VPP Driven Parametric Design of AC75 Hydrofoils

Abstract

Hydrofoils are a vital part of modern racing yachts such as the AC75, which was sailed in the 36th America's Cup and should hence be optimised thoroughly. The literature shows that hydrofoil design and optimisation usually focuses on the lift and drag characteristics in isolation of the yacht 'system'. Although these characteristics relate to hydrofoil performance, they do not directly translate to the performance of the yacht on the race course. In this paper we perform a parametric study of the main design variables of the hydrofoil that is based on a model of the entire yacht in the Velocity Prediction Program (VPP) FS-Equilibrium. The hydrofoil forces are modelled using an advanced lifting line method and empirical formulations for a bulb. This accurately captures the foil design influence on the boat's performance. The VPP is coupled to a parametric model of the foil based on NURBS surfaces (Non-uniform rational B-Splines) which was used to systematically generate 72 different designs. The candidates were tested in three wind speeds for up and downwind performance. The best performing design has maximum span and anhedral angle, and minimum chord with some of the weight stored in a bulb. The study shows that the assessment of hydrofoils where the performance is measured in boat speed is an extremely valuable tool.