From Clipper Ships to Sail Rockets: How Keels, Sails, and Foils Accelerate Sailing Speed

From the 1869 clipper Cuddy Sark to today’s high-speed hydrofoil yachts, this video explains how speed on the water is created and challenged. It traces the balance of thrust and drag, the hull-speed limit that governs traditional sailing ships, and the innovations that pushed racing into new realms. You’ll learn how copper sheathing reduced skin drag, why the Suez Canal shift reduced wind-powered ships, and how the America’s Cup fostered keel and hull innovations, culminating in the winged keel and modern foils. The story then moves to the Sail Rocket, a record-breaking project off Namibia that uses a cleverly designed sail, a self-ventilating foil, and cavitation control to push well beyond conventional limits. The video is presented by James Dingley of the Atomic Frontier.

Introduction: The Speed of Sails Through Time

The journey begins with the Cuddy Sark, launched in 1869, a ship that balanced massive sail area with a hull designed to reduce drag. The speaker explains how thrust from the wind on sails competes with water drag, and how the top speed of a vessel is where those forces balance. The Cuddy Sark’s 3,000 square meters of cloth across 32 sails produced an estimated 3,000 horsepower of thrust, yet speed was ultimately limited by hull dynamics and the sea itself. This section lays the groundwork for understanding how even with a strong breeze, a ship’s real speed is constrained by the interaction of wind, hull form, and water resistance.

"The point to which those two forces balance is known as your top speed" - James Dingley

Hull Speed and Hull Form: Why Speed Is Limited by Water

The video then dives into the two main forms of drag: skin drag, caused by water rubbing against the hull, and form drag, driven by the hull’s shape as water flows around it. The hull’s surface quality matters; copper sheathing reduced barnacle growth and skin drag, but sailors still had to scrub smooth hulls. Form drag is minimized by streamlining, but at the cost of cargo space, explaining why historical ships were not as sleek as today’s designs. The bow creates high pressure while the stern’s flow re-converges, effectively pulling the ship backward at high speeds. Hull speed is the practical speed limit for traditional hull forms, and pushing beyond it requires new ideas in hull shape and propulsion.

"A longer ship can go faster before sinking into its own bow wave" - James Dingley

America’s Cup and the Keel: From Pure Shape to Control and Torque

The narrative moves to the late 19th and early 20th centuries, where the America, Pilgrim, and later designs demonstrated how keel geometry could change performance. The America introduced a full keel for lateral resistance and righting moment, while 1870’s magic (the retrievable keel) reduced drag on downwind legs. By 1893 Pilgrim optimized torque by placing mass farther from the hull, creating a lighter, faster profile. In 1903 Daniel Hirschhoff attempted to bend rules with a longer waterline when he heel over, but rulers closed the loophole. The section emphasizes how keel design became a central battleground in the America's Cup.

"The board could be retracted to minimize drag" - James Dingley

The Winged Keel Breakthrough and the Foil Revolution

As hulls grew longer and rules limited waterline length, designers sought other ways to extract speed. In 1983 Australia II, Ben Lexson’s winged keel disrupted fluid dynamics and reduced drag while providing lift, enabling a stunning Cup victory after 132 years. This victory signaled a shift toward wing-inspired designs and foils, which would redefine competitive speed. The video then explains that foils work like airplane wings, generating lift with a side benefit of reducing hull drag. The keel’s lateral resistance remains essential, but foil-enabled boats can sail at angles to the wind to gain true speed.

"The winged keel proved to be superior" - James Dingley

Beyond the Cup: Cavitation and the Sail Rocket

The final act introduces the Sail Rocket, designed by Paul Larsson to chase speed in a controlled environment off Namibia. A carefully angled sail provides forward thrust and lift, while a hydrofoil underneath suppresses unwanted lift, effectively pinning the craft to the water. A self-ventilating foil is used to trigger and control cavitation, reducing drag at extreme speeds. The vessel demonstrates how engineers push the boundaries of what a sailing craft can do, going from traditional hulls to a vessel that can exceed 50 knots and approach 60 knots under specific conditions.

"Self-ventilating foil... reducing drag at these insane speeds" - James Dingley

On November 24, 2012, Larsson achieved a record of 65.45 knots, a milestone that has stood for more than a decade and inspired subsequent projects like SP80 and Sirico. The video closes by noting that today’s racing yachts still rely on wind, but use hydrofoils to glide above the waves, circumventing hull-speed limits, while the future may see even more radical concepts that redefine what a ship can be. The presenter signs off with a nod to the broader mission of Future Factual and the spirit of engineering exploration.