Foils are essentially airplanes in water, and aerospace is an extremely diverse industry. You can focus on structures as I did, or be a software engineer developing flight control systems, or spend an entire career reducing the drag of a winglet. Admittedly, I’m no expert in fluid dynamics, designing lifting surfaces, or configuring aircraft. I never had to, I was always given surfaces (known as the OML) by aerodynamics engineers and had to work inward, fitting structural components and optimizing layups to meet load requirements and minimize weight. There’s also more of an art to fluids and aero, as predicting the flow of water or air over a surface at 10-600kts is extremely challenging, despite modern computing power. Structures is easy, so I had to work with people a lot smarter than me to figure out some of the hydrodynamic challenges of Project Cedrus.
The section shape of the mast was optimized for weight and stability, not speed. Thanks to my non-structural edges, and the tiny discontinuity in the surface due to their transition, early prototypes of Project Cedrus were prone to ventilation. Ventilation occurs when a small air bubble develops along the surface and builds, particularly at high angles of attack, before traveling down the mast and separating from the section causing loss of lift. Together with an America’s Cup foil designer, we looked at a number of different combinations of chord, thickness, and shape to create a mast that was fully optimized structurally and hydronamicalljy. It’s always give and take, because the optimal aerodynamic section is essentially nothing (infinitely thin and short), while the optimal structure is thick and hollow with a long chord length. But this has been the story of my career, whether at Boeing or Apple. Getting engineers and designers to sacrifice a little and converge on the optimal solution is challenging but very rewarding.
My meetings with Tom were never dull, and my iPad Pro is full of scribbles from our racing minds as we looked for ways to optimize the foil. Relying on his decades of experience, but also his 2D CFD capability (the aero version of FEA), I learned a lot. Turns out the drag increase of a squared off trailing edge is actually quite minimal, a small price to pay for something less likely to cut through your flesh like a sushi knife. Also that hum you can occasionally feel and hear while you’re riding calm waters is quite common among all hydrofoils, including sail and power. It’s due to vortex shedding off the mast causing the trailing edge to vibrate at high frequencies. It can happen to steel foils supporting thousands of pounds. Ventilation is a bit harder to predict, as it can be due to water surface conditions (much more likely in choppy water as air can be captured) or even current. Nonetheless, by increasing the chord length and reducing thickness, we were able to increase the angle of attack before separation, or stall, by 2-3x over my prototype. Manufacturing quality will also be improved, which is arguably a much bigger influence on ventilation than section shape.