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Performance Racing Trim
by © Bill Gladstone

 

Chapter 3 - Introduction to Trim

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.

Stall

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.

Draft Position

Generally, the goal is to maintain the designed draft position (about 40%-45% in mains, 30-40% in jibs) to keep a smooth, even shape. A draft forward sail will be more forgiving steering in waves, and will create less drag; a draft aft sail will be better for pointing, but is a higher drag shape.

 

Horizontal Shape

There is more to sail shape then depth and position of draft. Horizontal Shape describes the shape from luff to leech. Most sails are designed with a fair, even curve to promote attached flow. Large overlap genoas are cut flatter in the aft sections where they overlap the main to allow for close sheeting without interfering with the main. Main sails are shaped evenly from luff to leech. Spinnakers are shaped round at the edges and flatter across the middle. Some sails have a kinked shape, though not by design. Fig. 14.

Vertical Shape Distribution

Sail shape varies vertically as well as horizontally. Different wind characteristics and sail dimensions necessitate a slightly deeper shape aloft than alow. This fact is counter to the intuitive response, which suggests more shape down low for less heeling moment. We want more shape aloft for three reasons: Fig. 15.

Fig. 15 - In light and moderate winds sails are trimmed to be deeper aloft than alow. In heavy air the top of the sail is flattened to reduce heeling forces.

There is stronger wind up high. More draft aloft helps pull extra power from this fresh breeze.

The short chord length necessitates a more powerful shape, to get all the available power over the short distance. The short chord length has a better lift/drag ratio, so the extra shape gives extra power but does not create excessive drag.

A deeper shape aloft keeps air from escaping up the sail in a path perpendicular to the leech. The air is forced into a more nearly horizontal path. The air stays on the sail longer for extra power, and there is less tip vortex as well.

In heavy air heeling moment does become a factor, and a flatter shape up high is desirable. In fact, much of our sail shaping effort is devoted to flattening (and spilling) the upper part of our sails as the wind speed builds.

Twist

Twist is the relative trim of the sail high and low. A sail has lots of twist when the upper part of the sail is open. The opposite is a closed leech, with little twist. Fig.16.

Fig. 16 - Twist is the difference in trim of the sail high and low. Sails are designed with some twist to match differences in wind high and low. We fine tune twist to match sailing conditions and performance goals.

Boat A has closed leeches, with little twist.

Boat B has open leeches, or lots of twist.

The stronger winds aloft necessitate some twist. The stronger wind up high creates a more open apparent wind angle aloft. The upper part of the sail is twisted out relative to the lower part of the sail to match the more open apparent wind angle. The sailmaker designs twist into the sail.

Twist can be fine tuned to match sail shape to the prevailing wind and sea conditions, and to match our performance goals.

Fine tuning twist is one the most powerful trim adjustments we can make. We';ll offer a few generalizations here; details will be covered in upcoming upwind trim chapters.

We increase twist by easing the sheets, and reduce twist by trimming. Generally, less twist will provide better pointing, more twist is preferred for speed and acceleration. For example, coming out of a tack sails are trimmed with extra twist, with final trim coming only as the boat accelerates to full speed.

In overpowering conditions power can be reduced by easing the sheets and increasing twist - spilling the top of the sail, or by flattening the sail shape. Either way, you reduce power. Which is preferred? Generally, in wavy conditions it is preferred to use twist to control power. In smooth water conditions reducing power through flatter sail shapes is preferred. One of the challenges of trimming is achieving the correct total power, and achieving the correct mix of power - the correct mix of shape and twist.

One final generalization: The shape and twist of the main and jib should be matched.

Mains and Genoas

Designed shapes in genoas and mainsails have a number of differences, some of which have already been mentioned.

Genoas are generally deeper than mainsails, and more difficult to adjust. When one genoa is overpowered we change to another. Large overlap genoas are designed with a flat or straight shape aft. This exit shape allows closer sheeting without clogging the slot or interfering with the main, and it also reduces drag. Consequently most of the shape in genoas is built in the forward section, and our trim efforts are concentrated in tuning this forward shape.

Mainsails, on the other hand, carry shape throughout. The turbulence from the mast reduces efficiency in the forward part of the sail, so we focus on the leech to trim for speed, pointing, helm balance, and heel.

Aspect Ratio.

Experience has confirmed the theoretical notion that tall sails with short chord length (high aspect ratio) are more efficient than low, wide (low aspect) sails. High aspect sails (and keels) create more lift with less drag. This efficiency is particularly strong when main and jib are considered together. Adding overlap offers diminishing returns. A 150% genoa, even with all its extra area, is not much faster than a 110% sail. Fig.17.

Fig. 17 - A tall narrow (high aspect) sail (or keel) is more efficient than a low aspect sail. A high aspect sail will be closer pointing, while the low aspect sail is more powerful. We are limited by materials and heeling forces in the design of rigs, and water depth in the design of keels.

Development of higher, narrower sails is limited by sail material strength, rig engineering constraints, rules, and righting moment (heeling forces).

In fact, new sail materials and rig designs have brought us to new heights in high aspect design. #3 genoas are now built to hold shape and withstand tremendous leech loads. The efficiency of various sail mixes will be discussed in more detail as we cover trim technique.

Keels

While we can';t trim ‘em (thank goodness - racing is complicated enough) the shape and condition of your keel is as important to your upwind performance as the shape and condition of your sails. See Chapter 14 - Boat Preparation for more on keels and keel shapes.

3.4 Performance Goals

So much for a theoretical framework. In practical terms, what sort of performance framework will lead us to competitive upwind performance? We need performance goals.

For upwind racing the goal is to achieve the optimum mix of boat speed and pointing. The figure shows a performance curve. The high point on the curve shows the best mix of speed and pointing. The boat sailing there is achieving optimum VMG - Velocity Made Good. Fig. 18.

Measuring Performance

How do you know if you';re at the right spot on the curve? Sometimes it is hard to tell.

The best and truest measure of performance is racing in a strict one design. If you are higher and or faster than the boats around you then you are on the high point of the curve.

You can also measure performance against other boats of different design, although differences are hard to attribute: Are we beating him because our boat is faster, by design, than his, in these conditions; or are we outsailing him?

Judging performance independent of benchmarks is tricky. Without other boats around we tend to sail a little low and a little fast. Interestingly, we tend to sail a little too high and slow when other boats are around.

You can also measure performance with instruments. If you change trim and improve pointing with no loss of speed, or increase speed with no loss of pointing, then you are doing better. Harder to judge is a change which trades in some of one (speed/pointing) for the other. Without other boats to measure against it can be difficult to tell.

With integrated instruments and performance predictions you can race against yourself. Sophisticated computer and instrument packages will show you your performance against predicted performance. The best packages will record real world performance and add it to the stored data. You then compete against yourself, trying to improve upon past performance in similar conditions. The power of these systems is daunting, and is covered in some detail in Chapter 16 - Performance Instruments.

Trim and Performance

As we study the details and techniques for upwind trim we will consider three positions relative to the optimum: Low and fast, high and slow, and below the curve.

Generally, if you are on the curve, but at the wrong spot, subtle changes in trim will get you where you want to be.

On the other hand, if you are simply off the pace then a fresh approach may be needed. With so many factors involved in upwind performance it can be difficult to know were to begin. We will provide a framework to help you work through the variables. Fig. 19.

Fig. 18 - The goal of upwind performance is to optimize the mix of boat speed and pointing to maximize VMG.

Fig. 19 - The trimmers and driver work as a team to optimize performance. If you are on the curve, but a little high or low, then subtle changes in trim can set things right. If you are off the curve then more dramatic changes are called for.

Performance Conclusion

The next section will explore the variables in upwind sail trim and relate changes in trim to changes in sailing conditions. The next chapter will start our exploration of the details of upwind trim technique.

3.5 Conclusion

We now have a basic understanding of sail shapes and the terms used to describe those shapes. This concludes the introductory section of the book. The next section - Chapters 4 through 8 - cover Upwind Performance.

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