow. Air passing over the outer, more curved side travels further and is deflected more than air on the inner side, and the net result is that the sail deflects the airflow sideways. By Newton's third law, pushing a mass of air one way pushes the sail the other, and the force produced is perpendicular to the wind direction rather than parallel to it. That is the crucial point: the force on the sail is mostly sideways relative to the wind, and it can therefore have a component pointing forward relative to the boat, which is what makes progress against the wind possible. The same analysis can be given in terms of pressure differences across the sail, and the two descriptions are equivalent; what matters is that the sail is generating lift, not absorbing a push.

Lift on the sail alone would simply slide the boat sideways, and a boat with no underwater fin does exactly that. The keel or centreboard is a second aerofoil, working in water rather than air, and its job is to resist sideways motion. Because water is roughly eight hundred times denser than air, a small underwater surface can balance a very large sail, and the combination is what converts sideways force into forward motion:

• •The sail generates a force largely across the boat, with a smaller forward component
• •The keel resists the sideways part almost entirely, leaving mainly the forward part to move the boat
• •The residual sideways slip, called leeway, is small and is one of the things a good hull design minimises
• •The sideways force also heels the boat over, which is resisted by ballast in the keel, by the crew sitting out, or in a dinghy by hanging off a trapeze
• •A multihull achieves the same stability from width rather than weight, which is why catamarans can carry more sail for their displacement and go faster

What the sail actually experiences is not the true wind but the apparent wind, which is the vector sum of the true wind and the wind created by the boat's own motion. Standing still in a fifteen-knot breeze gives fifteen knots of apparent wind; moving forward adds the boat's own speed as a headwind from ahead, so the apparent wind is stronger and comes from further forward than the true wind does. This has a remarkable consequence: as a fast boat accelerates, the apparent wind increases, which generates more force, which accelerates it further, so certain craft can exceed the speed of the wind driving them. Ice yachts, land yachts and modern foiling catamarans routinely sail at two to four times true wind speed, which sounds like something for nothing and is not, since the energy still comes from the wind and the boat is extracting it by moving across the airflow rather than with it.

No boat sails directly into the wind, because at that angle the sail cannot generate lift and simply flaps, a state called being in irons. The unusable sector, roughly forty-five degrees either side of the wind direction for a typical cruising boat, is the no-go zone, and its width is the single best measure of how well a boat points. To reach a destination upwind, the boat sails as close to the wind as it can on one side, then turns through the wind so the sail fills on the other side, a manoeuvre called tacking, and repeats, producing the zigzag course visible in any harbour. The distance sailed is longer than the direct line by a factor that depends on the angle, so a boat pointing at forty-five degrees covers about one and a half times the straight-line distance, which is why racing crews care enormously about pointing a few degrees higher. Going downwind, by contrast, the sail can simply be pushed, and the aerofoil analysis is replaced by straightforward drag, which is why a boat running directly before the wind can never exceed wind speed.

Square-rigged ships, whose sails are set across the vessel, work well downwind and poorly upwind, which shaped the entire pattern of early long-distance sailing: routes were planned around prevailing winds, and a voyage was often a matter of getting to the right latitude and then running with the trade winds. Fore-and-aft rigs, with the sail set along the boat's length, are far better upwind, and the crucial developments came from the Arab and Indian Ocean lateen sail, which spread into the Mediterranean, and from the independent evolution of the crab claw sail in the Pacific, where Polynesian voyaging canoes were capable of sustained upwind work centuries before European vessels were. Modern racing has pushed the principle further than the sails themselves, since hydrofoils lift the hull clear of the water and remove most of the drag, and the boats used in recent America's Cup competitions sail upwind at speeds far above the true wind, which would have been regarded as impossible within living memory.

A sail set for upwind work acts as an aerofoil, deflecting air sideways and generating a force mostly across the boat with a small forward component, which is why the push explanation fails. A keel or centreboard resists the sideways part in much denser water, leaving the forward part to drive the boat. The apparent wind strengthens as the boat accelerates, letting fast craft exceed true wind speed, and since no boat can point directly into the wind, upwind progress is made by tacking through a zigzag.