The Aero-Hydrodynamic Characteristics of Yachts Sailing Upwind in Waves

A common practice during the design of a sailing yacht is to assume an ideal environment. The wind is constant in intensity and direction and the sea is calm. In this situation, an equilibrium between the aerodynamic and hydrodynamic forces and moments can be found. The equations of equilibrium are thus solved to find the best setup (combination of sails, sail trim, etc.) in order to achieve the maximum boat speed. This procedure is the core of a 'traditional' Velocity Prediction Program (VPP), which was first introduced in 1976. Since then many improvements have been made and nowadays VPPs are capable of dealing with the most diverse designs. However, in nearly forty years of life, the nature of these programs has not changed and, still, important dynamic effects caused by the real environment, are not taken into account. Investigating the performance of a yacht taking into account these effects, is far from easy. Indeed, the equations of equilibrium should be replaced by the more complicated equations of motion which need to be solved in the time domain. However, in recent years, the dynamic characteristics of a yacht sailing in waves and shifting winds (real environment) have become a more popular topic in the sailing yacht community. In the wake of the increasing interest towards this subject, the present work aims to investigate some important dynamic aspects. The effect of the surge motion and of the heeling angle on the motions of heave and pitch, and on the mean added resistance, are studied by means of towing-tank tests. These are performed on two models of series 4 of the Delft Systematic Yacht Hull Series. Then, time-domain simulations are performed to study the effect of a pitch-induced oscillating aerodynamic force on the seakeeping of yachts sailing upwind in waves. The aerodynamic force is calculated using a quasi-steady as well as an unsteady method, whereas the hydrodynamic part of the problem is tackled using a strip-theory program which is capable of considering asymmetric hull shapes (e.g. hulls with a heeling angle). The results show that surge has a small influence on the motions and on the mean added resistance alike. The heeling angle on the other hand, prove to have a large effect on the seakeeping of the models. This effect can be qualitatively predicted by the strip-theory program, although the trend is not always correct. Finally, in same cases, the aerodynamic force significantly affects the pitch and heave motions and this results in a considerable reduction of the added resistance in waves.