The Wind the Sail Actually Meets

Stand still on a calm day and the air may scarcely be noticeable. Cycle forwards and it meets your face. Turn the bicycle and the felt wind turns with you, although the weather has done nothing of the kind.

A sail travels with its boat. The airflow reaching it therefore depends on two movements: that of the air and that of the boat carrying the sail. A sensible setting must answer the wind aboard.

Choose the observer

Relative motion is movement measured with respect to something specified. That reference, or reference frame, can be the land, the water or the boat. Current is the surrounding water’s movement relative to the Earth. Where there is current, an observer carried along with the water and an observer fixed to the bank can report different air movements.

Use the water as the reference for the first comparison. True wind here means the air’s motion relative to that water. The boat’s speed through water, often written STW, is measured relative to the same surrounding water. An instrument or weather report using a ground reference must say so; the word true does not settle the choice by itself.

A modern nautical mile is exactly 1,852 metres, and a knot is one nautical mile per hour. Five knots therefore specifies a speed, not a direction. Add a direction and we have velocity. Five knots north and five knots east have the same speed but different velocities.

A quantity with both magnitude, meaning size, and direction is a vector. Velocities are vectors. So are forces, though a force is not a velocity. The direction of the bow itself is the boat’s heading, which need not be its actual direction of movement.

For one simple example, let heading and movement agree: the boat heads north and travels north through the water. Meanwhile the true air moves east at ten knots. Sailors name a wind by where it comes from, so that is a wind from the west.

If the boat is stopped relative to the water, air crosses it at ten knots from the west. Move the boat north at five knots and the air, as measured aboard, gains a five-knot southward component. It now crosses the boat both eastwards and southwards. The combined speed is about 11.2 knots, and the wind arrives from ahead and to port, about 63 degrees from the bow.

This is apparent wind, the actual relative airflow experienced on the moving boat. Its apparent wind angle is measured from a stated reference, here the bow to the direction it comes from. True wind angle uses the bow and our defined true-wind direction instead. In this example the true angle is still 90 degrees while the apparent angle is about 63 degrees.

At ten knots of northward boat speed, the ten-knot eastward air and the equal southward component combine to about 14.1 knots. The apparent wind arrives 45 degrees off the bow. The true wind has stayed unchanged throughout. Increasing the boat’s motion has brought its apparent wind forwards.

Direction can change the result just as much. If the boat instead moves east at five knots, it follows the ten-knot air. Relative to the boat, the air passes eastwards at only five knots and arrives from behind. Going faster strengthens the apparent wind in some circumstances and weakens it in others. Subtract the boat’s velocity from the air’s velocity in the same reference frame, and both examples follow the same rule.

Sailors adjusted to these changing winds long before instruments calculated them. Nautical navigation texts later set out the velocity construction explicitly. Modern instruments add measurements and corrections, but a sail has always met the relative airflow at its own position.

Give the wind a side

The direction from which the wind comes is windward; the opposite direction is leeward. On the boat, the weather side is the windward side and the lee side the leeward one. In our north-heading example, the apparent wind comes over port, making port the weather side. Turn the boat through the wind and the fixed port and starboard sides remain, while their relationship to the wind changes.

Keep the reference clear when describing directions. An apparent-wind angle belongs to airflow aboard. Our broad descriptions of heading relative to the wind below use true wind over the water. They answer different questions, even though both are relevant to setting the sail.

Follow the air, then the force

A sail deflects the air flowing around it and exchanges force with it. Air, like water, is a fluid: both can flow around the boat’s surfaces. Most of the load across the cloth comes from differences in pressure on its faces. Pressure is force per unit area. The same force spread uniformly over twice the area gives half the pressure; the same pressure difference across a larger area produces a larger load.

Pressure acts at right angles to each small local patch of the sail. Friction between moving air and the surface adds a load along the surface, called viscous shear. This is the fluid-surface form of friction, related to the way layers of fluid and the surface move relative to each other. Combine these local forces over the whole sail and they give a net resultant force, the single force representing their combined effect for this comparison.

Look across a horizontal slice through the triangular sail. Join its leading and trailing edges with an imaginary straight line: the chord. The angle between that chord and the local incoming airflow is the angle of attack. The chord belongs to the sail section; the airflow belongs to the relative wind at that section. Neither the boom’s angle to the hull nor the boat’s heading alone supplies the angle of attack.

Angle of incidence also describes an angle, but its stated reference matters. For the horizontal sail slice, we could specify incidence as the chord’s angle to the boat’s fore-and-aft centreline. Hold that sail setting fixed and a change in wind direction can still change its angle of attack: the chord-to-hull angle stays the same while the chord-to-flow angle changes. Some writers use incidence for a flow angle. Identifying the two lines being compared is more reliable than assuming every use of incidence means angle of attack.

A flexible sail can work as an aerofoil, also spelt airfoil: a shaped surface or section arranged to produce useful force from air. Its edge tensions, cut, support and the air’s loading help determine the curve it actually takes.

When air substantially follows the surface, the flow is described as attached. Flow separation occurs where the flow leaves the surface and forms a separated region. A sail can have attached flow in one region and separation in another; a smooth-looking outline does not tell us the whole flow pattern.

Split the resultant according to the incoming relative flow. Lift is its component perpendicular, or at right angles, to that flow. Drag is its component along the flow in the downstream direction. Lift on a sail can be largely sideways.

A force component is the contribution along a chosen direction. Choose the boat’s fore-and-aft and crosswise directions instead, and the same sail resultant has a driving force component forwards and a side force component across the hull. Lift is therefore not another name for forward drive. When the wind is well forward, a useful sail force can act strongly sideways while retaining a smaller forward contribution.

The force and the changed airflow belong to one exchange. The sail deflects air; the air acts on the sail. Pressure and shear describe how that exchange occurs at the surface.

The deflection also extends beyond the cloth. In wing terminology, upwash describes approaching flow turning towards the side to which lift acts; downwash describes wake deflection in the opposite direction. For a simple sail producing lift mainly towards the lee side, that comparison gives approaching flow turning leewards and a wake deflected windwards. For a wing supporting weight, those names suggest familiar vertical directions. Around an upright sail, the relevant deflection can be mostly sideways.

Change the angle of attack and both flow and force change. Over a useful range, increasing it can increase lift. With sufficiently extensive separation, lift no longer develops as it did in the attached regime and drag can become large. This loss of useful lifting performance is stall. It can develop progressively and unevenly over a real sail, rather than switching the entire force off at one angle.

There is a different visible loss of working shape when the sail’s leading region flutters. This is luffing. A sail allowed too far towards the incoming wind can lose the pressure difference holding its front portion filled. The word also appears in turning the boat towards the wind, so the action or visible state must be clear from the sentence.

An over-restrained sail and a fluttering sail may both need attention, but they do not represent the same airflow. The crew adjusts the actual angle and shape to recover useful force.

Point the boat, set the sail

A course is a direction the vessel intends or is directed to follow. A point of sail describes its relationship to the wind. For the following terms, use the boat’s heading relative to true wind over the water; its actual path through water or over the ground will need a separate check.

Begin with the bow pointed directly towards the wind’s source. Within a sector around that direction, this ordinary sailing boat cannot maintain useful sailing propulsion. That is its no-go zone. Its width depends on the boat, sails and conditions, rather than one compulsory angle for every vessel.

Turn far enough away to sail usefully as close to the wind as the boat can manage: close-hauled. Turn farther so that the wind is still ahead of the beam but the heading is farther off than close-hauled: a close reach. With the wind approximately abeam, the boat is on a beam reach. With the wind behind the beam but short of directly astern, it is on a broad reach. Reaching covers those crosswind and oblique regions between close-hauled and running.

With wind arriving broadly from behind, the boat is running, or running downwind. Directly astern gives a dead run. These are regions of heading relative to the defined true wind. A fast boat can be on a broad reach while experiencing an apparent wind much farther forward.

Turning the bow closer to the wind’s source is heading up, also called luffing up. Turning farther away is bearing away, or bearing off. Older accounts also use bear up for bearing away. That historical expression cannot be decoded safely by treating its everyday word up as an instruction to head towards the wind.

The boat can alter heading without changing its sheet, or alter the sheet without turning. Sail trim means adjusting sail angle and shape for wind, course and the behaviour wanted. It is a different sense from hull trim, the relative depth of bow and stern. To ease, or sheet out, is to let a loaded control line out under control. To sheet in, or haul in, is to pull the sheet towards a tighter setting. The mainsheet changes the boom’s restraint, which changes the sail’s presentation to the apparent wind.

Hold the boat on a steady reach, with apparent wind coming from ahead and to port and the boom over starboard. Suppose the sheet has pulled the boom too close to the centreline, giving the sail an excessive angle of attack and extensive separated flow. Ease the sheet a little: the boom can move farther to starboard, bringing the chord closer to the incoming flow direction and reducing that angle. Under these stated conditions, that can restore useful attached flow. If instead the sail has been eased so far that its forward portion flutters, sheet in a little. The boom comes closer to the centreline, the angle increases and the sail can fill. These opposite corrections answer different starting conditions; the useful setting is judged from the sail’s response.

If the wind comes over the port side, the boat is on port tack; over starboard gives starboard tack. Tack here identifies wind side. It does not name the lower forward sail corner introduced earlier, and it does not yet describe the manoeuvre for changing sides. On a dead run, a common convention takes the side opposite the mainsail as the windward side. With the mainsail over port, that makes starboard the windward side and the boat on starboard tack; with it over starboard, the boat is on port tack. This supplies a designation even when the wind arrives directly from astern.

When the wind changes

The moving-observer example held the true wind steady. Real wind can change while the boat keeps the same speed and heading. A gust is a temporary increase in wind speed; a lull is a temporary reduction. A wind shift changes its direction. Each can alter the apparent airflow before the crew has moved a control.

For an upwind boat holding its heading, a shift bringing the wind closer to the bow is a header. The boat may have to bear away to recover its useful angle. A shift allowing it to head farther towards its upwind destination while keeping a useful wind angle is a lift. This is the wind-shift sense of lift, quite separate from the force component perpendicular to airflow.

There can also be a wind gradient, a change in wind speed or direction with height. Different levels of the same sail can therefore meet different airflows. The boat’s motion, the changing weather and the flow disturbed by its rig all contribute to the local conditions.

The crew’s controls answer this combined situation. Sail force changes the boat’s speed; changed speed changes apparent wind; changed apparent wind can require another setting. The hull is part of the same exchange. If it simply slides sideways under the sail’s side force, useful travel towards the wind remains elusive. The next part of the boat has to act in the water.