Ezzy Basic Sail Design Theory

On any given sunny or wickedly stormy day, when we step our feet onto our sailboards and with our booms in our grasp, sheet in, and the wind fills our sails, we start to move. Leaning out over the water, our eyes look ahead. We search for bumps to jump or smooth spots to glide over. We race our friends, or we search the shoreline ahead for today’s destination. If instead we were to look up at our sails, we’d see the foiling of the cloth as it fills with wind.

That may be all that we want to know about a sail: it’s there, it’s beautiful and it’s giving us freedom. But high-performance sailmakers look at other things going on in the sail that make or break our enjoyment of that watery ride.

The elements of a sail

1. Twist
2. Outline
3. Profile
4. Rotational vs non-rotational
5. Camber inducers vs no camber inducers
6. Materials
7. Features
8. Quality

1 Twist

What is it?

The sail’s leading edge is obviously aerodynamically important, but not so obvious is what is happening at the back of the sail above the boom, on the trailing edge, called the leech. A sailmaker knows that this area is either twisting or it is not. Let’s find out what that twisting means for you as captain of this personal ship.

Looking down the leech, you can see that it is either a straight line from clew to head, or convex with a smooth arc, or concave, or any combination of the above. The amount of sail area that projects backwards from the straight line between head and clew is called roach. When the sail is full of wind, this roach and trailing edge can open up, or twist, away from the front of the sail. Generally, more roach means more twist. Where the roach sits up the leech also dictates where the maximum twist will occur. And a flatter sail profile will twist more than a fuller one.

Why do we need twist?

Sailmakers are keenly aware of how their sails twist, for two reasons. Well designed twist depowers a sail to make it more controllable in gusts and strong winds. And twist can stop the upper part of the sail from stalling.

How does twist stop the top of the sail from stalling?

The wind angle at the top of the sail is different from the angle at the bottom. This comes from the wind gradient: wind is weaker near the surface of the water and stronger higher up. So the bottom of the sail sees more apparent wind, which comes from the direction the board is travelling. The top of the sail feels more of the true wind direction.

If the top of the sail were set with no twist, the airflow would not stay attached to the surface of the sail and would stall, with knock-on effects over the entire sail. In a sentence: a sail that does not twist is less aerodynamically efficient.

Where should the maximum twist be?

Since the wind angle widens as you go up the sail, the maximum twist should be at the top. If it sits below the top, either the sail stalls in the head or it loses power from the twist being too low.

How is twist different from a stall?

The twisted section of the sail still has airflow attached, it just produces less aerodynamic lift. A stall means the flow is not attached at all and that part of the sail is producing no lift.

2 Outline

The outline breaks down into three edges: luff, foot and leech.

Luff curve

The amount of curve built into the front of the sail. More luff curve requires more downhaul tension and gives a more rigid profile. Less luff curve is easier to downhaul and gives a softer profile. Luff curve also controls leech twist: the straighter the luff curve, the flatter the sail and the more the leech twists.

Foot curve

The curve along the edge of the sail from tack to clew. Wave and freestyle sails have less foot curve, so the foot stays out of the water during moves. Slalom and race sails have more, so they can be raked back to close the gap between sail and board. Closing that gap greatly increases the speed potential of a sail.

The leech

The back edge of the sail, running from clew to head. The area extending beyond the straight line from clew to head is the roach. The point of maximum roach usually corresponds with the point of maximum twist, and as we said above, that should be at the top of the sail.

3 Profile

Sail profile can be achieved in four ways:

1. Excess luff curve
2. Panel shaping
3. Battens
4. Camber inducers

A sail shaped with excess luff curve has more luff curve than the curve of the mast once rigged. This forms a cavity next to the mast, which in a full batten sail becomes batten rotation. The problem with sails shaped using only this method is that in strong winds the mast bends more, which pulls the shape out of the front of the sail.

Panel shaping is achieved by sewing together panels with curved seams. Once joined, they assume a predetermined aerodynamic profile. Used together with excess luff shaping, it produces a more stable profile. Panel shaping can be vertical or horizontal. Horizontal shaping, usually under the battens, is the better method because it controls the shape from luff to leech.

Battens hold out the roach of the leech and set the profile in the sail. One problem with traditional full batten sails is that they are asymmetrical, because all the battens sit on the same side of the sail. Flipped one way, the battens push against the firm sail body. Pushed the other way, they lift off the sail as they push against the soft pocket material. You can see it in many sails under high downhaul: the batten fronts want to S, and the foot batten only wants to set in one direction.

We solve that with the Endo-Batten pocket. Endo means inside: the batten sits symmetrically in the centre of the sail. We have eliminated the batten pocket and instead place the batten in the centre of a horizontal profile seam, where it works as a profiled seam and as a batten pocket at the same time.

The result is a perfectly symmetrical sail, and it saves all the weight of the batten pockets.

4 Rotational vs non-rotational

Most sails, sitting unloaded, have the battens rotated hard around the mast. That is a rotational sail, and it means most of the profile is built in through excess luff curve.

Over the years we have found that rotational sails are wrong. They produce exactly the opposite shape to the one you want. Unloaded or in light wind, the lower and middle battens rotate hard around the mast. With little profile in the sail body, they assume a very flat, draft-forward shape, which is not what you want in light air. Then, as the wind builds, the battens get pulled from beside the mast to behind it, putting more draft in the back of the sail, which is not what you want in high wind.

So the rotational sail works backwards: flat and draft-forward in light air, deep and draft-back in strong wind. On the water, that translates into a sail that jerks you forward in light gusty wind and then gives backhand pressure in strong puffs. It is better to start with more profile in light wind, and then have the draft stay put rather than deepen and move back.

Ezzy Sails are built on a different concept. When you tension the batten, the shape comes from the batten taper plus the profile in the seam. The result is a defined, exact, horizontal profile, the opposite of sails that have very little shape in the panels and take most of it from excess luff curve.

Our sails have very little batten rotation around the mast. Unloaded, we set the batten behind the mast, roughly where a traditional rotational sail would put it under load. Combine that with the Endo-Batten profile in the sail body and you get a far more stable shape, working the right way round: in light air the batten sits behind the mast with more profile back in the panels, and as the wind increases the batten moves beside the mast, drawing the draft a little further forward.

There is another benefit. We are less reliant on mast stiffness, because our profile comes from shaped panels rather than excess luff curve. That means we can use softer masts without losing profile. Softer masts are easier to downhaul and feel more alive on the water, because the luff keeps a bit more shape while the leech stays loose. It also means a wider range of masts will work.

Tuning our sails is very different. Set one like a traditional rotational sail and it will feel terrible. The main difference is that our sails take a fair amount of positive outhaul tension, needed to pull the battens away from the mast. A wave sail takes as much as 5 or 6 cm of positive outhaul. The outhaul not only flattens the sail, it tensions the lower leech and reduces unwanted lower leech twist. Every sail comes with a string gauge that makes setting it easy.

5 Camber inducers

Camber inducers define the sail’s profile further and make it more stable. These days all cams are sail driven, meaning the batten sits in a closed pocket and does not connect directly to the mast, with the cam acting as a spacer inside the sleeve. That kind of system is usually specific to one mast diameter, which limits you to the particular mast designed for that sail. It becomes a real problem if your sail was designed around a standard diameter mast and you want to use a reduced diameter one.

The Ezzy cam is also sail driven, but it fits any mast diameter, from a modern reduced diameter mast to a twenty-year-old standard one. It has two thin arms that insert into a pocket. The pockets come in ten lengths, which adjust the space inside the sleeve and keep it tight at the camber. Each sail also comes with three camber sizes to fit all mast diameters.

Camber induced vs no cam

• Cams extend the range of a sail. Light wind performance improves because the camber holds the profile out. High wind is better too, because the camber stops the profile shifting in big gusts. For racing, or for light fluky conditions, cams are a must.
• Cams make the sail feel more rigid. Some people like that feeling, others don’t. If you care more about manoeuvrability and easy jibing, no-cam is the way to go.
• With the right technique, our camber induced sails rig almost as quickly as one without cams.

Rigging steps

1. Slide the mast above the cams.
2. Thread the downhaul with no tension.
3. Outhaul to the high-wind setting.
4. Snap the cams onto the mast.
5. Adjust the downhaul.
6. Adjust the outhaul.

6 Materials

We develop our own sailcloth, because the cloth has to be matched to the specific loads in each part of the sail.

• Ezzy Spectra X-Film is the strongest and lightest windsurfing sailcloth in the world. We were the first sail company to laminate Spectra yarns between two films. Our X-Film weighs about the same as standard 5-mil monofilm, yet it is more than a thousand times stronger in tear strength.
• The luff takes the most static load in a rigged sail, so the material has to resist stretch and hold a needle well.
• The leech is under high load when the sail is in use, so it needs very low stretch along its edge. We use lightweight, ultra strong Technora cloth.
• The tack and foot take constant abuse from harness hooks and board noses, so they need a very impact resistant cloth. We use a high-density form of our Spectra X-Film.
• The window needs good visibility and impact resistance. Vinyl doesn’t scratch, so you can always see through it, and it is durable enough to make the sail last longer.
• The upper middle of the sail has less directional loading, so it can be a lighter cloth while still resisting impact. For that we use our Tri-Lite.
• The sleeve takes a lot of abuse from sharp rocks and broken masts, and has to resist tearing lengthways. We achieve that with a weft-stop construction.

7 Features

Head

Our patented Power-On Head regulates the twist in the head of the sail. It lets us have a loose head and a tight head in the same sail: if the sail needs to twist more, it can.

Luff

All Ezzy sails have a 5 cm wide luff reinforcement sewn up the luff, plus a folded 2.6 cm luff tape inside the sleeve. Our sleeves are made from two vertical curved pieces, with no horizontal seams that could come apart.

Tack

Our tack is reinforced with twelve layers of Spectra X-Film and two layers of polypropylene webbing. We engineered our own Ezzy pulley to make the sails easier to rig and fine-tune: made entirely from 316 stainless steel, CNC machined, with an inner bushing to turn on and dividers that stop the rope jumping out.

• We also make an optional D-ring for the tack. It simulates a grommet and suits anyone using a pulley hook or the North ratchet extension.
• Our tack strap allows fine-tuning of the foot and tack. Pull it until you see the tension along the edge of the foot.
• Our built-in mast pad, with 5/8 inch closed cell foam, protects your board and your feet, and flips up for easy rigging.

Foot

Board texture can destroy the foot of a sail. We counter that in two ways: a plastic bead sewn along the edge of the foot, and a special chafe resistant tape on each side.

Clew

Our clews have the best strength-to-weight ratio out there. No hard edges, no abrupt ending of layers that could cause the clew to blow out. A light clew makes a big difference to the sailing weight of the sail. We also reinforce the clew grommet with a webbing strap so there is no slippage.

Leech reinforcements

A lot of sail companies cut corners by using thin, unsewn sticky-back cloth on the leech. We don’t. Our wave sails have two-ply leech reinforcements sewn permanently in place, and all our sails use Technora cloth on the leech for lightweight strength.

Battens

We use only RBS epoxy battens: the lightest, strongest battens in the world. Each one is custom tapered for its exact location in the sail.

Batten tensioner

Our screw batten tensioner is no thicker than the batten and weighs less than 16 grams. The key is stored at the tack of the sail, so tuning is easy.

Sail weight

• Ezzy Sails have always had a reputation for being bombproof.
• We have worked hard over the years to lighten them without giving up strength.
• Inventions like the Endo-Batten have taken almost a pound off the sail, because we no longer need all the layers that batten pockets required.
• Spectra X-Film lets us use the strength and lightness of Spectra to make a bulletproof cloth that weighs very little.
• Our Tri-Lite weighs almost half as much as 7-mil monofilm.

8 Quality

We are obsessed with making the best quality windsurfing sails in the world. That is why we rig every single sail we produce. No other sailmaker can say the same.