How do Sailboats Work - Sailing Theory Aerofoil and Hydrofoil

Sections: Centre of Effort Angle of Attack Jib & Mainsail True & Apparent Wind Hydrofoils Leeway Resistance Upright

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How do sailboats work requires an understanding of hydrofoil and aerofoil sailing theory. This page discusses the effects of leeway and hull resistance as well as sail theory with the airflow over the jib and mainsail and air resistance. How sail camber effects the centre of effort of the sail along with the angle of incidence and apparent wind.

The Dynamics of Sailing a Sailboat

There is an incredible amount of aerodynamic and hydrodynamic theory involved in making sailboats work. Sailboats are equipped with aerofoils, above the water, and hydrofoils, below the water. The aerofoils above the water, the sails, create power to drive the sailboat forward; those hydrofoils below the water, the daggerboard, centreboard or keel, and rudder, prevent the leeway motion of the sailboat.

Sailboat sails work similarly to the wings of an aircraft with their shape producing a force from the passage of the wind. In aircraft, the engine pulls the horizontal wing through the air [creating lift ]; in a sailboat, the wind blows across the vertical sail producing a forward drive force.

Aerofoil Sailing Theory

The simple concept is that wind pushes a sail from behind known as running downwind. Sailing either side of downwind was the direction in which the traditional square-riggers could go.

If the wind was blowing from the wrong direction they would wait days or weeks for a change. Modern Bermudan rigs changed all that, and sailboats now sail in most directions with the exception of about [ 40 degrees ] either side of the wind.

The ability to sail towards the wind is achieved by the airfoil effect of modern sails. Sail design theory relies on a simple principle to enable sails to operate. Viewed in cross-section, the top surface of the sail's wing-like structure is convex and therefore longer from front to rear than the flat bottom surface.

Wind flowing over the sails comprises a moving mass of air particles that separate when hitting the front or leading edge of the sail. The airflow moving across the convex (leeward) surface travels further than that moving across the concave (windward) side and therefore increases its speed. Airflow moving faster causes its pressure to drop therefore the pressure on the convex (leeward) side of a wing is lower than the pressure on the concave side (windward).

The differential in pressure [ sucks the sail ] to leeward creating a force at right angles at all points on the sail’s surface with the sum of these forces driving the sailboat forwards overcoming the resistance of the water. This lift force is increased by the pressure of the wind on the windward side of the sail.

Forward Drive and Centre of Effort

All the forces acting on a sail's surface in theory are aggregated to a force acting at a single point, known as the centre of effort. This force theoretically is split into two: the driving force and leeway force. The degree of driving force depends on a sail's curved shape or camber. When this camber is altered through sail controls, driving force is optimized and leeway force minimized.

Sail Camber and Angle of Incidence or Angle of Attack

The cross-sectional shape of a sail determines its performance. The degree of curvature (camber) is aligned with the apparent wind (the angle of incidence) producing maximum drive. The [ optimum angle of incidence ] is considered to be at 15 degrees between the chord (an imaginary straight line connecting both ends of the sail) and the apparent wind.

  • As the angle of incidence becomes greater, airflow detaches from the leeward side surface creating turbulence which reduces drive.
  • As the angle of incidence becomes smaller, the sail stalls because positive air flow ceases to flow over the windward side.

An aid in setting the sail at the correct angle of incidence are pairs of tell-tales attached at regular intervals to both sides of the forward part of the [ mainsail ] and [ jib ] indicating the airflow over the sail. The tell-tales on the leeward side of the sail can be seen as the sailcloth is semi-transparent.

When the angle of incidence is smaller, the leeward tell-tale begins to flutter indicating the airflow at the back (leeward) side has broken away into turbulence instead of a smooth flow. Drive can be regained by either letting the sail out (toward the fluttering tell-tale) or by pushing the tiller toward the same side as the fluttering tell-tale. When the tell-tales on each side of the sail are flying together, the sail is again creating its maximum drive.

The primary control used to control the angle of incidence of the sail is the mainsheet, mainsheet traveller or jib sheet which adjusts the sail relative to airflow. The amount of camber and its position in the sail greatly affects the performance characteristics of the sail. Altering luff (leading edge) and foot tension maximizes or minimizes camber, along with changing the position of maximum camber fore and aft from the midpoint of the sail. Sail luff and foot tensioning controls adjust the camber of the sail, tuning it to maximize performance.

Forward Drive with Jib and Mainsail

A jib when added in front of a mainsail, creates its own drive similar to a single sail, but also increases the efficiency of the mainsail. Airflow through the slot between jib and mainsail is compressed between the two sails and accelerates, further reducing the pressure on the mainsail’s leeward side, therefore increasing its drive. Although the jib is smaller than the mainsail, it is more efficient because there is no mast in front disturbing the airflow. The jib is trimmed so that the slot between the jib leech and the mainsail luff is similar all the way up.

The jib of a two-sail boat influences the airflow over the mainsail, especially when sailing close to the wind; they act effectively as single aerofoil. The front part of the mainsail may not set correctly and cause flutter, but is acceptable as the majority of the sail will be working efficiently as part of the combined aerofoil.

As well as generating driving forces, individual sails have a marked effect on the directional stability of the sailboat. Sails in front of the mast turn the bow away from the wind, and sails behind the mast turn the bow toward the wind. When properly balanced, these sails cancel out the turning movements to produce forward drive.

True Wind and Apparent Wind

The wind's force in a sail is centralized in what is called the centre of effort (CE) with the outer sail area having the role of keeping wind power under control.

  • True wind is the direction from where the real wind blows and its speed and direction is measured in a stationary position.
  • Apparent wind is the wind direction experienced by any moving object.
For example a cyclist cycling into the wind at 16kph (10mph) and the wind has a true speed of 16kph (10mph), the apparent wind he has to contend with will total 32kph (20mph). Similarly, the cyclist cycles in the same direction as the wind with both moving at 16kph (10mph), the apparent wind will be zero.

True wind and apparent wind speed is never the same when a sailboat is moving. Apparent wind is least when the boat travels exactly the same direction or running dead downwind. True wind becomes progressively greater with the boat sailing closer to the wind. This phenomenon has an effect on the temperature difference when sailing downwind and upwind.

The direction of true and apparent wind is only the same when the sailboat travels in the same direction or running dead downwind. Sailing in other directions, the apparent wind comes from further forward than the true wind. The direction of the wind is induced as a result of the sailboat's forward motion.

With the speed of the sailboat increasing, the direction of the wind moves forward, this is why fast sail-powered craft such as windsurfers and catamarans have their sails pulled hard in when sailing at speed maximizing the apparent wind.

A sailboat accelerating causes the direction of the apparent wind to change coming from further ahead. To keep up speed, the helmsman steers away from the apparent wind.

Hydrofoil Sailing Theory

The combination and interaction of sails, keels, centreboards and rudders determine the sailboat’s sailing characteristics and it is important to understand that the sails act as a single unit and must be tuned together on every point of sailing. The keel and rudder also act as a single hydrofoil unit and should also be tuned together, although except in the case of a lifting keel or centreboard only the rudder angle can be changed to tune them.

Hydrofoil theory recongnizes that the keel or centreboard and the rudder act as a hydrofoil meaning that water flow acts on them in the same way as the airflow acts on the sails creating 'lift' as they move through the water at an angle to the forward motion of the sailboat.

Slipping Sideways or Leeway

As a result of their design, sails do not achieve a pure forward force with most of the force being sideways, becoming greater as a boat sails closer to the wind.

This leeway force, to be effective, must be converted into forward speed instead of the boat going sideways or blowing over.

This is achieved somewhat with using a hydrofoil such as centreboard or daggerboard providing leeward resistance. Along with minimizing hull and air resistance the sailboat moves forward with the crew's skills and weight to keep the boat upright.

A centreboard or daggerboard hydrofoil works with the rudder hydrofoil maximizing leeward resistance. The shape and area of the hydrofoil is important to performance, which depends on the length to thickness ratio, the position of maximum thickness and the radius of the leading edge.

Fixed keels can only be tuned by ensuring that the finished surface is smooth so it creates the minimum of drag. Centreboards enable you to adjust the [ amount of board ] that is exposed beneath the boat. This is important because each point of sailing requires different degrees of centreboard that move the centre of lateral resistance (CLR) to combat the leeway force. When sailing close to the wind you need the entire centreboard down, but sailing downwind it is not critical and a small amount helps to balance the sailboat.

Hull and Wind Resistance

Hull and air resistance relate to the speed and course of a sailboat, the wind speed and direction, and water conditions.

Individual layers of water passing beneath the boat cause friction and resistance under the boat and known as skin friction. The solution is to reduce the amount of boat hull in contact with the water, or wetted surface area, to a minimum, by sailboat design and by the crew trimming the boat.

Form Resistance

The shape of the hull or form resistance is usually caused by waves hitting the hull plus turbulence created by imperfect shapes of bows, stern and centreboard or daggerboard and rudder hydrofoil. Form resistance decreases in proportion to the weight of the sailboat, and is the main factor preventing displacement sailboats from accelerating beyond a specific point.

Displacement sailboats create a single wave, from bow to stern, and they cannot escape this, but planing boats break out of that wave and plane over the waves. Planing hulls found on most dinghies allows the hull to plane causing the resistance to decrease, once the sailboat rises off its bow wave, leaving its stern wave behind. This is the fastest form of sailing on a monohull or windsurfer, but the speed of the catamaran is due to the combination of the minimum wetted area on slim hulls with maximum sail area.

Reducing form resistance includes designing and sailing a boat with minimum weight, trimming the boat hull both fore, aft and sideways ensuring the stern does not drag creating turbulence.

Heeling resistance of the sailboat increases form resistance in direct relation to the [ angle of heel ] . Once a dinghy is heeled over, its underwater shape changes and form resistance increases dramatically at the same time its foils cease to function as efficiently. It is therefore efficient to sail dinghies on a flat plane at all times, while yachts should be sailed as upright as conditions allow.

Resistance due to leeway, or induced drag, creates turbulence on the leeward side of the sailboat with form resistance increasing. This is solved by sailing the boat on a flat plane, with the centerboard fully down, and driving the sailboat forwards, which is a specific skill.

Wind Resistance

Hull and rigging wind resistance is most apparent when sailing close to the wind. Counter this by making the outline of a boat as clean as possible by using internal halyards that do not break up airflow round the sail along with minimal rigging and no needless protrusions. Wind resistance caused by the crew is minimal and the need for the crew to sit in a place that trims the sailboat correctly takes priority.

Resistance caused by sailing through rough water is overcome by keeping the sailboat driving and preventing it stalling on waves.

Keeping Upright

Sailboat design requires boats to be sailed virtually upright, particularly dinghies that have flat bottoms maximizing their planing ability. A yacht has a fixed keel weighted or ballasted with lead but the centreboard of a dinghy is not ballasted and relies on crew skill to keep the boat sailing upright.

How problematic this is depends on hull stability and the power of the rig. Sturdy, wide-bottomed sailing dinghies with a modest rig are slow to heel over and easy to control. The ultimate high performance dinghy combines a super light minimum wetted surface area hull with a powerful rig, controlled using maximum leverage from racks and trapezes requiring lightning responses and technique.

Skills used in keeping a dinghy upright require a combination of depowering the sails when necessary and using crew weight to counteract the pull by the sails. This is achieved by using the crew’s centre of gravity to lever against that which pulls against the sailboat's centre of buoyancy, directly related to the wetted surface area. The lever’s power is increased by the crew hanging over the side of the boat (hiking) or standing out using a trapeze.

A dinghy heeling over causes a number of things to happen:

  • The angle of the centreboard hydrofoil results in greater side-slip.
  • The angle of the rudder hydrofoil creates less control.
  • The wetted surface area increases creating extra drag preventing effective planing.
  • The change in the wetted surface outline alters the control characteristics.
  • This causes the sails to become less efficient and drive is reduced.
  • The righting effect of the crew diminishes as their centre of gravity moves towards the centre line of the sailboat.

When a dinghy heels beyond a certain angle, its centre of gravity i.e. the crew, passes over its centre of buoyancy or the wetted surface area. At this instance, the righting moment is transformed into positive heeling moment with the rig weight pulling the sailboat over into a capsize.

The keelboat’s behaviour is different since its centre of gravity is much lower down because of hull weight and ballast keel, and remains lower than the centre of buoyancy which is within the yacht. The righting arm distance between the centre of gravity and centre of buoyancy gets longer when the angle of heel increases meaning the yacht's resistance to heeling increases.

A yacht may be blown flat but there is a minimal heeling moment induced by the sails, and the ballasted keel brings it back upright. There can be exceptions, if a yacht is being held down by a spinnaker or knocked down by waves, the cockpit may fill with water.

Hydrofoil and aerofoil sailing theory and how sail camber effects the centre of effort of the sail along with the angle of incidence and apparent wind. how does a sail work?

headsail tell tail
angle of heel
centerboard center of  lateral resistance
mainsail tell tail
angle of attack
sail air flow
points of sail