This chapter contains information about how the sails work and how you can control them. You can learn theory details from other, more comprehensive, sail theory manuals, but this will give the basic ideas and how to use them. Sometimes what is written below is not the whole truth, but partial truth which has been simplified to keep this guide short, and the ideas easy to grasp.
For more information on the topic of sail theory and aerodynamics, one excellent source is, "The Art and Science of Sails," by Tom Whidden and Michael Levitt.
There are two themes that are important to understanding the theory of sails: lift and flow. Lift is the force which makes the boat move, and the flow of air over the sails generates that lift. It also generates drag, which slows the boat down.
Sails, contrary to popular belief, are not merely "pushed" along by the wind. Have you ever thought about how the boat could go into the wind? This would not be possible if the sail was just pushed. Once, during the age of square-rigged boats, people thought this was true, and there's a story that says the first Viking that managed to go against the wind was burned at the stake for being a witch. How could he go against the wind? The idea is fairly simple to understand.
A sail, like a wing of an airplane, is an airfoil which changes the shape of the wind as it flows over the surface of the sail.
With this change in the shape of the wind come other effects such as pressure changes and directional changes. Looking at the top of the foil in figure 1, you can see an air particle will have further to travel than his little twin particle going over the bottom. These two want to reach the back of the sail at the same time, so what gives? The particle over the top needs to travel faster.
When the air is traveling faster over the top, a phenomenon known as the "Bernoulli Effect" kicks in. Since the speed is faster, the pressure drops, "sucking" the foil up. You can picture this by thinking about air coming through a fan. Watch a piece of dust outside the flow of the fan. It will drift slowly, then more quickly, toward the stream of the fan. Since the air in the stream is moving more quickly, it gets its particles through faster, calling for more. The area over the foil gets its particles out of the way faster, pulling the foil upward to fill the gap.
Of course, while thinking of a sail versus a simple airplane wing, you must see that the foil is on its side, actually being sucked forward, not upward.
The preceding and following discussions apply to a boat going upwind. When the boat travels dead downwind, many things change, such as the way the wind is used. When traveling downwind, the wind simply pushes the sail. I will leave more discussion of downwind aerodynamics out for brevity, and will concentrate on the first priority: beating.
The sail is the foil driving the boat. In order to maximize the speed of the boat in different conditions, you must change the depth of the foil. This depth is called the draft of the sail. Its size and location can be changed using the sail controls discussed below. These changes are important because the optimal draft position and size change with the condition of the wind and water.
Position: It will be best to have the draft of the sail a little forward of halfway.
Size: In general, the bigger the draft, the more power. A large draft is like first gear in your car, giving lots of power to accelerate, but topping out at a fairly low speed. A flat sail is fifth gear. The sail attains a higher speed, and will point higher into the wind, but it also takes longer to accelerate.
How is a flat piece of cloth transformed into the shape of an airfoil? Sails are made up of panels which are sewn together. There are two basic methods to get draft into a sail using these panels.
One method consists of sewing a flat sail with a curve in the luff. This is how Laser sails, as well as most windsurfer sails, are sewn. It is a very simple method of tailoring.
Once the mast is inserted, straightening out the luff, the extra material has to go somewhere, so it goes into the body of the sail, forming the draft.
The second method is called broadseaming. The panels, themselves, are cut with curves on the connecting edges. These edges are then sewn together and the draft is created. This is how FJ sails, as well as most other dinghy and keelboat sails, are made.
You will notice when laying an FJ sail on the ground that it will not lie flat, as opposed to the Laser sail, which will. The effect, once raised on the mast, is mostly the same, with a curve or draft in the middle of the sail.
It should not be difficult to understand that air, when traveling slowly, does not have the same high energy as when it is traveling quickly. When a light wind hits the sail, it will try to get around the sail as best it can, but it gives up quickly if the draft is too large. Keeping the sail flat will help a very light wind get around the sail. You should notice when a puff hits, those with fuller sails will pass you. However, if the air is steady enough, your set-up will make you faster in the long run, at least upwind.
In moderate winds, you can follow the rule: "The bigger the draft, the more power-the smaller the draft, the more speed." Make the decision how to set the sail based on your experience before and during the day you are racing. If you are on the course, and are finding your speed is okay, but the boat is not accelerating as quickly as the others around, put more "bag," or draft, in the sail, using the sail controls discussed below. If your acceleration is good, but you're lacking in top speed upwind, flatten the sail.
In heavy winds, there will be an excess of power in the sails. As you will see below, whenever the boat heels too much, lots of bad things happen. Heavy winds and a large draft make it easy to heel the boat too much, so any excess power should be "squeezed" out of the sails by flattening them. It won't be difficult to get the boat up to speed since there will be plenty of power. Remember that everyone else around will have flatter sails as well.
Lift is a product of the flow of air around the sails, which can be classified as either attached or unattached. The attached flow is a smooth flow of air that "sticks" to the sail. This is very desirable when going upwind as it generates much more lift than does unattached flow. Unattached flow breaks off the sails with little swirlies in it.
Certainly you can't directly see the action of the wind, but there are things you can do to help it stick to the sail. For instance, when the wind is light it separates from the sail when there is too much draft. Flattening the sail will help the air stay attached, generating more lift. Generally, the separation occurs when the wind has to make a sharp turn, like when the draft is too large:
There will be other instances mentioned below where separation occurs, which can be rectified through control of the sails.
Another product of airflow around the sails is drag, which slows the boat down.
There are basically two types of drag which are important to the sailor. These forces, holding the boat back, can be reduced partially. It is important to understand how drag is formed, in order to lessen its effect.
Frictional drag is what we all think of when we hear the word "drag." It is the scraping of a box on the ground as you try to push it across the driveway when you're moving. It is the reason there is oil in car engines, and one of many reasons we use edible body oils.
Frictional drag is generated from the side stays, from the seams in the sails, as well as from the skipper and crew. You can think of this force as friction on the form of each of the elements of the boat and crew.
Watch a movie sometime with an airplane flying through a cloud. In that footage, it is easy to see large vortices of fog swirling off the tip of the wing of the airplane.
What is happening is that the low and high pressure areas meet at the tips of the wings. Much of the air and fog "leak" suddenly from the high pressure side (the windward side) to the lower pressure side, creating big swirlies. These swirlies require a lot of energy to form-energy which could be better used in propelling the airplane or boat forward.
There have been numerous solutions proposed to slow this effect, such as small vertical winglets on the ends of airplane wings. On a boat, there will be two places this leakage occurs: the head and the foot of the sail. In one of the early 1900's America's Cup races, the American yacht had a 4 foot wide boom, dubbed the "Park Avenue Boom" for its size. The purpose of this was to slow the flow from the windward, high pressure side, to the leeward, low pressure side. This slowed the creation of the induced drag.
There are other, more reasonable solutions to this problem. For instance, on modern windsurfers, the sail is built so that at high speeds, the sail can be leaned back all the way to the board. This closes the gap between the sail and the board, using the board as an "end plate" to stop the flow, much as the fuselage of an airplane does for its wings. Another instance of this end plating is the "deck sweeper" jib, which is cut to come all the way down to the deck of the boat. Deck sweeping jibs are now very common on most modern sloop-rigged boats.
Neither of these solutions addresses the problem of the leakage at the top of the sail. There is no way, so far, to prevent this type of leakage.
One more place where vortices can form and sap energy from the sails is at the trailing edge, or the leech. The wind needs to have a smooth exit from the sail in order to keep it from getting angry and swirling. Two ways to make the wind angry are: 1.) put the draft too far back in the sail, so it has to make a sharp turn immediately before it exits; and 2.) curl the leech of the sail inwards with too much boom vang.
You will see below how you can use basic sail controls to fix this.
This section outlines the major controls for the shape changes you have seen above. Many times, in the beginning, it's not obvious how to change the shape, location, or size of the draft in the sail. However, with some practice, it will become second nature. All the myriad lines, blocks, and grommets in the boat should begin to make sense, instead of looking like part of a big bowl of spaghetti.
Before you learn to change the shape of your sail, you have to know what types of changes to make. Much of your sailing will be done by the feel of the wind, but there are also many visual aids for detecting wind direction, and how the wind is interacting with the sails. These aids are important when you begin to sail, and even after you have mastered the basics. They are especially useful in very light winds, when it's difficult to sense the wind by feel.
The windex is a small weather vane attached to the top of the mast, or in the Laser, in front of the mast about 1 foot off the deck. This is a very gross method for measuring the direction of the wind, and is most useful downwind, when fine adjustments are not needed. The windexes at the top of the mast are often distracting since you have to look away from where the boat is going, so it's best not to have one here. One of the best racers I've ever known once said, "Windexes are great, if they're on someone else's boat." You can watch someone near you, but don't forget where you're going.
Sidestay telltales accomplish the same task as the windex. They are good for a gross indication of the wind direction
The jib telltales are the most sensitive, accurate, and thus useful, of the bunch. These should be placed about 1/3 of the way back from the luff of the sail, and at 1/4, 1/2, 3/4 the distance from the bottom to the top. They should be made of a light material which does not stick to the sail, such as yarn or audiocassette tape.
Use these to see the attachment of the wind to the sail. Going upwind, with the sail mostly flat, there should be attachment on both sides. You will see both telltales flow straight back. Sometimes, as when reaching, it is not possible to make both flow back, because of a large draft in the jib. It's imperative that the flow be maintained on the outside surface, so keep the outside telltale streaming back.
When there is a jib, these indicators are not as important. The mainsail is used to keep the boat flat and it is best to trim it by feel, so it pays to ignore visual indicators on the mainsail. However, telltales can give a little information as to what's happening.
These give the same information as those on the jib. If they are flowing straight back, there is attachment, and if not, separation.
Attach these to the leech of the sail, at the points where the battens are inserted. When the air is leaving smoothly from the leech, these will flow straight back, as they do on the surface of the sails. The goal for these is to have them lifting (flowing) about 1/2 the time. The theory is a little sticky, but this is a good goal for speed and pointing.
If they lift more than 1/2 the time, there is too much air flowing freely off the leech, so you need to capture more by trimming the sail, or by tightening the vang (see discussions below on main sheet and boom vang). If they lift less than this, the leech is too tight, and you should let up on the mainsheet or the vang.
One of the most basic of all the controls is the outhaul. It controls the size of the draft in the bottom 40% of the mainsail. To flatten the sail, pull on the outhaul-to give the sail more "bag", or draft, let out the outhaul. It does just what the name says-it hauls the back of the sail out.
This is not a very interactive sail control on club FJ's or on Lasers. Often, it is set at the beginning of the race and left in that position. If it is possible to adjust on the fly, its major purpose is to make the transition between beating and going downwind. When reaching, more draft in the sail will give more power. Dead downwind, the outhaul is let off to make more bag in the sail. This bag allows the sail to generate as much drag as possible, which is the driving force dead downwind. Right before the leeward mark, the outhaul is then brought back in to allow pointing. This is very important- try pointing upwind sometime with the outhaul all the way off. You won't ever get where you're going.
In one of the 1920 era America's Cup races, a skipper named Cunningham designed his namesake sail control. The sails at that time were made of canvas and were apt to stretch depending on the wind conditions. Given the maximum sail size restriction, the sails had to be cut short to meet that maximum when it was windy, leaving them small in less demanding conditions. He realized if he put a grommet through the sail near the tack and pulled it down, the sail area could be increased in light air.
The cunningham is no longer used for this purpose, but it still remains an important control. The modern purpose for this little line is to control the location of the draft. When the cunningham is pulled on, the draft in the sail moves forward.
As the wind speed increases, the draft tends to blow back in the sail towards the leech. This is undesirable as it causes overpowering and extra drag. The draft should be somewhere between 40% and 45% back from the luff of the sail.
A by-product of a tighter cunningham is that the leech of the sail begins to "open up." Sighting straight up above the boom, the top batten of the sail should be parallel with the boom. If the cunningham is pulled on tight, the top batten will then be pointing outward, away from the boom. To bring it back in, you can put on more boom vang, which is discussed below.
Of course, another by-product is that the sail will be flattened, partially. However, the cunningham should not be used to flatten, except in special cases, as it moves the draft while flattening.
Referring back to figure 4 above, you can see the Laser sail laid out flat. When the mast is inserted, straightening out the luff, the sail gets its fuller shape. If that straight mast is bent back, closer to the shape of the flat sail's mast sleeve, the shape goes out. The main purpose of mast bend is to depower the sail, and this can be done in a variety of ways.
When sailing a Laser, you will soon find that the mast is quite bendy. One of the ways to induce bend and decrease the draft of the sail is to sheet very hard. In fact, while beating upwind in all but the lightest winds, the two main sheet blocks at the stern of the boat should be touching each other (called two- blocking).
Bend in the mast can be induced even before putting force in the sails by pre-bending it. There are several ways to do this, but they all have to do with forcibly pulling the bottom 1/2 of the mast forward, toward the bow. On International FJ's, as well as other modern, high-performance dinghies, there is often a fitting and a set of blocks mounted on the deck for this purpose.
Pre-bend is used when it is obvious that conditions warrant a much flatter sail-i.e., very windy. This way, the crew is never caught with too much power over their heads.
Ever wonder what the spreaders (the horizontal supports for the sidestays, located 1/3 of the way up the mast) on the FJ were for, or more particularly, why they are adjustable backwards and forwards? If the spreaders are pulled back, the force of the stays is directed more toward the stern, pulling the top 1/2 of the mast back, bending it. Alternately, angling them more forward reduces bend. Angling them forward might be done in lighter conditions when more power is needed.
The boom vang can be used on a bendy mast, just like the main sheet, to pull the top 1/2 of the mast back. In fact, in the Laser, this is often preferred, because the shape of the sail stays the same when it is sheeted in and out. If all the control were left to the main sheet, the sail would power up when eased and depower when pulled in. This way, only the angle with which the sail hits the wind changes.
The main job of the boom vang on most boats is to control the shape of the mainsail leech. When the vang is pulled on, the leech gets tighter, and when released, the leech gets loose, and "twists" off to leeward. The top batten, as mentioned above, should be approximately parallel with the boom. If pulled in too much, there will be excess drag. If left loose, too much power is lost out the back of the sail.
The jib does not have as many controls as the main. In fact, most of the time, the sheets offer the only interactive control. However, many of the above shape qualities can still be changed. One important point to keep in mind is that there is a "slot" between the jib and main. This slot should be kept fairly open to allow the correct flow to form. The jib leech should also be kept mostly parallel with the closest part of the main. That slot should be very nice and uniform, up its entire length.
If the slot is too wide at the top (i.e. leech of the jib is too open), too much of the air escapes through without affecting the jib. If it is too narrow, the flow is "choked," and the boat will not go as fast, or point as high. The way to control the slot is through the jib leads.
The two blocks for the sheets can often be moved forward and back. When moved backwards, think of the bottom of the sail being pulled toward the stern, flattening the sail. Also, consider that the leech of the sail will open up a little.
When the blocks are moved forward, the force of sheeting will mostly be down, closing the leech of the sail, and allowing more bag to go into the middle.
These controls are the most interactive of all. Their major function is to control the angle of attack on the wind. This is the angle at which the wind hits the sail, with respect to the boom.
When the sail is brought in, the angle increases and the power increases, but hopefully you already know that. You should also know that if the angle of attack is too large (the sail is pulled in too tight), the sail will "stall" and the lift will be destroyed. It will look the same, but it won't be working as it should any longer. If the angle is too small (the sail isn't pulled in enough), it will luff, generating no lift at all. You can use the telltales on the sail to judge whether the flow you need is being generated. Remember that this flow can be created and destroyed by changing the angle of attack.
Each boat will have its own rigging quirks. Some things will make one boat go faster, while making another go deathly slow. When you begin to sail in a particular class, make sure you discuss rigging with people who have raced the boat before. When they give their hints, find out why they work for this boat and not for others.
Listed below are some points that will keep your Laser rigging headaches to a minimum.
There are very few tricks to rigging an FJ. However, the boat seems to perform its best when the mast is raked back as far as it will go. This opens the slot immensely, and helps the boat's pointing ability, because you can pull the jib in tighter. Lots of mast rake means leaning it back by lowering the side stays as far as the forestay will allow. Do this before the boat is on the water.
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