How Sailboats Work


A large charter sailboat cruises in the Guadeloupe Islands. See more pictures of sailing.
National Geographic/Getty Images

Sailboats are one of humankind's first and most revolutionary transportation inventions. Powered mainly by the wind, these simple but incredible machines opene­d up new pathways for international trade, exploration and cultural exchange, which shaped the modern world.

Although no one knows when the first sailboat was built, archeologists have found remains of primitive canoe-like vessels dating back to ancient Egypt and Mesopotamia. Since then, boat design has developed steadily to enhance speed, ma­neuverability and cargo load, reflecting unique aesthetic and technological innovations.

­For example, to construct their signature dragon-headed boats, Vikings used axes rather than saws to cut longer, lighter pieces of wood that allowed for faster travel. These longboats, called drakkar, dominated the seas by taking advantage of wind in their square sail for long distances and of oarsmen for swift attacks [source: Hadingham]. Later, 15th-century Chinese junk boats with their distinct scalloped sails were so well-crafted to withstand regional typhoons that they reached the east coast of Africa and the Persian Gulf more than 50 years before European explorers [source: University of Calgary]. Today, specialized racing yachts slice through the water at speeds faster than the wind.

While these amazing ships range in size and capability, all are linked by the fundamental elements of the common sailboat. Whether large or small, vessels of the past and present share the same flotation and movement abilities. In this article, we are ­going to explore how the basic parts of a sailboat work together, how physics principles allow them to float and move and how sailboat design continues to evolve.

­

Basic Parts of a Sailboat

Diagram of a common sailboat
Diagram of a common sailboat

The common sailboat comprises eight essential parts: hull, tiller, rudder, mainsail, mast, boom, jib and keel. The hull is the shell of the boat, which contains all the internal components. Its symmetrical shape balances the sailboat and reduces drag, or the backward pull caused by friction, as it moves in the water. Inside of the hull in the stern, or back of the boat, is the tiller, which is attached to the rudder in the water. Think of the tiller as the boat's steering wheel and the rudder as the tire. To maneuver a sailboat to the right, for example, you pull the tiller to the right side of the boat, causing the rudder to alter its direction.

If you think of the tiller as the steering wheel, then the sails and the keel are the engines. The mainsail is the larger sail that captures the bulk of the wind power necessary to propel the sailboat. Its vertical side attaches to the mast, a long upright pole, and its horizontal side secures to the boom, a long pole parallel to the deck. Sailors can rotate the boom 360 degrees horizontally from the mast to allow the mainsail to harness as much wind as possible. When they pivot the boom perpendicular to the wind, the mainsail puffs outward. Conversely, it goes slack when swung parallel to the wind. This freedom of movement allows sailors to catch the wind at whatever angle it blows. The jib is the smaller, fixed triangular sail that adds additional power for the mainsail. The keel, a long, slim plank that juts out from the bottom of the hull, provides an underwater balancing force that keeps the boat from tipping over. In smaller sailboats, a centerboard or daggerboard serves the same purpose as the keel, but can be raised or lowered into the water to allow for shallow water sailing.

Before a boat can move in the water, it first must be able to float. In the next section, we'll discover how something as heavy as a sailboat can stay afloat.

How Sailboats Float

Floating depends on two things: displacement and density. Archimedes' principle, which explains the concept of buoyancy, states that in order for an object to float, it must displace an amount of water equal to its weight. As a sailboat's weight pushes downward and displaces water beneath it, an upward force equal to that weight holds the boat up.

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Here's where density comes into play. To displace enough water to remain afloat without becoming submerged, a boat must have an average density less than water. For that reason, the hull of the boat is hollow. Whether the boat is made of concrete or fiberglass, its average density is less than water. Think about it: If you put a basketball and a bowling ball in a swimming pool, the air-filled basketball has an average density much less than that of water, so it will float. The solid bowling ball, however, will sink immediately. This is how anything from a small sailboat to an aircraft carrier can manage to stay on top of the water.

Surface area also helps to keep the boat afloat. More surface area gives an object a better chance to displace enough water to offset its own weight. For instance, a small ball of clay likely will sink before it can displace the amount of water equal to its weight. But if you flatten the ball into a thin pancake, there is more surface area to distribute the weight across and displace the water, so it will float. For more information on precisely how a steel ship can float, read how boats made of steel float on water when a bar of steel sinks.

Now that we know how sailboats can float, we can learn how they zip through the water.

How Sailboats Move in the Water

The giant 43-meter schooner Eleonora tacks upwind.
The giant 43-meter schooner Eleonora tacks upwind.

Sailing a boat is simple when you're navigating downwind with the wind at your back. You let out the mainsail perpendicular to the wind to capture the most energy. As the wind presses directly into the sails to make them puff out, that natural force propels the boat forward.

Plotting an upwind course, against the wind, is much harder. Compare the difference between running with the wind behind you and running with the wind gusting at you. You exert more energy to run into it, rather than enjoying the gentle push of it at your back. In fact, it is impossible to sail directly upwind. Either the opposing force of the wind will push the boat backward if the sails are let out, or it will stall the boat if the sails are pulled in and slack. Sailors refer to this as being in irons. Instead, to reach an upwind destination, crews use a method calling tacking.

While the wind pushes the boat when going away from it (downwind) the opposite happens when going toward it (upwind). "When you sail upwind, the boat is actually being pulled rather than pushed by the force of the wind," says Bryan Kelly, sailing instructor at Sail Newport and membership assistant with US Sailing, the national governing body of sailing in the United States. That forward pull is referred to as lift. For that reason, sailors steering upwind must take a zigzagging path called tacking. By doing so, the wind approaches at an angle rather than head-on.

When tacking, the sails act as the engine of the boat, harnessing wind power. However, since the boat is moving angled to the wind, that wind power pushes the boat sideways. But remember that the wind isn't the only element the boat interacts with. There's also the water. As the boat tips to one side, the long, flat keel submerged underneath the hull, pivots upward with the motion of the boat, creating a sideways force in the opposite direction because of the amount of water it displaces as it moves.

When tacking successfully, these equal, opposing sideways forces cancel each other out. However, that collected wind power must go somewhere, so it is released in a forward thrust -- there is nowhere else it can go. This is the same type of effect that happens when you shoot a marble. Your finger and thumb press equally hard on either side of the marble, causing it to zip forward.

After this happens, the sailor would alter course and tack again toward the opposite direction to gradually move upwind.

In the next section, we'll dissect the physics of lift that pull sailboats forward into the wind and what they have in common with kites.

The Physics of Lift

Bernoulli's theorem helps to explain how sailboats can move upwind.
Bernoulli's theorem helps to explain how sailboats can move upwind.

When you see a kite catch the wind and swoop up into the air, you're witnessing lift. You can feel the forward acceleration in the pull on your end of the string. Likewise, the mainsail and jib harness wind energy with their aerodynamic shapes that puff out on one side when the wind hits them. You also might notice that the kite flies angled to the wind, just as the mainsail and jib capture wind when tacking.

Sailing aficionados use two prominent -- yet often disputed -- theories to explain how exactly the wind interaction generates lift: Bernoulli's theorem and Newton's Third Law.

Bernoulli's theorem, also called the Longer Path Explanation, explains lift in terms of high and low air pressures on either side of the sail. Imagine the front of the boat angled upwind, or into the wind. As the breeze hits the sails, the air particles rush over both sides. Theoretically, the air particles moving across the outer, convex side of the sail have a longer distance to travel in the same amount of time as the particles moving across the inner, concave side.

If the particles on the outer side are traveling farther in the same amount of time, they must have a higher velocity, or speed, than the particles on the other side. These higher-velocity particles have more room to spread out, forming a low-pressure area. On the inside of the sail, the slower air particles are packed together more densely, creating a higher-pressure area. This difference in the pressure on the sails acts as a forward suction, producing lift.

Lift also applies to airplane flight. For a more detailed explanation of lift and the Bernoulli and Newtonian theories, read How Airplanes Work.

Newton's Third Law at work on a sailboat

Newton's Third Law describes lift in terms of the reaction of the wind's air particles to the mainsail and jib. The law states that every action has an equal and opposite reaction. As the wind hits the sails from an opposing direction (remember, you're sailing upwind to tack), it generates drag, or backward pull. Drag is parallel to the original wind current [source: Glenn Research Center] and occurs naturally when something is moving against a fluid or gas. Swimmers wear specialized suits and caps to reduce drag as much as possible in the water.

Examining lift through the Newtonian lens, the air particles' movement creates an equal, opposite reaction -- or forward pull. It can also be applied to the interaction of the sails and the keel, described in the previous section. The sails and the keel create equal and opposite reactions to focus the boat's energy forward rather than sideways.

Now we'll examine the keel more in depth to see how it contributes to lift and keeps the boat from tipping over when tacking.­

A Closer Look at the Sailboat Keel

The long, white board on the right-hand side of the picture is the keel. Normally you don't see the keel since it's underwater.
The long, white board on the right-hand side of the picture is the keel. Normally you don't see the keel since it's underwater.

The keel has two main functions: to keep the boat from being blown sideways in the wind (lateral resistance) and to hold the ballast. The ballast is a weight traditionally at the bottom of the keel that keeps the boat right-side up.

When the sails interact with the wind, a lot is also happening underwater to help create lift and allow the craft to recover from tacking. When a boat heels, or tips sideways in one direction when tacking, the ballast prevents it from going completely over. Positioned beneath the sailboat toward the center of the hull's underbelly, the keel's broad, flat surface creates sideways force by displacing water in the opposite direction that the boat is tipping. Although the keel has a much smaller surface area than the sails, the density of the water allows it to initiate a force strong enough to cancel out the heeling motion. That resulting equilibrium is called the righting moment.

You're probably familiar with the strong force of the keel if you've used a canoe paddle to change a canoe's direction. Although the paddle has a relatively small surface area, when turned against the current, you can feel the strength of its resistance as it becomes harder to hold.

Given this delicate balance among the wind, water and boat, sailors must tack carefully to avoid capsizing the boat, monitoring the angle at which they tack. If they tack the boat at too tight of an angle, the force of the wind will be too great for the keel and the water to overcome.

The maximum angle that a boat can tack and recover from is 30 degrees [source: US Sailing]. Sailors can tell the angle at which they approach the wind thanks to telltales, or strands of yarn-like material attached to the mainsail. Depending on how they blow when the sails are pulled tightly, they reveal the angle to the wind. Ideally, they will blow straight out, indicating even airflow across the sails and the optimum tacking angle. This is referred to as banging the corners, or sailing efficiently.

Read on to learn just how fast these wind machines move.

Sailboat Speed

Fast Frenchmen and world record holder for nautical speed Alain Thebault talks about his 'flying boat' Hydroptere. He plans to build a maxi-hydroptere capable of sailing at 50 knots (around 58 mph).
Fast Frenchmen and world record holder for nautical speed Alain Thebault talks about his 'flying boat' Hydroptere. He plans to build a maxi-hydroptere capable of sailing at 50 knots (around 58 mph).

Now that we understand how the boat moves, let's get to the good stuff: speed. A vessel's top speed will vary, depending on its size and purpose. For instance, sleek racing sailboats are designed specifically to maximize speed, but larger, bulkier ships will plod along more slowly due to drag and friction.

The nautical measurement of speed is the knot. One knot is equal to about 1.15 mph. According to the World Sailing Speed Council, the current record holder for the fastest nautical mile is Alain Thebault from France with a speed of 41.69 knots -- or about 48 mph. Given that the wind was Thebault's sole power source, that's pretty fast! The knot dimension is based on the circumference of the Earth. You can read more about how nautical miles compare to miles and kilometers by reading What is a nautical mile?.

Since Thebault was going 48 mph, you may be wondering if that means the wind was blowing that fast. Probably not. Thebault likely was moving faster than the wind because when sailboats create lift, as we discussed earlier, they create their own additional wind, called apparent wind.

It's important to understand that there are two types of wind at work when sailing: true wind and apparent wind. You feel true wind when you're standing on the dock or if the boat anchors. This wind makes the waves in the water. Apparent wind is what you feel while the ship's moving -- a combination of the true wind and the wind that the boat's motion creates. This is the wind that powers the ship.

So it is possible for some boats to beat the wind, particularly slimmer, more aerodynamic models that have less drag or friction in the water, such as yachts and catamarans. But remember, to sail faster than the wind, these types of boats must travel at an angle to it, rather than straight downwind or upwind, to stimulate lift and accelerate apparent wind.

As sailing technology advances, boats are becoming faster and more efficient than ever before.

Modern Sailboat Design

A hydrofoil boat
A hydrofoil boat

Modern sailboat design has evolved overall to create a wider, yet sleeker, structure. While the width increases stability, other basic components have been refined to improve sailing efficiency.

Keels are often longer and leaner, allowing for a closer, quicker tack. Their larger surface area displaces more water for a tighter tack. The hydrodynamic, slender design cuts through the water with less drag. Likewise, designers have honed the hull to reduce pull in the water. New materials such as fiberglass have replaced wood, reducing weight and increasing speed capability. To improve efficiency, sailcloth is made of synthetic materials instead of cotton to better retain form against the wind and use that natural energy. Sail shape also has been continually fine-tuned to harness wind.

Due to such innovations, some sailboats, as previously noted, can actually exceed the speed of the wind. Hydrofoils, which elevate the hull above the water, drastically reduce friction between the boat and the water to maximize speed. In short, hydrofoils act like airplane wings on the bottom of boats [source: Getchall]. They have four appendages that resemble water skis with a rounded top attached to each corner of the boat. When stationary, the hydrofoils remain underwater. But as the boat speeds up, they create lift that eventually raises the hull completely above water, leaving only the lighter aerodynamic foils in the water. The same lift principles discussed earlier with the wind and the sails apply as the water rushes over the curved hydrofoils to create the same effect.

An ice boater zips across Ghost Lake, Alberta.
Todd Korol/Getty Images

In addition, specialized iceboats that sail on top of ice can travel up to three times faster than the speed of the wind. A Blade Runner iceboat, for instance, can move as fast as 75 mph [source: Blade Runner Ice Boat Company]. Like a boat on ice skates with a simple hull and sails, iceboats have very little drag as they glide across frozen bodies of water. Not surprisingly, they're especially popular in colder climates such as New England, Canada and Russia.

As the evolution continues as it has for centuries, sailboats will undoubtedly hold their integral place in human life, whether for utilitarian purposes or pure windblown pleasure.

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More Great Links

Sources

  • Boat Safe Kids. (Feb. 12, 2008) http://www.boatsafe.com/kids/033199kidsques.htm
  • Cox, David. "The Sailing Handbook." Stackpole Books. 1999.
  • Getchall, David R. "The Outboard Boater's Handbook: Advanced Seamanship and Practical Skills." McGraw Hill. 1994. (Feb. 12, 2008) http://books.google.com/books?id=YpMTd7-Mb3sC&pg=PA103&lpg=PA103&dq=hydrofoils+how+do+they+work&source=web&ots=wPcwsdSncw&sig=-Pvjyd6fSF7EHNJrZ2ZJmKSrU84#PPA104,M1
  • Glenn Research Center. "Bernoulli and Newton." (Feb. 12, 2008) http://www.grc.nasa.gov/www/K-12/airplane/bernnew.html
  • Hadingham, Evan. "Secrets of Norse Ships." NOVA Online. (Feb. 12, 2008) http://www.pbs.org/wgbh/nova/vikings/ships.html
  • Hodanbosi, Carol. "Buoyancy: Archimedes Principle." August 1996. Glenn Research Center.
  • Hyperphysics. "Bernoulli Principle." (Feb. 12, 2008) http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html
  • Kelley, Bryan. Personal interview. Conducted Dec. 14, 2007.
  • Lawler, Andrew. "Report of Oldest Boat Hints at Early Trade Routes." Science Magazine. June 7, 2002.
  • MacMillan, Douglas. "The Making of the $30 million sailboat." BusinessWeek Online. Oct. 17, 2006.
  • Naval Historical Center. "Nautical Terms and Phrases…Their Meaning and Origin." May 5, 2005. (Feb. 12, 2008) http://www.history.navy.mil/trivia/trivia03.htm
  • Pickthall, Barry. "The Color Guide to Sailing." London, Castle & Co. 1980.
  • Schryver, Doug. "Sailing School." McDonald/Queen Anne Press.1987.
  • University of Calgary. "European Voyages of Exploration: Asia." (Feb. 12, 2008) http://www.ucalgary.ca/applied_history/tutor/eurvoya/asia.html
  • US Sailing. "Points of Sail." (Feb. 12, 2008) http://www.smallboat.sailingcourse.com/points_of_sail.htm
  • Vawter, Richard. "Floating Diet Coke QT Movie." Western Washington University Physics Department. (Feb. 12, 2008) http://www.ac.wwu.edu/~vawter/PhysicsNet/QTMovies/PressureFluids/FloatingDietCokeMain.html
  • Wolf, Joe. "The Physics of Sailing." University of New South Wales. (Feb. 12, 2008) http://www.physclips.unsw.edu.au/jw/sailing.html
  • World Sailing Speed Record Council. "Nautical Mile Records." (Feb. 12, 2008)http://www.sailspeedrecords.com/