How a sail works: lift, suction and stall

Most people imagine a sail as a bag that catches the wind and gets pushed along. That picture is right when the wind is behind you. For every other course, it is wrong. When a boat sails across or into the wind, the sail works like an aircraft wing turned on its side. It pulls the boat more than it pushes it.

This guide explains how that pull is created, why it can suddenly disappear, and how to see it all happen in the simulator.

The two sides of a sail

Wind meeting a curved sail splits into two streams.

• The windward side (the side facing the wind) slows the air down a little. Slower air means slightly higher pressure.
• The leeward side (the side away from the wind, the outside of the curve) makes the air travel faster around the curve. Faster air means lower pressure: suction.

The sail sits between a small push on one side and a strong pull on the other. The pull from the leeward side does most of the work, especially near the front of the sail, just behind the mast.

Engineers measure this with the pressure coefficient (industry term: C_p). It compares the pressure at a point with the pressure of the oncoming wind. Negative numbers mean suction. In the simulator, the peak suction near the front of a well-trimmed sail reaches about C_p −4.5: the air there pulls more than four times as hard as the wind pushes on a flat wall.

In the simulator, blue shading shows suction and red shows pressure. The chart Suction along the sail plots both sides from the mast to the back edge.

Lift and drag

The total force from the air on a sail is split into two parts, named exactly as they are for aircraft:

• Lift: the part at right angles to the wind the boat feels. It has nothing to do with “lifting” upwards; on a sail it points sideways and forwards.
• Drag: the part along the wind direction. It pulls the sail downwind.

Good sails make a lot of lift for little drag. The ratio between them is called lift-to-drag ratio (industry term: L/D). The higher it is, the more efficient the sail.

The boat, however, does not care about lift and drag directly. It cares about two other parts of the same force:

• Drive force: the part pointing toward the bow. This moves the boat forward.
• Heeling force: the part pointing sideways. This tips the boat over and tries to push it sideways through the water. The keel resists it.

When you sail close to the wind, lift points mostly sideways, so most of the sail’s force becomes heeling force. As you turn away from the wind, the same lift swings forward and becomes drive. That is why reaching (wind from the side) is usually the fastest point of sail.

Angle of attack: the one number that matters

The sail’s behaviour depends mainly on one angle: the angle of attack (industry term: AoA). It is the angle between the wind the boat feels and the sail’s chord (the straight line from the mast to the back edge of the sail).

You control it with the sheet: the rope that sets how far out the boom is.

• Sheet in (pull the boom toward the centre of the boat): the angle of attack gets bigger.
• Ease (let the boom out): the angle of attack gets smaller.

In the simulator, angle of attack = apparent wind angle − boom angle. Turning the boat changes it too, which is why you have to re-trim every time you change course.

Three ways a sail can work

Here is the same boat, same wind (12 knots, wind 60° off the bow), with only the sheet changed. All numbers come from the simulator.

	Luffing	Well trimmed	Stalled
Boom angle	45°	23°	10°
Angle of attack	about 1°	about 15°	about 35°
Lift coefficient (CL)	0.34	1.28	0.91
Drag coefficient (CD)	0.11	0.23	0.54
Lift ÷ drag	3.0	5.6	1.7
Boat speed	4.0 kn	7.3 kn	4.4 kn

Luffing: too little angle

If the sail is eased too far, the wind meets it almost edge-on. The front of the sail gets hit from the wrong side and starts to flap. Sailors call this luffing. There is almost no suction, so there is almost no lift. In the simulator you can see the front of the sail ripple and the suction chart go flat.

Attached flow: the sweet spot

At a moderate angle, the air follows the curve of the sail smoothly all the way to the back edge. This is called attached flow. Suction is strong, drag is low, and the boat is fastest. For the simulator’s default sail this happens roughly between 2° and 17° of angle of attack.

Stall: too much angle

If the sail is sheeted in too far, the air on the leeward side cannot follow the curve any more. It peels away from the surface and tumbles into swirling eddies. This is flow separation, and the result is a stall.

In a stall:

• suction behind the separation point collapses to a weak, flat value;
• lift drops;
• drag jumps, because the eddies waste energy;
• the boat heels a lot while going slowly.

The simulator shows eddies (small spinning pockets of air, industry term: vortices) peeling off the back of the sail, and the chart marks the point where the flow breaks away.

Why stall happens

Air close to the sail’s surface is slowed by friction. This thin, slow layer is called the boundary layer. On the leeward side, the air speeds up around the front of the curve and then has to slow down again toward the back edge, moving from low pressure into higher pressure (industry term: adverse pressure gradient).

The slow boundary layer has very little energy. If the pressure rise toward the back of the sail is too steep, it simply stops moving forward and separates. A bigger angle of attack makes the suction peak stronger and the pressure rise behind it steeper. Past a certain angle, separation becomes unavoidable.

On a sail, separation usually starts at the back edge (the leech) and creeps forward as you sheet in more. That is why the leech is the first place to watch for trouble.

The lift curve

If you plot lift against angle of attack, you get the shape every sailor should know. The simulator draws it in the chart Lift and drag vs angle of attack.

1. Lift rises steadily as the angle increases.
2. It peaks just before the stall, at around 17° for the default sail.
3. Past the peak, lift drops while drag keeps climbing.

The best trim for speed is usually just below the peak. You want the most lift you can get without tipping into the stall zone, where a small gust or a wave can push the flow over the edge.

What sail shape changes

The simulator lets you change three shape controls. They move the lift curve around:

• Camber (also called draft depth): how deep the curve of the sail is. A deeper sail makes more lift and more drag. It is powerful in light wind and in waves, but harder to keep from stalling.
• Draft position: where the deepest point sits. Further forward is more forgiving to steer with; further back points higher but stalls more easily.
• Aspect ratio (AR): sail height compared with its width. Tall, narrow sails waste less energy in the swirl at the top (the tip vortex), but they stall earlier.

Key takeaways

• A sail works like a wing: suction on the leeward side does most of the work.
• Lift and drag become drive (forward) and heeling force (sideways), depending on the course.
• Angle of attack decides everything. Too little and the sail luffs; too much and it stalls.
• The fastest trim sits just below the stall.

The next guide explains why the wind the sail feels is not the wind that is blowing: Apparent wind explained.