Sfoil

An S-foil, short for "slotted foil," is a type of hydrofoil designed to reduce drag and improve efficiency in watercraft propulsion and support systems. Unlike traditional hydrofoils, which have a flat or slightly curved surface, S-foils feature a pronounced "S" or sinusoidal shape along their chord, creating a series of alternating high and low-pressure regions. This design enhances lift generation while minimizing drag, making them particularly useful in high-speed applications.

S-foils were developed in the mid-20th century as an advancement over conventional hydrofoils, which often suffered from cavitation—where low-pressure zones cause vapor bubbles to form, leading to performance degradation. The S-shape helps mitigate cavitation by smoothing pressure distribution along the foil’s surface. This makes them ideal for use in fast boats, hydroplanes, and some underwater vehicles where efficiency and speed are critical.

The design of an S-foil typically includes a curved leading edge that transitions into the sinusoidal profile, followed by a trailing edge that may be straight or slightly tapered. The amplitude and wavelength of the "S" curve can be adjusted based on specific performance requirements, such as lift-to-drag ratio or cavitation resistance. Modern applications often incorporate computational fluid dynamics (CFD) to optimize their geometry for particular operating conditions.

Beyond propulsion, S-foils are also used in marine stabilizers, where they help reduce rolling and improve vessel stability at high speeds. Their efficiency has made them a preferred choice in competitive sailing, where reducing drag is essential for performance. While more complex to manufacture than traditional foils, their advantages in speed and energy savings have led to widespread adoption in both recreational and professional marine applications.