Considerations of Hull Structural Deformation on Hydrodynamic Performance of Sailing Yachts

The increasing availability of affordable finite element modeling software and computational fluid dynamics codes have given today’s sailing yacht designers tools that, in the past, were only available to designers of yachts competing in the most high-profile events. This paper provides a case study on the structural design of the International America’s Cup Class Yacht Stars & Stripes USA-34, which won the U.S. defender series, the Citizen Cup, leading up to the 29th America’s Cup competition in 1995. The authors have recently been able to publicly share this information, and feel that the problem-solving approach is of direct use to many of today’s designers. The design approach is twofold, the first being the use of Finite Element Analysis to minimize drag-inducing local deformations. Over 80 finite element models were analyzed with varying configurations, starting with an initial baseline structure based on the previous boat, USA-11. These structural improvements reduced local hull deflection from5 to 6 mm per meter on the previous design to less than 1 mm per meter on the USA-34 design. The second part of the design approach isolates the hydrodynamic effects of global bending under rig tension. Illustrative tank tests are provided to demonstrate how the global sagging of the hull, caused by rig tension, can often increase the resistance of a yacht (especially one built for racing). As part of a series of steps taken to mitigate the effect of rig tension, a novel design was developed, with soft patches and flexible structure to permit low-cost modification of the hull in the boatyard, giving approximately a 225mm increase in waterline length and 75 mm reduction in beam if needed—a modification that was made to USA-34 prior to winning the finals of the Citizen Cup, and at a cost of under $500. The reduction of drag-inducing local hull form deformations, and the development of the cost-saving hull modification concept, demonstrate how a creative designer can take advantage of the finite element modeling tools that are now widely available.