Wingsail is the propulsion mean adopted by the America’s Cup and C-class catamarans. This rig has improved aerodynamic performance with respect to conventional soft sails enhancing the yacht performance. However due to the higher forces acting wingsails, the yacht stability can easily be compromised especially in heavy gust wind conditions. The wingsail response to a gust has been then investigated performing numerical analysis in a C-class catamaran in downwind navigation conditions. Both a LES and an unsteady RANS approach were used for the simulations. The solutions given by the two approaches have been compared analyzing both the aerodynamic coefficients and the flow characteristics. The effect of the wind gust on the wingsail has been further investigated at different gust frequencies. Stall cells appear on the flap surface when the gust is taking into account affecting the wingsail aerodynamic performance.

A Reynolds-Averaged Navier Stokes computational fluid dynamics (RANS-CFD) package will be one of the primary tools used during the development of a performance prediction program for Wind- Assisted commercial ships. The modelling challenge presented by large separated flow structures in the wake of the sailing ship points to a conscientious validation study. A validation data set, consisting of hydrodynamic forces acting on the ship sailing with a leeway angle, was collected at the Delft University of Technology towing tank facility, for bare-hull and appended cases. Four hull geometries were selected to represent of the Delft Wind-Assist Systematic Series. Appended cases were designed to represent a broad range of appendage topologies: Rudder, Bilge-keels, Skeg, and Barkeel. The direct validation exercise for the bare-hull case was successful, with the validation level for the sideforce equal to 9.5% (fine mesh: 9M cells). An extended validation statement is made for simulations for the entire series. This exercise was successful for leeway angles equal to = [3 , 6 ]. The validation level (base mesh, 3M cells) for each force component is: ′=12%, ′=17%, ′=10%. The validation for appended geometries was not regarded as successful, with the exception of the Rudder case. The numerical uncertainty is the dominant contribution for the validation level, motivating a proportionate refinement of the grid. Here, it is sufficient to achieve parity with other contributions to the uncertainty within the larger context of the project.