Over the recent years the Wolfson Unit has seen a greater impetus from yacht designers and their clients to quantify and compare the seakeeping qualities of their sailing yacht design choices. Modern high performance yachts, fitted with a wide range of appendages generating lift and creating large moments, provide a number of complex challenges for designers. Assessing seakeeping behaviour and performance in a seaway is, indeed, important during the design process since the motions cause unsteady effects on the yacht hydrodynamic characteristics, for instance on the lift generating capabilities of the appendages. Hence it may not be justifiable to assume during the optimisation process that the yacht outperforming other design candidates in calm water would also perform well in waves. Therefore, the Wolfson Unit developed an innovative experimental model testing approach that would be an improvement over existing methods, simulating the 6 degrees of freedom motions and accelerations. The unique un-restrained sailing test approach uses a mast mounted air screw device to simulate the aerodynamic propulsion from the sails allowing a scaled model of the yacht to be tested at a range of conditions, sea states and wave directions (from head seas to following waves). These tests can be used as a comparative tool to assess controllability and seakeeping characteristics of multiple configurations (e.g. hull shape, appendages, inertias) by quantifying induced motions and providing an estimate of added resistance in waves. Non-linear attitudes such as surfing can be investigated. Free-running model testing is a technique frequently used in the development of power vessels, but little adoption is made in the sailing yacht world. Furthermore dynamic and seakeeping studies are at present challenging for computational fluid dynamics based tools, encouraging an experimental based approach. This paper introduces an un-restrained model testing method on sailing yachts. Discussion will also be made on how this new method can be implemented in design decisions and add value during the design and performance evaluation process for sailing yachts.

A three-dimensional panel method developed for the prediction of the seakeeping properties of conventional ships has been extended to predict the motions and added resistance of IACC Yachts. The method solves the three dimensional unsteady potential flow around the yacht in monochromatic oblique waves. Predicted quantities include the heave and pitch motion amplitudes and phases and added resistance over a broad range of wave frequencies yacht speeds. Computations have been carried out for a series of IACC hull shapes studied by PACT (Partnership for America's Cup Technology) and correlations with experimental measurements are found to be very satisfactory. The same method was also used to study the added-resistance properties of hull shapes supplied by the America3 Foundation. A sensitivity analysis was carried out of the added resistance on the principal yacht hull shape parameters, including the slenderness, displacement, LCBLCF separation and pitch radius of gyration.