## Basic Concepts

If you are scratching your head right now, don't worry. Much of this theory is counterintuitive - again, its pretty controversial stuff - and involves some fairly esoteric aerodynamic theory. Still, if we break things down into separate parts, they are much easier to understand. There are also a number of ancillary points we need to come to terms with, including such concepts as lift, drag, boundary layers, attached flow, separation, pressure gradients, and angle of attack. Note that in the beginning we will be talking about headsails alone since, without the presence of a mast along the leading edge, they present a much simpler picture aerodynamically. Once we get the notion of how jibs and other simple foils work, we will move on to the mainsail, as well as how it is that the jib and main work together to create a situation in which the sum is substantially greater than its constituent parts.

### The Boundary Layer

This concept is central to modern sail theory and basically states that the surface of the sail, no matter how smooth, presents an area of friction to the air passing over it, a so-called boundary layer of slower-moving air around the sail. All air particles have a certain amount of viscosity, or stickiness to them, so

Air particles closest to the sail surface are slowed down by the surface. Each successive layer feels the effect.

These particles are far enough away from the foil to not feel any of its effect.

These particles are far enough away from the foil to not feel any of its effect.

BOUNDARY LAYER

### Figure 15.3

The influence of the fabric is diminished the further you get from the sail, until it has no effect on the passing particles and they move at the same speed as the rest of the surrounding air.

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 (Figure 15.2). This thin layer moves at the same speed as the surface to which it is attached, in other words the attached air has zero speed with respect to the airfoil. The influence of the fabric is diminished the further you get from the sail, until it has no effect on the passing particles and they move at the same speed as the rest of the surrounding air (Figure 15.3).