SYSTEMATIC STUDY OF THE HYDRODYNAMIC FORCES ON A SAILING YACHT HULL USING PARAMETRIC DESIGN AND CFD

In order to predict the velocity and attitude of a sailing yacht travelling in a given wind speed and wind angle, the hydrodynamic problem and the aerodynamic problem need most of the time to be decoupled. Two matrices are built to characterize the hydrodynamic and the aerodynamic behavior separately. Then a Velocity Prediction Program (VPP) interpolates the matrices and finds the equilibrium between the forces acting on the hull and appendages on one side, and the forces acting on the sails on the other side. This gives the velocity and attitude of the yacht depending on the wind speed and wind angle. Two main approaches are currently used to build the hydrodynamic matrix. The first method is to build a reduced or a virtual model with the proper hull shape and test it in a towing tank or a Computational Fluid Dynamics (CFD) program. This approach can lead to a very precise estimation of the matrix for a given hull shape, but it is time consuming and gives no indication on other possible hull shapes. The second method is to build and test various hull shapes and use this database to build analytical formulas describing the evolution of the hydrodynamic forces depending on the speed and attitude, but also on the hull shape, via several "shape parameters". During the early stage of design, numerous hulls are to be evaluated, and it is very valuable to understand the influence of the design parameters on hydrodynamic efficiency of the hull. Therefore, the second method should be much more efficient at this stage of the design process. The most used regressions have been provided by J.A. Keuning et al., based on the Delft Systematic Yacht Hull Series, [1], [2]. This work began in 1971; the sailing yacht hull shapes have changed a lot since then. The aim of the present work is to enhance these regressions, by using new shapes in the database and by adding new "shape parameters" to describe the hulls. A powerful loop driven by the commercial software Mode Frontier has been developed in order to build a database by means of CFD. Systematic morphing of the hull shapes, parallel computing, automatic meshing and automatic post-treatment will provide a large database in a relatively short time. The aim of the ongoing work is to improve the accuracy and sensitivity of the prediction of yacht hull performance during the early stages of the design process. The study will focus on flat water, steady predictions. The following results concern exclusively bare hulls, the interaction between the hull and its appendages will be treated separately.