Concepts and Definitions

3.1 Introduction

3.2 Theory of Lift

3.3 Tuning Shape to Conditions

3.4 Performance Goals

3.5 Conclusion

3.1 Introduction

What makes a boat go? Downwind, at least, it would appear to be pretty straight forward; but sailing upwind is quite another matter. While perhaps not a miracle, efficient upwind performance- working against the force that drives you - is certainly a marvel of modern design. The forces at work are complex, and they are not entirely understood - at least not by me.

In this chapter we will take a look at the theory of upwind sailing and attempt to create a theoretical framework to guide us in trim. We'll start with some definitions to establish a common language. We will also look at a variety of performance factors and see how they fit into our theory.

Sailboats are sometimes described as airplanes with one wing in the air and the other wing in the water. This is of course a lie. Wings provide the lift for planes, but not the thrust. Yet sails, like wings, are lifting foils. They are shaped to maximize lift and minimize drag for the expected conditions. Like adjusting flaps on a wing, sail shape can be fine tuned to suit particular conditions.

Basic sail shape is described in terms of the amount and position of depth or draft. A sail could be described as having a draft of 10% at 40% for example. This would mean that the draft at the deepest point would be one tenth (10%) the chord length or distance from the luff to leech; and that the deepest point is located 4/10 (40%) from the luff to the leech. Fig.1.

Fig. 1 - Basic sail shape is described by the amount of depth (draft) and the position of the depth along the length (chord).

In addition to basic concepts of draft amount and position, sail shape can be further described in several ways. Smoothness of shape, horizontal shape distribution and exit shape, vertical shape distribution, angle of attack, and twist. The overall dimensions of a sail can be described as the ratio of height to width, or aspect ratio. In a moment we will take a closer look at each of these concepts, and how variations in sail shape effect performance;
but first it is time to discuss the theory of lift (and other lies).

3.2 Theory of Lift

While the existence of lift and related forces are generally recognized (planes fly and boats sail) the theory of how lift is generated remains a point of contention. The old slot affect and venturi models have been debunked,, and replaced with Circulation Theory , which center on satisfying the Kuta Condition and so forth. Without getting into deep theory - which I don't grasp well enough to write about - let's take a look at what we know about sail shape, lift, and performance. We'll start with flow:

Flow

Air flows around a sail (or wing). The air flowing around the outside travels further, and faster, than the air inside. Wait right there: Why must it flow faster just because if flows further?

Why Faster Flow Around the Outside?

Imagine if the air flowing around the outside did not flow faster than that on the inside. As the inside air reached the leech the outside air would not be there yet. A vacuum would form on the outside of the leech. The air on top is then drawn in to fill the vacuum, which accelerates its progress - it flows faster to fill the vacuum. Fig. 2ab.

Incidentally, accelerating the air around the outside is not the only way to fill the vacuum. Air from the inside can double back around the trailing edge to fill void. This happens when the flow around the outside separates from the sail before it reaches the leech. When this happens the sail, (or wing) is stalled. (We see this on mainsails when the leech telltales disappear behind the leech.) Fig. 3.

How Much Further?

So the air does flow faster around the outside because it flows farther - but sails aren't very thick. It doesn't seem far enough farther to make a difference.

In fact the air flowing on the inside of the sail does not follow the exact contour of the sail. The high pressure on the inside creates a cushion, or boundary layer, that the air flows over. In effect, the air cuts the corner - taking a much shorter route. On the outside of the sail there is also a thin boundary layer. The low pressure on the outside pulls the flow to the sail, keeping it attached.

Forces of Lift

The faster moving air exerts less pressure on the sail than the slower moving air. (Bernoulli';s Principle states that a fast moving fluid exerts less pressure than a slow moving fluid). The relatively low pressure on the outside of the sail creates lift perpendicular to the chord of the sail. Fig. 5.

When we put these sail lift forces on a boat we find a large, unwanted, heeling force; and a relatively small forward force. One goal of trim is to improve this mix. Fig. 6.

Main, Jib and Upwash

The combined effect and interaction of the main and jib is a dangerous theoretical frontier. What is known is that the two sails work together to create a combined lifting force greater than the sum either could create alone.

We also know that as the air approaches the sails it is slowed and bent. Since this slowing and bending of the air occurs upstream of the sails it is called upwash. As a consequence of upwash the jib sails in a relative lift and the main in a relative header. This is manifest in the way we trim, since the main is often trimmed to center line while the jib is trimmed ten degrees off center (more or less). This lift makes the jib more efficient; that is, its lifting force is rotated further forward, creating more forward force and less heeling force. While the main sails in a relative header it benefits in that the jib helps shape the flow of air around the main. Thus, although it is trimmed to the center line, air flows all the way to the main leech. Fig. 7.

Fig. 7a - Air approaching the sail plan splits, putting the jib in a relative lift, and the main in a relative header.

Fig. 7b - Think of the sails as elements of a single foil.

The Slot

Not all the air flows outside the jib or inside the main. Some flows through the slot, but not as much as you might imagine. Upwash steers air around the slot. The air which does flow through the slot is slowed as it approaches. It accelerates through the slot and is bent to flow onto the back of the main.

Add it Up

You can even view the main and jib as inside and outside sections of a single foil. The full shape of the foil is filled out by a pressure bubble around which the upwash flows.

No matter how you look at it, when you takethe main and jib together we find a combined force which is predominantly heeling force, with a very small forward force. Fig.8.

Fig. 8 - The total force from the sails can be broken down into a large heeling force and a small forward force.

Keel Lift

Were it not for the boats underbody, particularly the keel (or other foil), the side force would be dominant; and we would not be able to sail upwind. Fortunately, the keel generates lift which nearly offsets the side force of the sails and allows us to sail to weather (with only a few degrees of leeway).
Fig. 9.

But wait a minute, how can the keel generate lift when it is symmetrical? I hear you ask.

Ever see a plane fly upside-down? I reply cleverly.
Fig. 10.

The issue here is angle of attack. While the keel is symmetrical the water does not hit it straight on; due to leeway the water hits the keel from a few degrees to leeward and does not see a symmetrical shape. It sees a foil with a long and a short side; and lift is generated perpendicular to the angle of attack. Fig. 11.

Speed First

In order for the keel to generate lift it must first be moving through the water. You need speed first, before you try to point. Look again at the forces on the boat: Only the keel takes you upwind. The sails push you downwind. The keel will take you upwind when you are moving fast. Speed First.

The Combined Force of Keel and Sails

The combined forces of the keel and sails drive us forward. Note that only a very small fraction of the forces generated are actually translated into forward force. Most of our trimming and fine tuning effort is directed at improving this mix of useful and useless forces. Even a slight improvement in the mix can make a big difference in performance. Every little bit counts alot. Fig. 11.

This small forward force must then fight a tremendous amount of friction (drag) to push the boat through the water. Here again, a very small reduction in friction through better bottom preparation and refined keel shape can result in a significant gain in speed.

Theoretical Conclusion

Most races are won or lost by minutes, or even seconds, over many miles and hours. The margin of victory is the sum of many small things. Every detail is important. Everything shows up in the results.

3.3 Tuning Shape to Conditions

The sailmaker';s goals in designing and building a sail are two; first, to create a fast shape, and second, to create a shape which can be fine tuned to perform well in a variety of conditions.

As sail trimmers our goals parallel those of the sailmaker; first to achieve the designed shape, and then to fine tune to conditions. We must consider each element of sail shape in striving toward these goals.

Draft

The depth, or amount of draft, in a sail controls the power, acceleration, and drag of the sail. More depth creates more power and acceleration; while a flatter sail has less drag and a narrower angle of attack for closer pointing. A deep sail is best to punch through waves and chop, and after tacking. A flat sail will be faster in smooth water. In overpowering conditions a flat sail is also best. Fig. 12.

Fig. 12 - DRAFT. Depth equals power.

A deep or full shape is best for power and acceleration.

Airliners create a deep shape with the flaps down for extra lift at low speeds during takeoff and landing; but pull the flaps in for a flatter shape and less drag for high speed cruising.