Early Theory

When I first learned about boats and sails, I read a number of the simple theories that were eventually upgraded by Mr. Gentry. They certainly seemed to make sense at the time. But while many of them were grounded in hard scientific fact, they overlooked some basic problems that both research and common sense later revealed. For example, while most sailors learn that a sail is shaped like the wing of an plane, and that since planes can fly it must hold true that a sailboat will sail, this line of reasoning ignores one big difference, namely that planes have an engine, and that without it, no matter how hard the wind blows and how much you will it, the plane will never get off the ground. Sailboats, of course, although they have them, don't use their engines when sailing, so how can they move, sometimes at good speeds, using just the wind?

Taking a closer look at the airplane analogy, the common theory behind powered flight is that the air passing over the curved upper surface of an airplane's wing has to travel a longer distance than the air travelling under the flat lower surface, and if the wind has further to travel it must move faster to get to the end at the same time as the wind on the bottom (Figure 15.1). According to the renowned eighteenth-century Swiss scientist, Daniel Bernoulli, for any fluid - and

Air flowing over the top of a wing travels further than wind on the bottom.

To arrive at the end of the wing at the same time means that the air on top must travel faster = more speed = less pressure.

Figure 15.1

The common theory behind powered flight is that the air passing over the curved upper surface of an airplane's wing has to travel a longer distance than the air travelling under the flat ther to travel it must move faster to get lower surface, and if the wind has fur-

to the end at the same time as the wind on the bottom.

DIFFERENT WIND SPEEDS

remember air is a fluid - pressure plus speed is constant. Bernoulli further declared that if speed increases then pressure will drop, and conversely that if speed drops then the pressure will increase. The good news is that Bernoulli was absolutely correct and more than 250 years later his theory remains unchallenged.

Applying the airplane analogy to a boat's sails, the theory held that, like the airplane wing, the air moving over the leeward surface of the sail simply had to travel farther and therefore went faster than the air going across the windward surface of the sail. These differences in speed created a difference in air pressure between the two regions. And since an area of high pressure always tries to move toward an area of low pressure, a resultant force was applied to the windward side of the sail with the result that the boat started to move. Like most sailors, I loved this theory because it was easy to understand. The top of the airplane wing was the same surface as the outside, or leeward side of the sail, and combined with the keel to stop the boat from slipping sideways, it simply moved forward. You can imagine how I felt when it was pointed out that if this theory was true, how then could a plane fly upside down? Rack up one point for common sense.

So how does an airplane fly upside down? As a small child I used to love traveling in our family car. I would sit in the back with my hand out the window and feel the pressure of the wind against my palm. Once my Dad got the old Valiant up to a decent speed, I could ever so slightly tilt my fingers upward, and suddenly the pressure of the wind started to force my hand up. The faster the speed of the car, the less I had to tilt my hand to get it to rise. The same thing happened when I reversed the angle and pointed my fingers down. Suddenly my hand was forced downward. I was actually proving Sir Isaac Newton's third law of motion, which states that for every action or force in nature there is an equal and opposite reaction. This action of deflecting the air down by tipping my hand slightly, was producing an equally forceful action upward. In other words, it was giving my hand lift. The same can be done by offering an angle of attack by any surface, even one curved on one side and flat on the other. With the engine helping, the plane can fly upside down, probably less efficiently, but upside down nonetheless.

If that's the case then why aren't wings just flat boards? If it's the engine that helps a plane fly, why are the wings curved in an aerofoil shape? Good question, the answer to which is that, at least in part, the wing of a plane is curved so that the wind hitting the wing has a more gentle introduction to the surface, which helps keep the flow attached to the wing. If there were hard edges the wind would have a difficult time attaching itself, there would be increased turbulence and the wing wouldn't provide as much lift. And it turns out that the same is true for sails.

Ultimately, the difference comes down to a contrast in power and wind speed. In other words, there are lots of things that airplane wings can do that sails cannot do, simply because of the power supplied by an airplane's engine and the speed of the air flow over the top and bottom of the wing. Made of thin pieces of fabric or membranes, the pressure differentials generated according to the classical theory of sail dynamics would never be enough to carry a sailboat through the water. In order for a sail to function there needs to be something more.

"The same thing happened when I reversed the angle and pointed my fingers down. Suddenly my hand was forced downward. I was actually proving Sir Isaac Newton's third law of motion, which states that for every action or force in nature there is an equal and opposite reaction."

"The result of this circulation is that the air passing along the leeward side of the sail is given a boost in speed while the air on the windward side gets bunched up, making the depth of the foil that much more effective."

Figure 15.2

All air particles have a certain amount of viscosity, or stickiness to them, so while the air particles closest to the solid mass of the sailcloth adhere to the fabric, they in turn have an effect on each subsequent layer of air.

That "something more" is called circulation, and involves the movement of air over the surface of the sail in such a way that it increases the aerodynamic efficiency of the overall foil. Specifically, when a sail is oriented for sailing close-hauled, the wind not only passes over the sail from front to back, but a thin layer of air actually travels in the opposite direction, powered by vortices off the leech, which causes the pressure differential between the windward and leeward sides to be that much greater. The result of this circulation is that the air passing along the leeward side of the sail is given a boost in speed while the air on the windward side gets bunched up, making the depth of the foil that much more effective. In many ways the basic theory is left unchanged, i.e., air speed and pressure differentials on either side of the sail are causing it to create lift. It's just that now we are filling in the extra parts that actually make the theory work.

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