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www.sname.org/sname/mt April 2013 adjusted in three dimensions to match the complex curva- tures of a custom airfoil. e bespoke ber placement, in the case of 3Di, is deployed in the pre-forms or complexes before the material is located on the molds, but when the ber reaches the molds, the rest of the process remains sim- ilar to the classic 3DL molding and thermoforming strategy. e end result is that these modern sails are more aerody- namically rigid due to a more sophisticated structure and use of material. e surface and structure are now one; the two are fabricated simultaneously, and they are indistinguishable and inseparable. is unitary monolithic structure becomes a true monocoque. is new use of materials requires that the ber architecture will be based on a philosophy quite dierent to that of 3DL. is new structural typology is quasi- isotropic in nature, rather than load path. Fiber, and therefore modulus, runs in all directions, instead of only in the direc- tion of the primary loads. e resistance of the membrane to stretching is thus much improved. Along with this innovation likely comes another new real- ity in terms of what happens to a high-performance sail at the end of its life. With 3DL membranes and other string sails, the weak links in the longevity cycle are the lm and resin systems. Because of the fact that there is a lot more ber in the structure than is required for breaking strength, the ber tends to outlast the other constituent components. String sail membranes therefore tend to come apart rather than break, as the lms or adhesive degrade; this is generically referred to as delamination. 3Di is not a laminate sail, so there is nothing to come apart or delaminate. Lamination and the failure thereof arise because of the need to bond dissimilar materials to one another. In the case of laminate sails, one tries to bond adhesive and fiber onto film, which is inherently chal- lenging and always the weak link in a lm-based sail. As 3Di sails are ber and resin only, the lamination problem eectively disappears. e nished part is a consolidated structure of matrix and reinforcement, and while it is still early days for this product, it appears that it will not come apart as it ages. Progress and greater control We have progressed from using materials to produce cus- tomized sails, to now also producing our own customized materials. is gives us more control over those base constit- uent materials, and an almost unlimited number of material congurations to choose from for any individual aerofoil. Sail designers can choose from pre-preg lament tapes produced from Dyneema, Aramid, Carbon, or varying combinations of each. As an example, 100% Dyneema tapes can be clustered into corners and at load points for both greatly increased breaking strength and added exibility for furling. Stier car- bon tapes are located where required for shape holding and aerodynamic rigidly. In this way, there is an almost innite variety of pre-preg lament tapes that can be selected for the base materials of a given sail membrane. erefore, by carefully choosing the constitution of each layer, sail designers can prescribe exactly the mechanical properties of the resulting composite. Placing material only exactly where it is needed, on an as-needed basis, enables us to produce the lightest and most ecient possible structure for a given application. With our continuing investment not only in new materials, but in digital manufacturing strate- gies and sophisticated automation, the process 3DL started, whereby the distance between design and fabrication is nar- rowing, continues apace. MTJB Braun is the project leader for 3Di at North Sails Design Services, re- sponsible for the engineering and analysis of structures. The full three-dimensional mold of a sail design set up just before vacuum bagging and the thermal forming process.