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

Cavitation can occur during takeoff at low speeds, making it essential to maintain an adequate speed threshold. In high-speed conditions, it is also important to push the limits to avoid constraints imposed by maximum speed. Typically, addressing these aspects resolves the central portion of the performance curve, which is generally not a limiting factor.

To manage cavitation effectively, we constrain the maximum mean coefficient of pressure (CP) using the Chryselmeyer-Steinhauser (KS) function. This aggregation function has been utilized by my team for various applications over the years due to its versatility.

The accompanying contour plot illustrates this aggregation process. The two red lines represent constraints, and their intersection indicates a discontinuity. While this poses challenges for Sequential Quadratic Programming (SQP), the KS function smooths out this discontinuity, providing a conservative yet effective solution. By tuning the parameter ρ in the KS function, we can achieve results that closely align with our optimization goals.

In a sample optimization scenario, the baseline exceeded the cavitation limit, but after applying the KS constraint, the results remained below this threshold. In this case, the tuning of the ρ parameter ensured that the optimization was not overly conservative.

For the AC 75 class boats, it is noteworthy that they can achieve speeds up to four times that of the wind.