Design Optimization of Subcavitating Hydrofoils for America's Cup Class Yachts

Cavitation on foils and appendages is a significant design consideration for sailing yachts like the America’s Cup AC75 monohulls and the SailGP F50 catamarans because they achieve speeds in excess of 50 kts. As speed or lift increases past cavitation onset conditions, the vapor cavities grow and can adversely affect lift and drag. For design optimization, it means we need constraints to control the minimum surface pressures on the foil, which we do using the Kreisselmeier–Steinhauser (KS) aggregation function. This approach works with our viscous fluid solver and gradientbased optimization toolchain. Multipoint optimizations with the cavitation constraint function were used to optimize hydrofoil sections over a range of flap angles, lift coefficients, and speeds starting from baseline ‘seed’ profiles that were considered good subcavitating profiles. Consideration of multiple flow conditions makes resulting profiles more robust against flow separation. We also explore the effect of plain trailing edge (TE) flaps (and flap chord length) as part of the section design where we optimize deflection angles in addition to the profile shape. Independent analyses of the optimized profiles using the XFOIL viscous 2D solver helped verify that the optimizer was not exploiting flaws in the computational models. Our results demonstrate that the multipoint, cavitation-constrained methodology works for gradient-based optimization of subcavitating hydrofoil sections with flaps. When combined with judgment and some expertise for setting objectives and constraints, this methodology can be used to design high-performance hydrofoils.