Sailors have been trying to go fast for a long time. In the old days, sailing speed was important for moving cargo. It was also critical for the survival of warships. A good example is the famous defeat of the Spanish Armada in 1588. The English ships were more maneuverable and better able to sail to windward. This allowed them to stay upwind of the bigger Spanish ships and shoot their cannons downwind.
Today sailors have different reasons for going fast. Sailing as a recreational sport has grown incredibly in the past several decades, and speed is now a means to win races and set records. In the past few years, almost all the old sailing speed records have been smashed. And sailboat technology has improved so much that the racing boats of today would undoubtedly blow their counterparts from even a decade ago out of the water. We are clearly sailing in a world of speed.
The Secret of Sailing Upwind
Sails function a lot like the wings of airplanes. However, the sailing environment is much more complicated than the sky. Unlike a plane, which is completely submerged in an ocean of air, a sailboat floats in one ocean, its sails move through another, and it creates waves on the interface between the two.
This explains why the early sailboats were not very efficient. In fact, it wasn't until relatively recently that boats could sail upwind. For a long time, sailors had to be content with going where the wind took them.
Perhaps the biggest breakthrough in all of sailing history was the addition of underwater surfaces to boats. These surfaces -- keels or centerboards -- counteracted the sideways forces generated by a boat's sailplan. They permitted boats to take advantage of the forward component of the sail forces, and finally make ground to windward.
As we have since learned, your keel is actually just as important as your sails for sailing upwind. The early sailors proved that without a keel, the boat would be pushed in the direction of the wind, no matter which way it was headed. With a keel, the force generated by the wind on the sails is counteracted by the force of the keel pushing against the water.
As Sir Isaac Newton said, "For every action, there must be an equal and opposite reaction." The boat's underwater surfaces provide the reaction to the sail forces that allows the boat to move forward instead of sideways. This is why the keel is essential for sailing upwind.
The vector that separates the forces of the sail against the keel shows the balance that must be achieved to beat to windward. As the length of an arrow is proportional to the force in that direction, we can see that most of the wind's energy is expended on creating and counteracting sideways forces. Very little of the wind is converted into forward force, or drive. It doesn't take much backward force, or drag, to counteract it. Racing upwind is the quest to lengthen the drive arrow and shorten the drag arrow.
A Bit of Fluid Theory
In 1738, Daniel Bernoulli established a relationship between the velocity and pressure of a fluid. For our purposes, we can summarize it by saying, "The higher the velocity, the lower the pressure." The reasoning behind Bernoulli's Principle is that the total energy in a mass of moving fluid must remain constant.
A simple example of Bernoulli's theorem at work is a house chimney. On a windy day, the smoke goes up the stack more readily than if it were calm. Why? Because the velocity difference between the outside and inside of the house causes a corresponding pressure difference. Bernoulli's principle states that the still air inside the house is at a higher pressure than the high-velocity air outdoors blowing by the top of the chimney. The pressure difference pushes smoke up the chimney.
Another example: Have you ever noticed movement in the water level of your toilet bowl on a windy day? That's the same phenomenon. Most toilets are ventilated to the outdoors by a pipe on your roof. When a gust blows across the roof, it temporarily lowers the outside air pressure. High-pressure air indoors pushes the water level down the bowl and up the pipe.
The same principle is used by baseball pitchers to throw curve balls. Note that a fastball is thrown with minimal spin so it will go straight and quick into the catcher's mitt. When a pitcher wants the ball to curve, he releases it with lots of spin. This spinning makes the air velocity different on each side of the ball. As a result, the ball will tend to "curve" toward the higher velocity side, where the fluid is exerting less pressure.
Have you ever sliced a golf shot and had the ball tail off into the woods somewhere? The same principle is at work.
How an Airplane Flies
Now we're a little closer to understanding how air foils and water foils generate lift. Lift is the total force that's exerted by a foil in a direction perpendicular to the fluid flow.
Let's start with an airplane. If you look at the cross-section of a wing, you'll see there is more curvature on the top than the bottom. As the air stream approaches the front of the wing, it splits, with some air going over the wing and some going under. Because the wing is curved on top, the air that follows this route has a longer way to go; therefore it must travel faster to meet up with the air on the bottom.
According to Bernoulli, faster air velocity on top means there is less pressure there and more pressure underneath. Therefore, the wing tends to lift, which is where this term came from.
A sail works in much the same way. As wind approaches the leading edge of the sail, it separates and flows along both sides of the sail from luff to leech. The sail is curved, so the airflow curves along its outlines. Naturally, the air on the windward side reaches the leech first, since it has a shorter route to travel. The result, without getting into a deep analysis, is lower pressure on the leeward side of the sail, and the resultant lift (force perpendicular to the fluid stream).
All of the theory about lift applies to the keel as well, but only because the water stream is hitting the keel at an angle (due to the boat's leeway). If the water hit the keel head-on, it would see a symmetrical surface, and there'd be no lift.
Even in the best of all possible worlds you can't get lift for free. Drag always accompanies lift, dragging the object downstream. For example, the baseball slows down as it streaks to the plate; the aircraft needs its engine to maintain a constant altitude; and every sailboat has a certain top hull speed limited by drag. The trick, as we will see, is to get the most lift for the least drag.
Describing the Shape of a Sail
If you were looking at the mainsail on your boat, how could you describe to me how it is shaped? This isn't an easy task, because sails are three-dimensional creatures with many different curves. The easiest way to do this is by using several two-dimensional measures to approximate a three-dimensional shape. The first thing we want to know about a sail is its depth. In other words, how much fullness is in the sail?
To measure this, we pick any of the draft stripes and draw an imaginary line between luff and leech. This line is called the chord length (C). We also have to find the amount of maximum draft (D). Sail depth is then D/C, expressed as a percentage. Typical sail depths range from 10% to 18%. This number simply tells us, at that draft stripe, how deep the sail is relative to its width.
The second thing we want to know about a sail's shape is the draft position. In other words, is the maximum fullness up at the luff, or way back by the leech? Using the same procedure as above, we calculate how far aft (as a percentage of C) the point of maximum draft is. Typical draft positions range from 40% to 50% aft.
Another useful description of a sail is the leading edge angle. This is particularly important for genoas. To find it, we measure the angle between a tangent to the curve of the luff and the chord line. The leading edge angle is a measure of the roundness of the luff. The wider the angle, the rounder the entry. A narrow leading edge angle means that the sail has a fine entry.
A more difficult way to describe a sail is by its vertical shape distribution, or how the sail's shape changes as you proceed vertically up the sail. We don't yet have a good quantifiable way to measure this (except on the computer). Suffice it to say that the fastest sails generally have more depth at the top of the sail than at the foot.
A final numerical measure of sail shape is twist. Twist refers to the change of each chordline's angle to the centerline of the boat from one height to the next. More on this later.
Seeing and Recording Sail Shape
Quantifying sail shape, once the realm only of sail designers and theorists, has made its way into the ranks of competitive sailors. Many sailors have realized that in order to achieve and repeat fast sail shapes, they need some objective way to see and record those shapes.
Sailmakers began to measure shape shortly after cloth construction allowed them to willfully design it. The pivotal advance came in the late 1950s with the introduction of consistently produced Dacron cloths. Now that low-stretch Mylar and Kevlar fabrics are more widely used, measuring shape makes even more sense.
The most accurate measuring technique is to photograph the draft stripes on your mainsail or genoa from underneath the middle of the sail. You can then draw chordlines and make your measurements directly on the photos (larger prints are best), and the photos themselves can be passed onto others or filed as permanent records. Make sure that on each photo you write the sail, true wind velocity, backstay tension and any other pertinent data.
Photography does have its limits. For one thing, the prints are never available on the water when they're needed most. Instant cameras have been tried, but their prints are too small. Also, photographs freeze the sail shape at one instant, losing the subtle shape changes that happen when the rig flexes and the sail breathes (especially with spinnakers). One idea that a North sailmaker developed is the North U. Sailscope. This plastic card will instantaneously give you a fairly accurate sail shape measurement right on the boat.
Optimizing Sail Shape Performance
The task of designing and setting sails to a given shape only makes sense to the extent that we understand the performance characteristics of the shapes we choose. The performance of a sail is measured in terms of the lift and the drag it generates, and by the ratio of these two values.
The L/D ratio is the most important measure of upwind efficiency. If you want to increase your velocity made good (VMG) to windward, you cannot simply add more lift. Beyond a point, drag builds up faster than lift, and the boat actually makes more leeway, points lower and goes slower. The better solution is to improve the L/D ratio as much as possible by making the sailshape more efficient.
Lift is almost always desirable, for it is the power produced by the sail. Only in heavy air, when the boat is overpowered, do we want to reduce lift. By shortening sail, we reduce lift and drag very directly.
Drag is the backwards pull of the wind on the sail. When closehauled, drag slows you down and hurts your pointing ability. On a run, however, drag pulls you where you want to go anyway, so the more drag the merrier. Let's consider how each shape factor affects performance:
Depth -- A deeper sail generates greater forces, both lift and drag, than a flatter section. For a given sailplan and sailing condition, there is depth that provides the optimal lift-to-drag ratio. Too flat a sail has minimum drag, but also produces little lift. Too full a sail produces much more lift, but is so bulky that its "form drag" (caused by its shape) is high. Again L/D suffers.
Draft Position -- Draft-aft shapes generally produce higher L/D ratios than draft forward shapes, and they can handle lower angles of attack. However, draft-aft shapes are also considerably more prone to stall if not sailed just right. In other words, draft-aft shapes are more efficient and more critical at the same time. They offer a narrow groove with high risk and high rewards. Thus, they are best suited for ideal conditions -- medium air and smooth water. Draft forward shapes work better in rough water when their forgiving characteristics help generate a dependable amount of power even when they're temporarily out of trim. They offer a wider steering groove.
Leading Edge Angle -- The angle of the sail's leading edge determines how high you can point before the sail starts to luff. A wide angle is more forgiving, but limits your pointing ability. A narrow angle allows you to point higher, but at the risk of getting too critical a shape and too narrow a groove.
Vertical Depth Distribution – An efficient sail is always somewhat deeper in the head than in the foot. However, in heavy air the added heeling force of the deep sections aloft is too great a penalty, so the head must be flattened to keep the boat on her feet.
Understanding the Wind
Before we discuss twist, and why every sail needs twist, we need to explain a few things about how the wind behaves.
Apparent Wind -- The apparent wind is the wind you feel on a moving boat. The true wind is what you feel when you are stationary (relative to the bottom). The apparent wind's speed and angle is the vector sum of the true wind plus the boat speed and heading.
When you're sailing upwind, the apparent wind is always stronger and shifted more toward the bow than the true wind. Once you get the wind aft of abeam, the apparent wind will usually be less than the true wind. A notable exception is iceboats, which go so fast that their apparent wind is almost always coming from the bow.
The apparent wind strength can be measured directly by a masthead anemometer on a big boat. However, true wind is usually a better guide for sail selection because it is not affected by wind angle.
Velocity Shift -- A velocity shift is a shift in the apparent wind angle caused by a gust or a lull, not by a shift in the wind direction. When the true wind velocity changes, so does the apparent wind velocity. However, a velocity change in the true wind also shifts the angle of the apparent wind.
For example, a lull in the true wind will temporarily feel like a header, and a puff will feel like a lift. This temporary direction change wears off as soon as the boat has sped up or slowed down to its target speed for that breeze.
Wind Gradient -- Wind gradient is the wind velocity difference from the water surface to the masthead. Wind gradient and wind sheer usually go together; the higher the gradient, the more likely there is to be an associated sheer. A high gradient condition is often a good sign of future conditions. It indicates the presence of more breeze aloft, and usually this will indicate a freshening breeze, such as a filling seabreeze that has not yet reached down to the water.
Wind Sheer -- Wind sheer is the angular difference between the wind on the surface and the wind at the masthead. At times wind sheer is quite dramatic and noticeable, at other times you will encounter virtually no sheer.
The signs that wind sheer exist are asymmetries in wind angles and boatspeed from tack to tack. Because the instruments are only reading wind information at the masthead, wind angle information will have seemingly large errors. The top of your sails are headed on one tack and lifted on the other, and because the sails are working better on one tack, boatspeed will also differ from tack to tack. Like wind gradient, sheer is also a good sign of windshifts to come.
The Reason for Twist
Now that we've discussed apparent wind and wind gradient/sheer, we can finally discuss sail twist. As mentioned before, twist measures how much the back of the sail falls away from the centerline of the boat. Twist is necessary in order to keep the same angle of the sail to the apparent wind from foot to head.
We know that the apparent wind "twists". The reason is that the wind at the masthead blows stronger than the wind at deck level (due to wind gradient). This produces two effects: an obvious freshening of the breeze higher up in the rigging, and a less obvious lift (shift aft) of the apparent wind angle up high. This shift can be explained by comparing the wind vector triangles. Note the change in velocity and direction of the apparent wind. Twisting the sails corrects for this effect.
Luckily for us, setting the proper twist is much easier than explaining it. The twist in the apparent wind is there whether we understand its origins or not. If you set your sail to luff evenly from head to foot, you are will generally get the correct amount of twist, subject to the following qualifications:
- • Twist can be minimized in smooth water and moderate air.
- • Over-twisting is necessary in heavy air to depower the head and reduce heeling force.
- • In light air, try to develop enough twist to avoid stall in the head.
How Sails Affect Helm
If you let go of your wheel or tiller in moderate or windy conditions, your boat will most likely round up into the wind. This is called windward, or weather, helm. When you have windward helm, steering a straight line requires angling the rudder slightly to create a counteracting lift.
All boats perform best to windward with a small amount of helm. Measured in degrees off centerline, 3 to 5 degrees of rudder angle seems best. This gives the helmsman a solid feel, while the lift generated by the rudder helps prevent the boat from sliding to leeward, and reduces the leeway angle as well. Too much windward helm requires too much rudder force, which produces too much drag.
The center of the sail plan, or the center of effort (CE), is the point where the sail's driving force is centered. The corresponding center of the underwater surfaces is called the center of resistance (CR). If the CE lines up over the CR, the boat will have a neutral helm. If the CE is behind the CR, then the boat will have windward helm.
This is actually a simplification of a more complicated situation. The CE is almost never right over the CR because of heel. When a boat heels, the CE moves even farther away from CR, and windward helm increases. Another reason heel produces more windward helm is that when a boat heels, its underwater hull shape becomes asymmetrical. Now the water flowing past the hull tends to push the bow to windward.
The "Target" Concept
One more thing before we move on from the basics. The use of instruments is becoming more widespread these days at all levels of sailboat racing, and anyone who races should be familiar with these. Undoubtedly the most important is boatspeed. If you're lucky enough to have this, it will be invaluable for the helmsperson and trimmers.
The next most important idea, in conjunction with boatspeed, is "target speed." Targets are boatspeeds used as guidelines for attaining maximum performance both upwind and downwind. A few grand prix boats have instruments that will read out their targets at any point of sail and wind velocity. However, most sailors will have to create their targets and post them somewhere for the driver to see.