Sailing the Wind page 1
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Windsurfer

Although flight is a recent human achievement, the wind was harnessed for transportation long ago. The exact date is uncertain, and there is no historic person to credit, but an engraving on an Egyptian pot more than 5000 years ago clearly illustrates a major invention: the sail. Navigation of the water has four basic components that were more or less developed one after another: floating, rowing, sailing, and motoring. The first travel on water was simply by floating, and in different parts of the world today we see continued use of ancient methods. The Tamil people of Sri Lanka simply place a log under an arm to achieve flotation; in New Zealand the Maoris lash bundles of reeds together to form a raft; the Sindhi of Pakistan bob 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. These various flotation devices were gradually combined and refined to accommodate groups rather than individuals. Reed platforms, buoyed by inflated skins, were built large enough to transport elephants into battle. Entire villages were carried to a new destination upon a single boat.

Once afloat, humans developed means to use or resist the currents of the water. Poles were utilized to push against the river bottom. In deeper streams, paddling by hand gave way to wooden paddles, then oars that were pulled in unison. But rowing was certainly more pleasant with the current than against it. In fact, many early water travelers preferred a one way voyage - the ease of return by land was worth the extra time.

The invention of the sail, and its many refinements, is another great example of human observation and employment of natural phenomena. By harnessing the power of the wind, people were able to travel with greater agility and for longer distances than ever before. It is also interesting to note how the materials and designs of sailing ships varied according to the characteristics of the local body of water, wind patterns, and surrounding environment. Archaeologists have discovered much evidence around the Mediterranean sea, including the Nile, Tigris and Euphrates rivers, that documents the development of the sail from both native experience and cultural encounters.

The first sail unfurled on the Nile at least as early as 3100 B.C. From the drawing that survives from that date, the sail appears to be no more than a square patch of material fastened to a stick near the front of the boat. The artistic rendition may have been oversimplified, but it's plausible to imagine the first sail as indeed being rather primitive. The importance of this contraption was immediately recognized, however, and modifications continued throughout the Egyptian dynasties. By 2400 B.C the sail had become oblong and rose on a tall mast, allowing it to catch the breezes that flow between the many cliffs along the Nile. Everywhere else in the Mediterranean the sail was low and square, and eventually due to ease of management the Egyptian design conformed. Rigging, the means of support and control, consisted of several lines that were either fixed in place to strengthen the mast from which the sail was hung, or movable so that the sail could be raised, lowered or otherwise positioned.

Although refinements continued, the shape of a sail remained the same for nearly 4000 years. Its biggest aerodynamic drawback was that, although it could be angled to achieve maximum thrust, the square sail received air only from the rear - the boat was pushed by the wind. Sailing was revolutionized in the 9th century A.D. by the lateen or triangular sail, probably invented by Arab seamen. Hung fore and aft (front and back) of the mast and easily shifted, the lateen sail received wind on either side - in effect, the boat was pulled as well as pushed. The inventors of the triangular sail recognized that their design greatly improved boat speed and responsiveness, but they did not understand the aerodynamic principles which were being applied. We will investigate these principles now, and in the process learn more about how airplanes as well as boats manage to sail the wind.

Sailboat

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 accomplished, we first need to identify some of the parts of a sail.

The leading edge of a sail is called the luff; it's positioned at the front, or fore, of the boat. The trailing edge at the back, or aft, is called the leech. An imaginary horizontal line from luff to leech is called the chord. The amount of curvature in a sail is called the draft, and the perpendicular measurement from the chord to the point of maximum draft is called chord depth. The side of the sail that the air fills to create a concave curve is called the windward side. The side that is blown outward to create a convex shape is called the leeward side. We'll return to these terms as we proceed.

A boat is moved in a windward direction by using forces that are created on each side of the 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, 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 air flow velocity in relation to the surrounding free air stream causes a decrease in pressure where the faster flow occurs. This is what happens on the leeward side of the sail - the air speeds up and creates a low pressure area behind the sail. Why does the air speed up? Air, like water, is a fluid. When the wind meets and is divided by the sail, some of it sticks to the convex (leeward) side and hitches a ride. In order for the "unstuck" air just above it to move past the sail, it has to bend outward toward the flow of air unaffected by the sail. But this free air stream tends to maintain its straight flow and acts as a kind of barrier. The combination of the free air stream and the curve of the sail creates a narrow channel through which the initial volume of air has to travel. Since it can't compress itself, this air has to speed up to squeeze through the channel. This is why the velocity of flow increases on the convex side of the sail.

Once this happens, Bernoulli's theory takes effect. The increased air flow in the narrow channel is faster than the surrounding air, and the pressure decreases in this faster flowing area. This creates a chain reaction. As new air approaches the leading edge of the sail and splits, more of it flows to the leeward side - air flow is attracted to low pressure areas and repulsed by high pressure areas. Now an even larger mass of air must travel faster to squeeze through the channel caused by the convex sail and the free air flow, causing an even lower air pressure. This continues to build until the maximum speed is achieved for the existing wind condition, and a maximum low pressure area is created on the lee side. Note that the air flow increases only until it reaches the deepest point of the curved shape (the chord depth). Up to this point the air is converging and speeding up. Beyond this point the air diverges and slows down until is again the speed of the surrounding air.

In the meantime, just the opposite is happening on the windward side of the sail. As more air travels to the leeward side there is less air on the windward side to travel through the expanded space between the concave side of the sail and the free air stream. As this air flow spreads out it slows down to a speed less than the surrounding air, creating an increase in pressure.

Now that we know about these potential forces, how do we actually develop them in order to move our boat? We need to create an ideal relationship between the sail and the wind that will allow the wind to both speed up and flow along the convex curve of the sail. One part of this relationship between sail and wind 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, the air does not speed up to form a low pressure area on the lee side, and there is no movement of 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 very precise. If the angle remains too close to the wind the front of the sail "luffs" or flaps. If it's angled too wide the flow lines along the curve of the sail detach and rejoin the surrounding air. This separation creates a "stall zone" of whirling air that causes a decrease in speed and an increase in pressure. Because a sail's curvature will always cause the aft end of the sail to be at a greater angle to the wind than the leading edge, the air at the leech is unable to follow the curve and returns its direction to that of the surrounding free air. Ideally, separation shouldn't start until the airflow reaches the leech. But as a sail's angle of attack widens, this point of separation gradually moves forward and leaves everything behind it a stall zone.

You can see that, along with having the correct angle of attack to allow air to pass smoothly onto it, the other important factor in the wind to sail relationship is that the sail must have the correct curvature so the air attaches all the way aft. If the curve is too slight the air flow will not bend out, and there will be no squeezing effect that increases the speed. If the curve is too deep the flow cannot remain attached. Therefore, separation can occur from too much curvature as well as from too wide an angle of attack.

So now we know how pressures on the sail are developed in theory and in practice. But how do these pressures move a boat forward? Let's take a closer look, which we can do because air pressure can be measured rather precisely.

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 that air pressure decreases. Suppose it decreases 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 even though the leeward pressure is negative and the windward is positive, they both work in the same direction. So now we have a total of 6 pounds per square foot. Multiply that by a 500 square foot sail and we've created a total force of 3000 pounds on the sail.

Each point of the sail has different pressures working on it. The strongest force is at chord depth, 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 direction of these forces changes also. At every point in the sail the force is perpendicular to the sail's surface. 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. In the rear part of the sail the force grows still weaker as wind speed decreases, and causes backward or drag direction.

Each force on a sail can be calculated to determine the relative strength of its forward, heel and drag components on either side. Since the forward forces are also the strongest, the total force acting upon the sail is in a slightly forward, but mostly sideways, direction. Increasing the power of a sail to gain more forward drive also results in a much greater increase in the heeling force. So how does one move forward into the wind when the greatest force is to the side? This involves the angle of attack of the sail to the wind, and the resistance of the boat to the other fluid involved here: water.

The direction of the total force is nearly perpendicular to the sail's chord. When a sail's chord is parallel to the boat's centerline, the main force is almost completely to the side. But if the sail is angled a bit so the sail force is in a slightly more forward direction, 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 - it keeps the boat from simply going in the direction of the sail force. And although total sail force is always to the side when sailing into the wind, a proper angle of attack will move the boat forward.

Suppose you hold a mop perpendicular to 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 rather 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. Add to this irony the fact that 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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