Loads Catenary Versus Isotropic

There are two principal issues at work here, and they represent the primary differences between molded sails and paneled sails made from Cuben Fiber fabric. Since the early days of sail engineering there has been an underlying assumption that the loads in a sail radiate out from the three corners of the sail and then travel along catenary curves much like cables strung between two endpoints. The common thinking was that if you placed a fiber or bundle of fibers along this catenary, such as in a molded sail, and if they were of sufficient strength, then the sail would not distort when the load came onto the fabric. This thinking was most certainly valid and followed common engineering practices. But the problem was that as soon as the helmsman fell off from a beat the sail had to be eased out, and the loads no longer travelled along predictable lines. Catenary loading might be valid in general terms for primary loads, but it is not realistic when you know that a sail will be used through a wide range of conditions.

Sail engineers have long understood this problem and generally relied on films to take up on the off-threadline loads. This is a reasonable solution, but loading film results in its own set of issues. Film is weak relative to fiber, and when the film is loaded in a certain direction and caused to stretch, it shrinks on the axis opposite the load. The manufacturers of Cuben Fiber believe that no threadline model can ever be perfect, i.e., there will always be some load where there is no fiber unless the sail is built so heavy that it is impractical to use. Instead they

"The job facing the sail designer is challenging. He needs to take two-dimensional pieces of fabric and turn them into a three-dimensional aerodynamic shape."

Wind Tunnels Versus Computer Programs

Modern computer programs allow the sail designer to create a computer-simulated wind tunnel that subjects the molded shape to forces of wind, heeling angle, and other mechanical loads. In the days before powerful computers, all critically important sails, like those for the America's Cup, would be scaled down and built, and those scaled versions would be tested in a proper wind tunnel. This kind of testing gave the sail designer some important information, although these days computers often do a better job. The goal of all wind tunnel testing and flow programs is to optimize the lift and drag ratios for the sail. Unfortunately, these flow programs only treat the sail as a solid shape and do not take into account the natural stretch and deformation that will occur when real loads are exerted on a sail. Computer-simulated wind tunnels are able to factor this information into the design. As a result, for really important projects sail designers use a combination of wind tunnel and computer analysis.

believe that it's important to engineer a sail that is "isotropic," in other words using a fabric that is strong in all directions. Cuben Fiber fabric relies on thousands of tiny filaments of fiber to introduce stability into a fabric rather than bundles running along primary load lines. The argument is clearly that since the loads in a sail are infinite and depend upon many variables, you can either attempt to engineer a sail that will do a reasonable job on all points of sail, or you can engineer one to manage the principal loads and suffer a little when they travel off the main catenaries. Since weight and cost also factor into engineering, some compromises have to be made. Molded sails continue to outnumber Cuben Fiber paneled sails so perhaps this argument does not have as much merit as it seems. Only time will tell how these two different approaches play out.

Bearing in mind that sails will undergo various loads and the design process includes dealing with these loads no matter how they occur, let's start with a blank piece of paper (or more to the point, blank computer screen) and work through basic sail design. We will look at the following steps:

Step 1 - Geometry

Once the sailmaker has your rig measurement details and understands the kind of sailing you plan to do, he can start the design process by figuring out the geometry of each sail. Perfect aerodynamic shape and engineering amount to zero if the sails do not fit, and this means more than just getting the luff lengths right and having the sail sheet to the tracks. The sail designer needs to take into account details like the location and length of the mast spreaders and where the standing rigging fits into the overall rig plan. For example, he needs to be careful that the design shape for a headsail does not have the sail going right through the spreaders when trimmed for sailing hard on the wind. He also needs to be sure that once the sailor bears away onto a reach that the sail can still be sheeted to the boat. The same points apply to the mainsail. The designer needs to take into account the location of the backstay and design the roach profile accordingly. There is no point in adding roach to the sail and then not being able to tack the sail through the backstay. He also needs to be careful when designing a full-batten mainsail that the batten locations do not coincide with the spreaders, both when fully hoisted and reefed. Point loading a batten on a spreader is looking for trouble.

The kind of sailing you plan to do also plays an important part in the geometry of the sails. If you are strictly inshore racing, the sail designer will keep the clew of the headsails fairly low and have the foot of the sail "sweep" the deck. On the other hand if you are heading offshore it might be useful to raise the clew so that waves can pass under the foot. This will also allow some visibility under the sail. Finally, the sail designer needs to be sure that there is some correlation between the sizes of different sails so that the sailor can reduce sail area and still keep the center of effort of the sailplan in the right location so that the boat remains balanced.

Step 2 - Sail Shape

There are two theoretical design shapes for each sail. The first is the molded shape; in other words, the static shape of the sail before it is subject to any loads. The second is the flying shape, i.e., the shape of the sail after it has been subjected to the force of the wind. The design process incorporates both the molded and flying shapes, and it becomes the designer's job to take both into account before moving on to Step 3, which is the part of the process that analyzes the interaction between molded and flying shapes.

Molded Shape - This shape is usually drawn from a data bank of known sail shapes and serves as a jump-off point for the design process. It is illustrated by horizontal and vertical cross sections of the surface of the sail and the measurements are called offsets. Offsets are a two-dimensional way to describe a three-dimensional curve. These offsets show the important design features of a sail, namely the chord depth, the position of the maximum draft, the angle of leading edge and the amount of twist in the sail. They are created for each section, or horizontal "slice" of the sail. Think of the sail design as a huge stack of individually created cambered shapes each with its own chord-depth ratio, twist, and maximum depth location. Stacking them on top of each other creates the overall sail shape.

Some sail designers have their own software that allows them to enter the boat's rig dimensions and deck hardware into the actual sail design program. This is fairly sophisticated and can be a great help in terms of the next step in the process, since it allows the designer to manipulate the sail and rig as one, and to address wind-flow issues over the entire plane, not just over individual sails. The mechanical properties of the mast and rigging can also be entered, including moments of inertia and a material's stiffness or resistance to stretch. Using this information, the designer can determine the deformation under load for the sail and every piece of standing and running rigging right down to the stretch in sheets and halyards. This kind of precise information is vital. If halyards stretch or the mast bends more than the designer anticipated, the shape of the sail will be affected. Being able to have some control over parts of the boat previously out of his realm allows the designer to have more control over his design. This control may only be in the form of knowing what to expect and designing the sail accordingly, but in any event it's useful knowledge.

Flying Shape - Once the designer is satisfied with the molded shape, he needs to subject the sail to the forces of the wind. This can be done by integrating it into a load program that exerts various loads on the sail. In addition to the rig information, the sail designer can input information about the fibers and fabric he plans to use. Drawing from a database of known stretch characteristics for different styles of fabric, the sail designer can see how a chosen fabric will stand up to the anticipated loads. The amount and orientation of fibers in a sail will have a tremendous effect on the flying shape of the sail.

North Sails tests their America's Cup sails in a modern wind tunnel facility that can even simulate twist in the wind.

Wind flow over sails can be displayed graphically on a computer allowing the sail designer to work on sail shape.

Wind flow over sails can be displayed graphically on a computer allowing the sail designer to work on sail shape.

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Thayer School or Engineering Dartmouth Colleae

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Thayer School or Engineering Dartmouth Colleae

Despite advances in sailmaking, some of the process like sticking seams together still needs to be done by hand.

Despite advances in sailmaking, some of the process like sticking seams together still needs to be done by hand.

These load programs take the design and "flow" air over the surface at predetermined settings. These can include true wind speed (TWS), true wind angle (TWA), leeway, boat speed, and angle of heel. The settings can be changed at random and the result displayed in a series of pressure maps that show the various pressures on the sail at any given time. By entering the fabric information, the designer can determine just how effective the fabric choice will be in resisting stretch. This leads the sail designer to the next and most important stage, the analysis.

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