Now that we've nailed down both some basic terms as well as the immediate effects that wind has on a sail, it's time to try to get our heads around the theory behind why a boat sails. First, let's look at a simple representation of what hap-
FLYING A FLAT SURFACE
Pressure forces on either side of a flat surface are the same; there is no lift.
pens to the wind when it is presented with a flat surface (Figure 15.9). This is actually an incomplete representation of what happens in the real world, but it serves as an instructive starting point. In the diagram you will see that the flow lines have been drawn so that, although reversed, they are the same on both sides of the flat surface. Because the flow lines are the same on both sides, the pressure forces on either side will be the same. This being the case, without a pressure gradient there will be no lift, and therefore this particular surface, at least as illustrated, should not be able to fly. A further indication that the flow did not exert any net force on the surface is seen by the direction of the flow after it exits the surface. It is exactly the same as it was before it hit the surface. These lines were drawn with the help of mathematical equations that calculated how they would look relative to the surface. We know the calculations are correct, yet we also know that even flat surfaces do provide some amount of lift. If this is the case, what are we missing in our analysis?
In fact, two key points are missing. On closer examination you will note that the air flowing around the edges of the surface make a pair of sharp turns, and we know that when any fluid makes a sharp turn there is an increase in speed, especially on the outer edge. Think of how when a river makes a sharp bend the water on the outer edge speeds up. Same thing here. The air speeds up as it makes the turn around the leading and trailing edges, in a situation that is analogous to the luff and leech of our sail. In fact, this speeding up, especially at the back end, is the key to the overall theory, but before going into it in full detail, let's go back to the flat surface.
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