How a Sail Works: Basic Aerodynamics

The more you learn about how a sail works, the more you start to really appreciate the fundamental structure and design used for all sailboats. It can be truly fascinating that many years ago, adventurers sailed the oceans and seas with what we consider now to be basic aerodynamic and hydrodynamic theory.

When I first heard the words “aerodynamic and hydrodynamic theory” when being introduced to how a sail works in its most fundamental form, I was a bit intimidated. “Do I need to take a physics 101 course?” However, it turns out it can be explained in very intuitive ways that anyone with a touch of curiosity can learn.

Wherever possible, I’ll include not only intuitive descriptions on the basic aerodynamics of how a sail works, but I’ll also include images to illustrate these points. There are a lot of fascinating facts to learn, so let’s get to it!

Basic Aerodynamic Theory and Sailing

Combining the world of aerodynamics and sailing is a natural move thanks to the combination of wind and sail. We all know that sailboats get their forward motion from wind energy, so it’s no wonder a little bit of understanding of aerodynamics is in order.

Aerodynamics is a field of study focused on the motion of air when it interacts with a solid object. The most common image that comes to mind is wind on an airplane or a car in a wind tunnel. As a matter of fact, the sail on a sailboat acts a bit like a wing under specific points of sail as does the keel underneath a sailboat.

People have been using the fundamentals of aerodynamics to sail around the globe for thousands of years. The ancient Greeks are known to have had at least an intuitive understanding of it an extremely long time ago. However, it wasn’t truly laid out as science until Sir Isaac Newton came along in 1726 with his theory of air resistance.

Fundamental Forces

One of the most important facets to understand when learning about how a sail works under the magnifying glass of aerodynamics is understanding the forces at play. There are four fundamental forces involved in the combination of aerodynamics and a sailboat and those include the lift, drag, thrust, and weight.

From the image above, you can see these forces at play on an airfoil, which is just like a wing on an airplane or similar to a sail on a sailboat. They all have an important role to play in how a sail works when out on the water with a bit of wind about, but the two main aerodynamic forces are lift and drag.

Before we jump into how lift and drag work, let’s take a quick look at thrust and weight since understanding these will give us a better view of the aerodynamics of a sailboat.

As you can imagine, weight is a pretty straight forward force since it’s simply how heavy an object is. The weight of a sailboat makes a huge difference in how it’s able to accelerate when a more powerful wind kicks in as well as when changing directions while tacking or jibing. It’s also the opposing force to lift, which is where the keel comes in mighty handy. More on that later.

The thrust force is a reactionary force as it’s the main result of the combination of all the other forces. This is the force that helps propel a sailboat forward while in the water, which is essentially the acceleration of a sailboat cutting through the water. Combine this forward acceleration with the weight of sailboat and you get Newton’s famous second law of motion F=ma.

Drag and Lift

Now for the more interesting aerodynamic forces at play when looking at how a sail works. As I mentioned before, lift and drag are the two main aerodynamic forces involved in this scientific dance between wind and sail. Just like the image shows, they are perpendicular forces that play crucial roles in getting a sailboat moving along.

If you were to combine the lift and drag force together, you would end up with a force that’s directly trying to tip your sailboat. What the sail is essentially doing is breaking up the force of the wind into two components that serve different purposes. This decomposition of forces is what makes a sailboat a sailboat.

The drag force is the force parallel to the sail, which is essentially the force that’s altering the direction of the wind and pushing the sailboat sideways. The reason drag is occurring in the first place is based on the positioning of the sail to the wind. Since we want our sail to catch the wind, it’s only natural this force will be produced.

The lift force is the force perpendicular to the sail and provides the energy that’s pointed fore the sailboat. Since the lift force is pointing forward, we want to ensure our sailboat is able to use as much of that force to produce forward propulsion. This is exactly the energy our sailboat needs to get moving, so figuring out how to eliminate any other force that impedes it is essential.

Combining the lift and drag forces produces a very strong force that’s exactly perpendicular to the hull of a sailboat. As you might have already experienced while out on a sailing adventure, the sailboat heels (tips) when the wind starts moving, which is exactly this strong perpendicular force produced by the lift and drag.

Now, you may be wondering “Why doesn’t the sailboat get pushed in this new direction due to this new force?” Well, if we only had the hull and sail to work with while out on the water, we’d definitely be out of luck. There’s no question we’d just be pushed to the side and never move forward. However, sailboats have a special trick up their sleeves that help transform that energy to a force pointing forward.

Hydrodynamics: The Role of the Keel

An essential part of any monohull sailboat is a keel, which is the long, heavy object that protrudes from the hull and down to the seabed. Keels can come in many types, but they all serve the same purpose regardless of their shape and size.

Hydrodynamics, or fluid dynamics, is similar to aerodynamics in the sense that it describes the flow of fluids and is often used as a way to model how liquids in motion interact with solid objects. As a matter of fact, one of the most famous math problems that have yet to be solved is exactly addressing this interaction, which is called the Navier-Stokes equations. If you can solve this math problem, the Clay Mathematics Institute will award you with $1 million!

There are a couple of reasons why a sailboat has a keel. A keel converts sideways force on the sailboat by the wind into forward motion and it provides ballast (i.e., keeps the sailboat from tipping). By canceling out the perpendicular force on the sailboat originally caused by the wind hitting the sail, the only significant leftover force produces forward motion.

We talked about how the sideways force makes the sailboat tip to the side. Well, the keep is made out to be a wing-like object that can not only effectively cut through the water below, but also provide enough surface area to resist being moved. For example, if you stick your hand in water and keep it stiff while moving it back and forth in the direction of your palm, your hand is producing a lot of resistance to the water.

sailboat wind vector forces

This resisting force by the keel contributes to eliminating that perpendicular force that’s trying to tip the sailboat as hard as it can. The wind hitting the sail and thus producing that sideways force is being pushed back by this big, heavy object in the water. Since that big, heavy object isn’t easy to push around, a lot of that energy gets canceled out.

When the energy perpendicular to the sailboat is effectively canceled out, the only remaining force is the remnants of the lift force. And since the lift force was pointing parallel to the sailboat as well as the hull, there’s only one way to go: forward!

Once the forward motion starts to occur, the keel starts to act like a wing and helps to stabilize the sailboat as the speed increases. This is when the keel is able to resist the perpendicular force even more, resulting in the sailboat evening out. This is exactly why once you pick up a bit of speed after experiencing a gust, your sailboat will tend to flatten instead of stay tipped over so heavily.

Heeling Over

When you’re on a sailboat and you experience the feeling of the sailboat tipping to either the port or starboard side, that’s called heeling. As your sailboat catches the wind in its sail and works with the keel to produce forward motion, that heeling over will be reduced due to the wing-like nature of the keel.

The combination of the perpendicular force of the wind on the sail and the opposing force by the keel results in these forces canceling out. However, the keel isn’t able to overpower the force by the wind absolutely which results in the sailboat traveling forward with a little tilt, or heel, to it.

Ideally, you want your sailboat to heel as little as possible because this allows your sailboat to cut through the water easier and to transfer more energy forward. This is why you see sailboat racing crews leaning on the side of their sailboat that’s heeled over the most. They’re trying to help the keel by adding even more force against the perpendicular wind force.

By leveling out the sailboat, you’ll be able to move through the water far more efficiently. This means that any work in correcting the heeling of your sailboat beyond the work of the keel needs to be done by you and your crew. Apart from the racing crews that lean intensely on one side of the sailboat, there are other ways to do this as well.

One way to prevent your sailboat from heeling over is to simply move your crew from one side of the sailboat to the other. Just like racing sailors, you’re helping out the keel resist the perpendicular force without having to do any intense harness gymnastics.

A great way to properly keep your sailboat from heeling over is to adjust the sails on your sailboat. Sure, it’s fun to sail around with a little heel because it adds a bit of action to the day, but if you need to contain that action a bit all you need to do is ease out the sails. By easing out the sails, you’re reducing the surface area of the sail acting on the wind and thus reducing the perpendicular wind force. Be sure to ease it out carefully though so as to avoid luffing.

Another great way to reduce heeling on your sailboat is to reef your sails. By reefing your sails, you’re again reducing the surface area of the sails acting on the wind. However, in this case the reduction of surface area doesn’t require altering your current point of sail and instead simply remove surface area altogether. When the winds are high and mighty, and they don’t appear to be letting up, reefing your sails is always a smart move.

How an Airplane Wing Works

We talked a lot about how a sail is a wing-like object, but I always find it important to be able to understand one concept in a number of different ways. Probably the most common example’s of how aerodynamics works is with wings on an airplane. If you can understand how a sail works as well as a wing on an airplane, you’ll be in a small minority of people who truly understand the basic aerodynamic theory.

As I mentioned before, sails on a sailboat are similar to wings on an airplane. When wind streams across a wing, some air travels above the wing and some below. The air that travels above the wing travels a longer distance, which means it has to travel at a higher velocity than the air below resulting in a lower pressure environment. On the other hand, the air that passes below the wing doesn’t have to travel as far as the air on top of the wing, so the air can travel at a lower velocity than the air above resulting in a higher pressure environment.

Now, it’s a fact that high-pressure systems always move toward low-pressure systems since this is a transfer of energy from a higher potential to a lower potential. Think of what happens when you open the bathroom door after taking a hot shower. All that hot air escapes into a cooler environment as fast as possible.

Due to the shape of a wing on an airplane, a pressure differential is created and results in the high pressure wanting to move to the lower pressure. This resulting pressure dynamic forces the wing to move upward causing whatever else is attached to it to rise up as well. This is how airplanes are able to produce lift and raise themselves off the ground.

Now if you look at this in the eyes of a sailboat, the sail is acting in a similar way. Wind is streaming across the sail head on resulting in some air going on the port side and the starboard side of the sail. Whichever side of the sail is puffed out will require the air to travel a bit farther than the interior part of the sail.

This is actually where there’s a slight difference between a wing and a sail since both sides of the sail are equal in length. However, all of the air on the interior doesn’t have to travel the same distance as all of the air on the exterior, which results in the pressure differential we see with wings.

Final Thoughts

We got pretty technical here today, but I hope it was helpful in deepening your understanding of how a sail works as well as how a keel works when it comes to basic aerodynamic and hydrodynamic theory. Having this knowledge is helpful when adjusting your sails and being conscious of the power of the wind on your sailboat. With a better fundamental background in how a sailboat operates and how their interconnected parts work together in terms of basic aerodynamics and hydrodynamics, you’re definitely better fit for cruising out on the water.