Anhedral foil

Engineering the Edge: The AC75 Foil Architecture Victory in the America's Cup isn't just about the crew's coordination; it's about the brutal physics occurring beneath the waterline. The AC75 class has pushed hydrofoil design into a realm once reserved for aerospace engineering. We are seeing a distinct split in philosophy between Emirates Team New Zealand (ETNZ) and the challengers. While the Kiwis have doubled down on the **T-foil** configuration, competitors like Luna Rossa Prada Pirelli and American Magic have leaned into **anhedral** or **Y-shaped** designs. This isn't aesthetic—it's a calculated gamble on lift-to-drag ratios and control stability. The T-Foil Advantage: Depth and Resilience Emirates Team New Zealand operates with a flat, deep lifting surface. This design keeps the foil’s core pressure center further from the surface, which is a masterclass in risk management. By maintaining depth, they achieve a superior **lift-and-drag coefficient**. The tactical brilliance here lies in the intersection angle. A T-foil enters the water at roughly 24 degrees, significantly mitigating the risk of **ventilation**—the catastrophic event where air is sucked down the foil arm, destroying lift. This allows ETNZ to sail with portions of the foil piercing the surface during upwind legs, shedding drag without losing flight stability. Anhedral Complexity and the Writing Moment The challengers' preference for **anhedral (Y-shaped)** foils stems from a different strategic priority: the **writing moment**. By using angled wings, teams can effectively increase the boat's leverage. The hull acts like it is "trapezing" further out from the center of pressure. However, this comes with a steeper technical price. These shapes create more aggressive flow acceleration at the junction points, creeping dangerously close to the **cavitation** limit—where water vaporizes due to low pressure, typically around the 50-knot mark. Strategic Trade-offs and Future Execution INEOS Britannia even experimented with a **W-shape**, attempting to blend the ventilation resistance of the T-foil with the span efficiency of the anhedral. Every design choice is a trade-off. A wider span reduces induced drag during takeoff but increases profile drag at high speeds. With a strict limit of only six foils for the entire campaign, a single design miscalculation can derail a multi-million dollar program. The winning team won't just be the fastest; it will be the one that mastered the mechanical engineering required to articulate these complex shapes under immense load.