Lift and Drag

Any time wind passes over an object – be it an airplane, a sailboat, a golf ball, a baseball, a rock, or even a person – the wind exerts aerodynamic forces on that object. Without delving too deeply into the physics of fluid dynamics, the forces created by wind passing over an object are due to acceleration of the wind air mass.

Essentially, when moving air molecules bump into or pass over an object in their path, they change direction or speed in some way. This change in direction or speed – called acceleration – of the air mass creates a force. You may be familiar with Newton’s 2^nd law of motion, simply stated as force equals mass times acceleration.

Some objects, such as parachutes, tend to primarily slow down or decelerate the wind air mass, while others, such as an airplane wing tend to deflect the air, changing its direction. Sails can act like parachutes or airplane wings, depending on their configuration.

The aerodynamic reaction force is most easily thought of as the sum of two components. Lift is the component of force that acts perpendicular or normal to the direction of the wind. Drag is the force component that acts parallel to the wind. Using the previous examples, airplane wing forces are dominated by lift, while parachutes create mostly drag.

Lift and Drag Forces

+In the context of sailing, the size of the total wind force on a sail and the relative contributions of lift and drag are dependent on:

1. The apparent wind
2. The angle of attack of the sail
3. The size and shape of the sail

The sailor determines the apparent wind felt by the boat (or at least its direction) by steering the boat.

The sailor can also control the sail angle of attack, the shape of the sail, and even the size of the sail that is exposed to the wind (limited by the full physical size of the sail).

Sail Angle of Attack

The angle of attack is simply a physics term that describes the angle between the chord of the sail and the apparent wind direction. A smaller angle of attack means the sail is aligned more parallel to the wind, while a sail with a large angle of attack is at more perpendicular to the wind.

The figure above illustrates how lift and drag contribute to the total wind force for different sailing directions. Notice that for a small angle of attack (the leftmost illustration), the total wind force is dominated by lift, while for a large angle of attack (right illustration), drag dominates. Also notice that when the boat is aligned nearly perpendicular to the wind (middle illustration), the sail angle – and the relative distribution of lift and drag – is nearly identical to that of the boat sailing more into the wind.

These concepts are fundamental to sailing. When sailing in the region from a direction pointing into the wind down to a point about perpendicular to the wind, the sail acts primarily as a lift generator, much like an airplane wing or airfoil. When sailing away from the wind, the sail harnesses drag forces, much like a parachute. In fact a spinnaker – which only functions on downwind tacks – is often referred to as “the chute.”

Sail Area

Obviously, the size of a sail also affects the total aerodynamic force, but fortunately, the effect is fairly simple and intuitive. All else being equal, larger sail produces more force than a smaller sail.

The size of the sail area exposed to the wind can be controlled by either of the following:

1. Changing the actual sail, i.e., replacing the jib with a smaller jib or a genoa
2. Reducing the amount of the sail that is exposed to the wind, achieved by either reefing the mainsail or furling the headsail

Sail Shape

The shape of the sail also effects the airflow around a sail and thereby the efficiency with which the sail captures wind forces. By shape, we mean the curvature of the sail in terms of:

1. The depth or draft of the sail
2. The twist of the sail

The aerodynamic effects of draft and twist very complicated, and the most efficient settings cannot be broadly defined. The simplest way to determine the best sail shape is to use feel or telltales – as we’ll discuss later. In general however, there are a couple rules of thumb that can be used:

• Twist is best kept to a minimum.
• A flatter sail is better in stronger winds and while close hauled.

The shape of a sail is controlled primarily by the angle at which the sheet pulls on the sail. This angle is determined by the traveler for the mainsail and by the position of the fairlead for the foresail. Improperly set halyards, clew outhaul, topping lift or Cunningham can also lead to excessive draft or twist of the sail, but these are not typically changed while under sail to control sail shape.

Multiple Sails

So far, our discussion has only considered a single sail, but most cruisers have two sails, a mainsail and a foresail. The addition of the second sail complicates the airflow somewhat, but the basic theory for understanding wind forces are the same.

Practically speaking, when setting the sails, the only thing that needs to be kept in mind is that the foresail should not block wind from the mainsail. Interference from the foresail can occur if it is pulled too close to the mainsail on a close hauled tack. This can be corrected by loosening the foresail sheet slightly.

Forward and Side Forces

Lift and drag are useful for understanding the mechanisms of how wind forces are created, but to understand their effects on a boat, it helps to consider wind forces differently. Total wind force can also be separated into the component acting in line with the boat’s heading and that acting perpendicular.

The driving force is the component of total wind force that acts parallel to the boat’s forward motion, while the side force acts normal to the boat’s forward motion, as shown in the figure below. Notice that the total force for each sailing direction is the same between the lift/drag figure and the side/driving force figure, the only difference being the orientation into which the total force is decomposed.

Driving and Side Forces

When sailing into the wind, only a small portion of the total force is harnessed as driving force. On the heading that is near perpendicular to the wind, although the total wind force is about the same, a much larger portion of it acts as along the boat heading. On the downwind tack, almost all of the wind force is captured as driving force and the side force is nearly negligible.

When the sail is operating primarily in lift, for the upwind and perpendicular headings, the total force is largest, but it is most efficiently captured on the perpendicular heading. On the downwind heading, the total wind force is even more efficiently captured as driving force. However, since the sail is now acting primarily in drag, the total force is smaller.

These two factors – the magnitude of the total force and its alignment with the boat heading – combine to affect the driving force. As it turns out, the heading with the largest driving force is the one that is nearly perpendicular to the wind.

Stability

The driving force component of the total wind force is the desirable quantity that propels the boat, but the side force is parasitical, causing the boat to heel. The figure below illustrates this effect by viewing the side forces previously discussed in the vertical plane along the boat’s beam.

As the boat travels through the water, the keel (or centerboard and rudder) generates lift and drag forces just like the sail. The side component of the total keel force reacts the side sail force equal in magnitude and opposite and direction (otherwise the boat would move sideways). These two forces act to rotate the boat (clockwise in this particular illustration) and are known as heeling forces.

Heeling forces are offset by righting forces – acting to rotate the boat in the opposite direction – including the hull buoyancy, the crew weight, and the keel weight.

As the boat heels, the size and location of buoyancy force changes. Buoyancy is proportional to the volume of water displaced by the hull and acts at the center of this volume. As the boats orientation changes, so does this volume of displaced water, which can be seen in the figure below as the portion of the hull that is beneath the waterline. Notice how it changes with the heeling of the boat.

Heeling and Righting Forces

Heeling also causes the location of the keel weight (with respect to the center of rotation of the boat) to change. When the boat is flat, the keel weight acts in line with the rotation point and does not produce a moment. As the boat heels, the keel weight begins to produce a righting moment that becomes larger as the heeling angle increases.

The crew weight – or in reality the weight of any moveable object aboard the boat – can be adjusted to change its distance relative to the rotation point and thereby effect the size of the righting moment. More generally, the weight of the crew or other onboard objects is called ballast. To prevent or limit heeling, ballast is moved to the windward side of the boat, a.k.a. topside.

For each sailing direction and distribution of heeling and righting forces there is some heeling angle where an equilibrium is reached and the boat stops rotating. If not, the boat continues heel over until it is knocked down or capsized.

The illustration shows the equilibrium for each of the three heading discussed in previous sections. As the side force becomes smaller, the heeling angle becomes smaller until a heading downwind where the side force is so small that the entire heeling moment can be canceled by only a small ballast or a minimal displacement of the keel weight.

Notice that by far the greatest righting force comes from keel weight. Imagine what would happen if the keel weight were removed. In that case – which is the configuration of most small dinghies and racing boats – the righting moment now depends primarily on the location of the crew. In order to offset the heeling moment, the crew must increase its distance from the rotation point, often beyond the boundary of boat. This process – which you might have seen in pictures of small racing boats where the crew hangs off the side of the boat using straps and ropes – is known as hiking out. As you can imagine, even the smallest changes in the location of the crew have a large effect on the balance of forces, the boat is much less stable, and capsizing is a common occurrence.

While heeling is arguably the most exhilarating aspect of sailing, it can be dangerous in high winds, and contrary to common belief, it reduces boat speed, all else being equal. Heeling changes the shape of the hull that is underwater, usually increasing the drag and thereby slowing the boat down. However, unless the boat has a very large base, such as a catamaran or trimaran, heeling is inevitable and the resulting speed reduction should not be given much thought.