Although flight is a recent human achievement, the wind was harnessed (used) for transportation long ago. No exact date is known for the invention of the sail, but an engraving on a 5000 year old Egyptian pot clearly shows a boat with a sail.

Four ways of getting around in the water (not including swimming) are: floating, rowing, sailing and motoring. The first travel on water was simply by floating. Today we see continued use of the old methods. The Tamil people of Sri Lanka simply place a log under an arm to float. In New Zealand the Maoris lash (tie) bundles of reeds together to form a raft. The Sindhi of Pakistan bob (float) while inside pots, while some Iraqis use goatskins filled with air. At one time, standard gear of a Roman soldier included an inflatable skin for river crossings. Some of these ideas were combined to make larger floating devices. Reed platforms buoyed (floating) by inflated skins, were built large enough to carry elephants into battle. Entire villages were carried to a new destination (place) upon a single boat.

Once afloat, humans had to find a way to go against the water currents. Poles, to push along the river bottom, or paddling by hand or wooden paddles were some of the methods used. Later, oars that were pulled in unison (together) were used. Still, it was easier to go down stream with the current. Returning upstream by land was not quicker, but saved a lot of energy.

The invention of the sail showed how humans could use a force of nature (wind) to move their boats, instead of only human muscle. The first sail was probably put on a boat on the Nile river more than 5000 years ago. A simple square sail fastened to a stick near the front of a boat appears in a drawing from that time. Even this primitive (simple) sail must have worked as new designs appeared for the next several hundred years. By 2400 B.C. the sail had become oblong (like a rectangle), and could be seen on very tall masts (poles). This was to catch the winds that came down the Nile over the cliffs. Everywhere else in the Mediterranean the sail was low and square. This type of sail was easier to manage and the Egyptians later changed to the low square sail. Rigging, the means of support and control, developed during this time also. This allowed the sail to be raised or lowered as needed.

Although there continued to be small improvements, the shape of the sail remained the same for nearly 4000 years. The biggest problem with a square sail was that it could only be faced forward with the wind coming from the rear - the boat was pushed by the wind. This changed in the 9th Century A.D. when the lateen or triangular sail was invented, probably by Arab seamen. Hung fore and aft (front and back) of the mast and easily shifted (moved), the lateen sail received wind on either side. The boat was pulled as well as pushed. The inventors of the triangular sail knew their design greatly improved boat speed and responsiveness (easier turns in the water), but they did not understand the aerodynamic principles being applied to the sails. We will now investigate (discuss) these principles to learn how boats manage to sail the wind.

One would think that a boat could only move in the direction that the wind was blowing - that is, downwind. But a triangular sail allows a boat to move toward the wind (windward). To understand how this movement is possible, we first need to identify some of the parts of a sail.

A boat is moved in a windward (toward the wind) direction by using forces that are created on each side of the (triangular) sail. This total force is a combination of a positive (pushing) force on the windward side and a negative (pulling) force on the leeward side (opposite of windward), both acting in the same direction. Though you wouldn't think so, the pulling force is actually the stronger of the two.

In 1738 the scientist Daniel Bernoulli discovered that an increase in airflow velocity (speed) in relation to the surrounding free air stream causes a decrease in pressure where the faster flow occurs. The air flow farther from the sail tends to go straight since it is not influenced by the curved sail. That air flow is referred to as the "free stream". The free stream air and the curve of the sail makes a little channel. This is what happens on the leeward side of the sail - the air speeds up and creates a low pressure area. The air speeds up because of the curve of the sail. The low pressure area created "in front" of the sail pulls the boat along. Meanwhile, on the windward side pressure is increasing and pushing the boat along.

How is this possible since the boat is moving into the wind? This is possible only if the sail is presented to the wind correctly. This is called the angle of attack. Picture a sail pointing straight at the wind. The air will split evenly to each side - the sail sags instead of filling to a curved shape. Therefore there is no "push" or "pull" on the sail or the boat. But if the sail is angled to the wind to just the right degree, the sail suddenly fills and the aerodynamic forces develop.

The angle of attack must be precise (perfect). If the angle is too close to the direction of the wind or is too wide then the sail will not fill.

But, how do these pressures move a boat forward? At sea level air pressure is 2,116 pounds per square foot. When the air flow on the leeward side of the sail is increased, you recall, air pressure decreases. Suppose it decreased by 4 pounds per square foot. Likewise, air pressure on the windward side increases - let's say by 2 pounds per square foot (remember, the pulling pressure is stronger than the pushing pressure). And remember, both pressures are working in the same direction - forward. So now we have a total of 6 pounds of force per square foot (force per unit area is equal to pressure). Multiply that by a 500 square foot sail and we have a total forward force of 3000 pounds on the sail! Of course, there is not equal pressure over the whole sail. The strongest force is where the curve of the sail is the deepest. This is where air flows fastest and pressure drops most. Force weakens as it moves to the rear and separates. The strong forces in the forward part of the sail are also in the most forward direction. In the middle of the sail the force changes to a sideways, or heeling, direction. The rear part of the sail actually causes drag.

As the forward force is increased, by changing the angle of attack, the heeling or sideways force is also increased. Since the angle of attack must always be slightly to the side (and not directly into the wind) the sideways force becomes the strongest. So how does one move forward when the greatest force is to the side? Again, this involves the angle of attack of the sail to the wind, and the resistance of the boat to the water.

As long as the angle of attack gives some forward movement the boat itself moves forward at once. Why? The centerline, or keel, of the boat acts against the water in a manner similar to that of the sail against the wind. The keel produces a force that opposes the heeling force of the sail. The keel keeps the boat going straight.

Suppose you hold a mop perpendicular (straight up) from the floor and push down - there is no movement. But if you angle the mop just a little and push again with the same amount of force, the mop slides easily across the floor. This is what occurs when the sail's angle of attack is altered. The farther the sail is angled from the centerline of the hull, the more the force points forward than to the side. Combine that slight adjustment in forward force with the opposition of water to air, and we have a boat shooting windward because it is now the course of least resistance. Using this combination of principles a boat moving against the wind can go faster than if it were to turn and travel with the wind. Such is the power of aerodynamics: definitely not magic, and hopefully not quite such a mystery.

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Last modified: Mon May 4 19:19:56 PDT 1998

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