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Performance Sailing and Racing
All You Need to Know to Sail Faster and Smarter
By STEVE COLGATE The McGraw-Hill Companies, Inc.
Copyright © 2012Steve Colgate
All rights reserved.
ISBN: 978-0-07-179345-2
Excerpt
CHAPTER 1
Sails
SAILS AND SAIL SHAPE
A boat's speed depends largely on how its sails are set and trimmed. And there are an infinite variety of sail types, shapes, and materials. Today's sails are made of many materials, including Dacron, a material that stretches constantly as the forces on it change. With the invention of Kevlar and spectra—sail material that doesn't stretch—sails are now being built with a set shape, and it is not as necessary to control the stretch as before but sail shape understanding is still critical to performance sailing. Forces affecting sails include wind-strength increases or decreases, and also pressure changes on the sails when the boat slows down as it plows into waves or speeds up when it is surfing or sailing in smooth water. As you sail in these conditions you can control this stretch, and thereby the shape of the sail, using numerous adjustment devices. We'll get to those in a bit, but before we look at the specifics, let's discuss the desirable end result.
Sails power a sailboat much like an engine powers a car with a manual transmission. When a car is moving slowly, uphill, or over a bumpy terrain, you keep it in low gear to add power. As it picks up speed and the ground levels, you shift to a higher gear. When the car is moving fast on a smooth road, you shift to an even higher gear. So, too, with a sailboat. Full sails are the low gear and flat sails are high gear. When seas are heavy and the boat is sailing slowly, almost stopping as it hits each wave, the sails need power. Full sails are the answer. In smooth water and high winds when the sailboat is moving fastest, flat sails are desirable.
DRAFT
The mainsail is a very versatile sail and can be made flat or full at will. But, you may ask, what is a "full sail" or a "flat sail"? The terms are relative. A sail is flatter or fuller than another based on the relationship of the maximum depth of the curvature (the draft) to the distance from luff to leech (the chord). Figure 1-1 shows the cross section of a mainsail. An imaginary line drawn from luff to leech is the chord. A line drawn perpendicular to the chord at the point where the sail is the greatest distance from the chord is the "draft" or "camber." The "camber-to-chord ratio" is the relation of this distance to the chord, usually expressed as a percentage. If the chord is 120? and the draft of camber is 12? deep, the camber-to-chord ratio is 10 to l or 10%. Sails can be used effectively as flat as 5% or as full as 20% at the center of effort, depending on the class of boat and the sailing conditions. The draft varies at different heights up the sail.
Of even more importance is the position of maximum draft in the sail. Figure 1-2 shows three sails all with the same camber-to-chord ratio, but with quite different locations of the maximum draft. Sail A has the draft in the desirable location for a mainsail—40% to 50% aft from the leading edge (the luff). Sail B shows the draft forward, near the mast. This can happen when a sail is designed to accept a certain amount of mast bend, but the sailor doesn't bend the mast enough as in Figure 1-1. Sail C in Figure 1-2 shows the maximum draft aft, near the leech of the mainsail. As the breeze freshens, sail material stretches and the draft tends to move aft toward the leech. This movement will cause the battens to cock to windward in the mainsail and produce a less efficient airfoil. Increased tension on the luff can keep this movement to a minimum.
The sailmaker puts draft into the sail in two ways: by a "luff and foot round" and by "broadseaming." If you laid a mainsail on the floor and "luff and foot round" was the only draft producer, it would look like the gray area in Figure 1-3. However, when it is put on a straight mast and boom, the excess material becomes draft (the white area). As the material stretches in the wind, this draft moves aft toward the desired location in the middle of the sail.
In light winds on a straight mast, the draft created by luff round will be forward, near the mast. If you bend the mast and boom to conform with the designed edge round, then the sail will be flat as a board. The other method of obtaining draft, broadseaming, is simply narrowing the panels of cloth before they are stitched together. To understand how this creates draft, imagine a football that has been taken apart. It looks somewhat like Figure 1-4. Sewn together, it becomes a football. The same method is practiced in sailmaking as in Figure 1-5. Draft created in this manner is placed exactly where the sailmaker wants it and does not depend on mast bend or stretch to place its location. A combination of both methods is used in the manufacture of all sails, except in some high-tech modern systems that use a molding process with exotic materials.
SAIL CONSTRUCTION
But first, just a bit about how a sail is constructed. The threads that run across a panel of sailcloth are called the filling threads, otherwise known as the "weft" (also called "woof") or the "fill." The threads that run lengthwise are called the "warp." Warp stretches more than weft, but the greatest stretch comes in a diagonal direction, called the "bias." Most sails are designed with this stretch in mind. For example, the mainsheet will exert the greatest force on a mainsail, and most of it will fall on the leech. Consequently, the panels of cloth are sewn together so that the crosswise threads, or filling threads, lie along the leech of the sail (see Figure 1-6).
This means that all the panels along the luff of the sail must be cut on the bias, where stretch is greatest. If we were to blow up a small section of the sail along the mast, we would see that the threads look like a whole bunch of little diamonds at the bias (Figure 1-7). As we pull down on the luff and increase the tension, each diamond elongates (the dotted lines) and pulls material in from the center of the sail (see Figure 1-8). If we pull down hard on the luff when there is not enough wind to warrant it, vertical troughs or creases will appear, running parallel to the mast (Figure 1- 9).
You can simulate this effect by taking a handkerchief and pulling it at two diagonally opposite corners, as in Figure 1-10. The same troughs will appear just as they will when there is too much luff tension. Figure 1- 11 shows that as the corners are stretched apart on the bias, the material moves upward. The lower corner was even with the person's waist and is now a few inches higher.
PROPER MAINSAIL ADJUSTMENT
Tensioning the Luff
There are two ways to tension a mainsail's luff—with a downhaul and with a cunningham. In the days of cotton sails, you would buy a sail that was actually too small in light air. This would allow you to stretch it with the halyard to flatten the sail when the wind velocity increased. Of course, this meant that you would automatically penalize yourself in light air by having reduced sail area.
To solve this little dilemma, Briggs Cunningham, developer of the Cunningham racing car and skipper of Columbia, winner of the 1958 America's Cup, chose the simple expedient of placing a grommet above the mainsail tack fitting in a full-sized sail. When the luff of the sail was stretched as far as it could legally be, a block-and-tackle arrangement was attached to a hook running through the grommet. Tightening it added further tension to the luff. Though some wrinkles do appear along the foot below the grommet when the cunningham is in use, they don't seem to make an appreciable difference in the efficie
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Excerpted from Performance Sailing and Racing by STEVE COLGATE. Copyright © 2012 by Steve Colgate. Excerpted by permission of The McGraw-Hill Companies, Inc..
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