There are two equally important aspects to sail design: aerodynamic shape and engineering. Aerodynamic shape refers to the curved foil that the sail will present when it is flying under certain conditions. Engineering refers to the various fabrics and fibers that will be used in building this foil and the precise manner in which they will be put together. In fact, these two aspects of sail design go hand in hand since a perfect shape is useless if it distorts when a load comes on the sail. Similarly, an over-engineered sail is equally useless if its shape is not conducive to good performance. This balance between shape and engineering is a delicate one, and the process starts with a careful analysis of the various loads that the sail will undergo when it is being used. Once the sail engineer knows precisely what loads the sail will encounter he can build it accordingly using just enough fiber and fabric to handle the anticipated loads without any unwanted stretch or extra weight.

This load analysis has two parts to it. The first is called finite element analysis in which a sophisticated computer program simulates the sail flying in various wind strengths and angles, and then graphically represents the different loads in the sail. The second involves calculating the exact strength and stretch resistance of each individual fiber and the finished fabric that is to be used in the sail. As discussed in chapters 2 and 3, a good fabric engineer will know both the tensile strength and yield strength of the various fibers, and will be able to provide the sail designer with this information so that it can be factored into the engineering process. The designer can then take full advantage of the strength and stretch attributes of the sailcloth and then incorporate them into his design. The goal is to keep the sail as light as possible, but still strong enough to be usable throughout its designed wind range without changing shape. Panel layouts and corner engineering are also an important part of this process, although before we look at this area in more detail, we need to take another look at the critical subject of load analysis.

Finite Element Analysis
Finite element analysis programs came about because of the complex nature of some of the engineering problems faced not only by sail engineers, but by engineers in general. For example, it’s fairly easy to calculate how much a steel beam will deflect when it is suspended between two points. It’s also fairly easy to calculate the loads a sail will be subjected to under specific set of conditions. The problem however is that the conditions never remain the same for more than an instant. Wind strength, waves, heel angle, air density and a myriad of other factors come into play. Therefore, in order for the engineer to calculate how the fibers and fabric will respond to the different conditions, he needs to break the entire problem down into small, solvable problems, and then feed this information into a finite element analysis program that will be able to solve the larger complex problems that result. The small, solvable problems can be expressed as mathematical algorithms that can then be interpolated into the engineering requirements for the more complex problems. Only by knowing the answer to the most basic of equations can the larger engineering problems be solved.

The sail designer starts with some basic information. He has the geometry of the sail, the designed shape and the anticipated wind range for the sail. All this information can be entered into the finite element analysis program, which will then represent the sail graphically on a computer screen. Once he has all this information in place he can begin to manipulate the conditions. He can increase the wind strength and see what result it has on the fabric. He can move the sheet lead position and see how the load paths in the sail change. He can also alter the wind angle and ease the sheet to watch how the loads in the sail will travel along different catenaries. Since a sail is an object that can be infinitely manipulated, and the wind is an infinitely variable element, another job of a sail engineer is to decide which parameters to engineer the sail around. For example, if he is designing a headsail for an America’s Cup boat that he knows will only be used on an inshore course for sailing as close to the wind as possible, he would not bother too much about wider wind angles. Instead, he would have the computer program simulate the sail being used within its designed wind range sailing hard on the wind and see what loads the sail encounters. If, on the other hand, he is designing a headsail to be used on an Open 60 sailing in the Southern Ocean he would know that the sail would never be used in a hard-on-the-wind situation. Rather, the sail would be used reaching and running. Therefore, engineering the sail for dead upwind conditions would not make any sense. The same applies to designing a sail for a cruising boat or a smaller one-design boat like a Soling or a Flying Scott. The amount of analysis for these sails will not be as much because the uses are less complex, and the time spent doing the analysis adds to the cost of the sail. But some kind of analysis is still important to creating the best possible all-around sail.
 

Figure 4.1 When sailing on the wind, a high-aspect sail like a blade jib will have the bulk of the load travel almost directly up the leech of the sail with less stress along the foot.

Figure 4.2 A low-aspect sail will have the loads travel more toward the center of the sail rather than concentrated along the leech. It will also experience greater loads along its foot than the high-aspect sail.

Note that the loads in a sail are also affected by the geometry of a rig, and in particular the aspect ratio or height-to-width ratio of the sail. For example, when sailing on the wind, a high-aspect sail like a blade jib (Figure 4.1) will have the bulk of the load travel almost directly up the leech of the sail with less stress along the foot. A low-aspect sail like a No. 1 genoa, on the other hand, will have the loads travel more toward the center of the sail rather than concentrated along the leech. It will also experience greater loads along its foot than the high-aspect sail (Figure 4.2) By knowing where the loads fall and combining this information with the strength and stretch resistance of the individual yarns, the sail designer can begin to develop an overall picture of an optimal panel configuration.

As you will see in future chapters the art of aligning yarns along specific load catenaries has become a sophisticated process, far more complicated than in the early days when there were only a few ways to configure a panel layout. As noted in Chapter 3 many different styles of fabric are available, and a number of different construction techniques are also available. Fortunately, it’s not rocket science, and understanding even a little about how and why sails are built in a particular manner will go a long way toward helping you make the appropriate decisions the next time you are in the market for a new sail.

Mitre- and Cross-cut Sails
Before we look at some of the latest sail designs, however, it’s once again important to look to the past. As was the case with sail fabrics and yarns, if we understand how we got to where we are today, we will have some idea of where we might be going. Sail engineering is all about making good use of raw materials, both raw fibers and sailcloth.

Figure 4.3 Back in the days of square-riggers and trading schooners all sails were made with their panels laid parallel to the leech of the sail in what was referred to as a Scotch-cut pattern.

Figure 4.4 The panels meet in the body of the sail with adjoining panels cut at an angle to both warp and fill yarns on what was called a heavy bias.

Back in the days of square-riggers and trading schooners all sails were made much the same way, i.e., with their panels laid parallel to the leech of the sail in what was referred to as a Scotch-cut pattern. This was true for both square sails and triangular headsails, despite the face that they were subjected to markedly different forces. (Figure 4.3) Then in the middle of last century a company by the name of Ratsey and Lapthorn Sailmakers, based in Cowes on England’s Isle of Wight, realized that fill yarns had less stretch than their warp counterparts, and that this fact could be used to some advantage in terms of sail shape. Specifically, the company discovered that by rotating the fabric 90 degrees, it was suddenly able to achieve a moderate amount of leech control, something that had until that point eluded sailmakers. For mainsails, where two out of three edges are supported by rigid spars, they ran the fabric all the way across the sail from the leech to the luff in what came to be called a cross-cut pattern. For the headsails, which were only supported along the luff by a headstay, they ran the panels perpendicular to the leech, and perpendicular to the foot as well so that both parts of the sail would benefit from the stretch-resistant fill yarns. (Figure 4.4) The panels met in the body of the sail with adjoining panels cut at an angle to both the warp and fill yarns on what was called a heavy bias. Fortunately, the middle of most sails is a low-load area so this bias didn’t result in too much distortion, although it could be very difficult to get the sail to look good when there was so much opportunity for stretch. In the old days, when sailmaking was more art than science, sailmakers were often judged by how well they could sight and cut the mitre line.

Cross-cut Dacron sails are the most common sails seen on weekends as small cruising boats head out for a sail.

Figure 4.5 As sailmakers gained some say in the way fabrics were woven, they were able to get fill-oriented and balanced fabrics made that could be used to build increasingly efficient cross-cut sails, both headsails and mainsails.

Figure 4.6 Choose a fill-oriented fabric for a No. 3 blade jib where the loads run right up the leech of the sail.

As designs developed and sailmakers gained some say in the way fabrics were woven, they were able to get fill-oriented and balanced fabrics made that, depending on the aspect ratio of the sail, could be used to build increasingly efficient cross-cut sails, both headsails and mainsails. (Figure 4.5) For example, as Figure 4.2 shows, a low-aspect sail like a No. 1 genoa of the kind used on an old IOR racer has the loads fairly evenly distributed throughout the sail. Therefore, if you had to choose a single fabric it would likely be a balanced one. If, on the other hand, you were choosing fabric for a No. 3 blade jib where the loads run right up the leech of the sail, you would definitely choose a fill-oriented fabric. (Figure 4.6) As fabrics became more sophisticated and sailmakers gained a better understanding of their craft, sail designs improved, and the demands for better sails increased as well. The quest for light sails that did not stretch when they came under load remained a top priority for sail engineers.

A Case for Multiple Plies
The fact that the fabric along the luff of sails was overkill was not lost on sailmakers, but until they figured out that fabric could be plied, or made up of multiple layers stacked on top of one another, there was little they could do about it. Sails were engineered for the loads on the leech and the excess fabric strength at the front end (generally in a low load area) got a free ride. When I worked for Hood Sails in the early 1980s we had a lock on the maxi-boat market. Maxi boats back then were 80 feet long, giants for their time, and the sail inventories consisted of five or six headsails of varying sizes and weights, plus a number of other sails like trysails and spinnakers. Some of the boats carried 15 to 20 sails on board each time they left the dock to go racing, which represented a tremendous bulk. It did not make any sense to remove excess weight from the boat in the form of spare tools and unneeded toothbrushes when the bilge was stuffed with heavy, overbuilt sails. When they were wet, which was much of the time, it was even more of a problem.
 

Figure 4.7 The first designs had the second ply joining the single ply along an imaginary line that ran parallel to the leech.

Crescent-cut and Saw-tooth Sails
Partly out of a desire to save overall weight in the boat, and partly in an effort to engineer more efficient sails, sailmakers started to manufacture sails that had a relatively light base fabric, then added a second ply of woven Dacron along the high load area, i.e. the leech. This development took place in the early 1980s and initial forays into two-plied sails, or sails that had heavier fabric up the leech were not a total success. The point where the different fabric strengths came together, for example, was an area that caused problems since a crease or gutter quickly formed where the seam had to try to accommodate the contrasting stretch characteristics of the different fabrics. The first designs simply had the fabrics joining along an imaginary line that ran parallel to the leech (Figure 4.7). But as sailmakers began to recognise that there was more load toward the head and clew, and less in the middle of the sail, some started to end the second ply in a crescent shape (Figure 4.8), while others tried their own configurations with the result that the saw-tooth mainsail (Figure 4.9) and other aptly named sails became part of sailmaking jargon. In each case, the basic idea remained the same. Use heavier fabric where it was needed and keep the rest of the sail light.

 Another advantage of these two-ply sails was that they allowed the designers to combine fabrics in a sail. For example, they could use a balanced Dacron for the base fabric and a fill-oriented Dacron for the second ply up the leech. This way the yarns were being used to their fullest potential. They even tried three-ply sails, but the added expense of building the sails did not show commensurate gains in performance. After much analysis and trial and error, sailmakers found which fabrics could be plied and matched with others and that if they carried the transition point further into the body of the sail there was less chance of a problem occurring. It was a cumbersome process and sail engineers knew there had to be a better way. Fortunately, it was just about this same time that laminated fabrics began gaining a foothold in the industry.
 

Figure 4.8	Figure 4.9
As sailmakers began to recognize that there was more load toward the head and clew, and less in the middle of the sail, some started to end the second ply in a crescent shape (Figure 4.8), while others tried their own configurations resulting in the saw-tooth sail (Figure 4.9).

A Primer of Panel Layouts will continue in the August 2004 issue of Northern Breezes.

Brian Hancock is an expert in sails, sailmaking, and offshore ocean racing, having made a career as a professional sailor for almost three decades. He apprenticed at Elvstrom Sails in South Africa before leaving the country to sail around the world. In 1981/82 he sailed as a watch captain aboard the American yacht, Alaska Eagle in the 27,000 Whitbread Round the World race. Four years later he returned for a second Whitbread, this time aboard the British yacht, Drum. In 1989 he sailed as Sailing Master aboard the Soviet Union’s first, and by happenstance last, Whitbread entry, Fazisi. With more than 200,000 miles of offshore sailing to his credit Brian is uniquely qualified to write about sails and the business of making sails.

Brian also owned his own boat, Great Circle an Open 50 carbon-fiber, water-ballasted sailboat designed and built for single-handed sailing and Brian did a number of solo offshore passages. Some of his experiences are recounted in his book, The Risk in Being Alive, published by Nomad Press. These days he works on special sailing projects and writes for magazines around the world while raising a family in Marblehead, Massachusetts.