High-Performance Sailing Aerodynamics: An investigation of the International Moth Class

Abstract

High-performance sailing is pending towards a large portion of foiling boats being sailed and built. To reach a maximum level of performance, the whole foiling boat needs to be optimised and this means optimising both hydrodynamic and aerodynamic parts. In the development of sailing classes such as the International Moth Class, the optimising of aerodynamic parts is mostly limited to trial-and-error assessments of equipment by sailors, designers, and manufacturers. There have been investigations into the stability and dynamics of the International Moth using Velocity Prediction Programs. However, these types of assessments lack the ability of seeing the influence of small design changes to the aerodynamic performance of the boat. To asses the impact of certain design changes to the aerodynamic performance of the International Moth, a parametric Computer Aided Design (CAD) model is build, which is then used in a Computational Fluid Dynamics (CFD) investigation. A simplified model of the Moth is built. Various parameters such as sail camber, aspect ratio and wing angle can easily be adjusted to investigate the aerodynamic effects of these different parameters. Furthermore, different shapes and sizes of a “deck sweeper” are analysed, as this is a widely developed part in recent years. The model is used to perform a parametric study of the aerodynamic performance in the CFD software STAR-CCM+. Some aerodynamic aspects that are looked at are tip vortices under the sails and endplate, as well as the optimisation of the lift to drag ratio. The CFD simulations are done for a number of apparent wind angles, to account for the effect of different parameters in different sailing conditions. Given that the cruise speed of cars is approximately 85 km/hour (23.6 m/s) and that the average car height is nearly 2 m, it can be observed that the flows induced by cars and sails are similar, where the sails cruise at around 20 knots (10.3 m/s), and the mast height is about 6 m. The setup of the CFD investigations for these similar flow conditions would be approached in the same manner. Thus, the methodology and findings explored in the present study are generally also applicable for the aerodynamic design of cars.