Hydrodynamic Modeling of Sailing Yachts

Abstract

In modem yacht design geometric modeling is regarded to be directly related to the hydrodynamic performance of the shape of the hull and its appending elements -usually the keel, often with winglets, and the rudder. While the traditional way of shape design - i.e., draw­ing, model building, tank testing, modifying . - is both time consuming and expensive, a complementing ap­proach shall be discussed within this paper. The ap­proach is called hydrodynamic modeling since it tightly combines the hydrodynamic analysis and the geomet­ric modeling in the design process. It is based on ad­vanced Computational Fluid Dynamics (CFD) methods for flow field analysis and unique parametric modeling techniques for shape generation. The geometry of a yacht is entirely described via im­portant form parameters as discussed in detail by the authors at the 1999 CSYS. The canoe body of the yacht is modeled from a small set of longitudinal curves which provide all parameters needed for sectional design. The longitudinal curves themselves being created via form parameters, a fully parametric description of the hull is achieved which allows to create and modify the geom­etry in a highly sophisticated manner. The fairness of the shapes is an intrinsic part of the form generation procedure. Apart from the canoe body the keel repre­sents the most pronounced hydrodynamic design ele­ment, dominating lift and righting moment of a yacht but also causing a non-negligible resistance component called induced drag. Keel, bulb and winglets are also specified in terms of form parameters. An application of hydrodynamic modeling is given for an IACC-yacht. Formal optimization can be suc­cessfully employed to identify improved and, eventu­ally, optimal configurations. A reasonably small set of parameters (free variables) was selected and systemati­cally varied making use of a fully automatic optimiza­tion scheme. Two optimization examples are presented in order to demonstrate the potential of the approach: (a) the optimization of a keel-bulb-winglet configuration so as to find a minimum drag solution for a given side force and (b) the optimization of the bare hull with respect to wave resistance. The examples can be regarded as representative for both racing and touring yachts with draft restrictions and illustrate the methodology of hydrodynamic modeling.