Aren’t you changing the boat? Get new sails! First installment

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Rolex Swan Cup 2014Changing the sails of your boat is like replacing the engine in your car. So if you have decided to replace your old sails and make new ones to renew the “thrust” of your boat, it is useful to take stock of what the sailmaker market offers today. A general overview allows you to understand where technology has gone today in the area of sailmaking, but more importantly it helps you understand what the sailmaker can do today to design the sail tailored to your needs. We are obviously proceeding on a different plane than the megagalactic projects of the America’s Cup and VOR, where there are incredible resources in the field, aiming to pull the design to the limit to design the sails of the future. We are in everyday life, in which the sailmaker has to deal with the production, technologies and materials that enable him to build a certain type of sail.

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OUR EXPERT FABIO VITALI
We asked Fabio Vitali, sailmaker at Elvstrom Sails, an expert in practical sail design and construction since 1979, to guide us on this journey. “People who have a boat should understand what is behind the construction of their sail. And be aware that renewing the sails can be a way to breathe new life into the boat with a custom-designed project”-he explains. – “Today, the sailmaker’s life has improved; I started making sails by laying a string on the ground to mark the outline and then unrolling horizontally cut fabrics over it. The sails were designed parametrically. Today, however, we have computerized programs at our disposal, which have revolutionized the way we work by being able to minimize error rates not only in terms of shape, but also in terms of structure.”

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HOW THE SHAPE OF THE SAILS IS BORN geometric_profile


Sail shape is the system that generates the force that moves the boat and is made up of the combination of a geometric and an aerodynamic profile. The geometric profile is set by the boat’s designer and corresponds to the measurements of the mast, boom, shrouds, catenary, etc. “When a customer arrives,” Fabio Vitali explains to me, “the boat is the one described by certain measurements of its geometric profile. To give an example: the situation is similar to going to a tailor to have a suit made. He will never ask you to shorten your arms. Similarly, the geometric profile of a sail, as if it were a dummy made from the mast, boom and shrouds, is entered as is into an aerodynamic model that generates a three-dimensional sail plan through computer programs. Walls, ashlar and pen are the supports on which the sailmaker will ‘spread’ the sail, which will thus take exact measurements: the three-dimensional shape is in fact dimensionless, thus only percentually similar to the real one. The great revolution of these computer programs is that once you have an aerodynamic profile drawn, you can apply it to different media even if they have different sizes, because the program automatically changes the dimensions of the 3D aerodynamic model.” To summarize and further clarify the concept: the aerodynamic and dimensionless sail shape, based on appropriate and mathematically repeatable models, is applied to a support that is dimensional thus taking its measurements (mast curvature, luff and leech length, etc.). Continuing the comparison with the work of the tailor: it is as if the latter has a magic wand that can make any dress take the specific measurements of the person who is going to wear it. The greatness of these programs lies right here, in not having to redesign the airfoil of the wing from scratch every time. “At this point,” Vitali continues, “I have my three-dimensional sail plan that allows me, for example, to go and see how close the genoa comes to the spreader, and if necessary implement adjustments. So each boat model has its own three-dimensional sail plan model to which specific measurements are applied.”

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WHAT INFLUENCES SAIL SHAPE

The forestay catenary and mast curvature are two elements that influence 70 percent of a sail’s shape, because they affect the final result in the arrangement of the “fat” on the surface. “For a sail to work well it should never be too fat, at most a little thinner than it should be, because over time it will tend to warp. Before ordering a new set of sails, it is important, therefore, to make sure that when taking measurements the mast is in its correct position, as all masting affects the shape of the mainsail and genoa. A sail only works well if it is designed on a certain sail plan.”

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THE FLUID DYNAMICS PROGRAMS
Let’s take this process of sail profiling a step further. Once an aerodynamic model is applied to a geometric profile, I can fit my three-dimensional sail into a virtual wind to observe how much power is generated by it. In this way, the areas of the sail that are subjected to the greatest stress will be evident: thus, thanks to these simulations, it is possible to further improve the sail plan. “When I design a sail, it is static: it is therefore important to have the ability to know what happens when the wind blows on that particular sail shape that I have designed. With a fluid dynamics program I can know how that sail I have designed deforms, and thus get the so-called ‘in-flight’ sail shape. I have to be clear about what I want to get out of a sail: for example, that a mainsail should be at 7 or 10 percent in terms of fat. When I know where a sail is deforming, I go to see where the highest stress points are and try to contain them. Where are the material elongations and figure out which ones to use. These programs make it possible to minimize errors, not only in terms of form but also in terms of materials.” This is where the structure of the sail comes in.

 

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