Hydrofoils Explained: The Science Behind Faster, Smoother Boating

Mention “foil” to most boaters, and they think of the crinkly aluminum stuff used to wrap lunch for a day on the water, or folded into a hat for receiving radio messages from the Pleiades. Today we’re not talking about bologna sandwiches or extraterrestrials, but the foils that make boating more enjoyable. There are more of them than you might think, starting with the blades on your propeller. Here’s a brief, and unscientific, intro to having fun with foils.

First, what is a foil? It’s basically a plate that develops lift when moved through a fluid, usually air or water. The design of any foil is determined by its intended use and the fluid in which it will operate. An airplane wing is the quintessential airfoil, shaped to maximize lift and minimize drag. Every science student is taught that an airplane wing works because when the wing moves through the air, the pressure on the wing’s curved upper surface is lower than that on the flatter lower surface due to the increased speed of the air moving across the top of the wing. The pressure differential causes the wing to lift toward the lower-pressure area, and the plane flies. Sails on a modern sailboat operate much like airplane wings. While a pilot changes the effective shape of the wing with the flaps, a skilled sailor can change the curvature of the sail to match the wind speed, direction and other variables.

The principle is the same for hydrofoils operating underwater, although they are shaped differently than airfoils because water is both much denser than air, and non-compressible: Unlike air, water cannot change its volume as the pressure increases or decreases. Instead, reducing the pressure can induce cavitation in a liquid. Bubbles form on the low-pressure face of the foil, reducing its efficiency—more on that in just a bit. Engineers shape hydrofoils for maximum thrust and minimal cavitation. But even a flat board can act like a hydrofoil when positioned at an angle to the fluid flow—the “angle of attack.” Foils, both air- and hydro-, work better with an angle of attack.

Props and Rudders

If your boat has a propeller, you’ve got foils. Prop blades are hydrofoils that rotate, and are very tricky to design for ultimate performance: When creating a propeller for a new boat, engineers consider the weight of the boat, its horsepower, its projected top speed, its engine rpm and gearbox reduction ratio, and the maximum permissible diameter of the prop; crunching all these numbers, and more, produces a rough idea of the correct diameter, pitch and number of blades. From there it’s a matter of fine-tuning to spec just the right prop. Sometimes it takes trial and error, too. Propeller design is a combination of hydrodynamics, physics and alchemy; sometimes alchemy is most important.

One thing engineers consider is the cross-section of the propeller blades. A propeller pushes the boat forward by generating lift on the low-pressure (forward) face of the blade and thrust on the high-pressure (aft) face. The blades need to work efficiently in only one direction—performance in reverse isn’t usually a consideration—so the blade shape can be asymmetric, often with a flat pressure face and a curved lift face. On a typical prop, the curve on the lift face is a section of a circle rather than an airfoil; this is called an ogival cross-section. The blade section is so thin and the curvature so slight, it’s often hard to tell what the shape is; the twist of the blade makes it even more difficult.

Some props have blades curved on both faces, others have airfoil blades that produce more lift. Sometimes the airfoil shape is too effective, and the low pressure so low it causes cavitation—the water boils along the back of the blade, increases vibration and drag and gradually pits and erodes the metal. The same situation occurs on the highly loaded propellers of super-fast sport- and raceboats; but rather than prevent cavitation, these boats spin surfacing cleaver props with wedge-shaped blades that throw off the cavitation bubble when the blade breaks the surface. Besides blade shape, a primary cause of cavitation is overloading of the blades: In that instance, the total area of the blades is too small relative to the horsepower delivered to the propeller. The solution is typically adding blades, so a muscular 40-knot sportfisherman might carry five- or even six-bladed propellers in order to increase blade area to solve the cavitation problem.

Propellers are complex; rudders are a lot simpler. Rudders are often just bronze plates, flat on both sides, but when rotated relative to the water flow they develop lift that pushes the stern one way or the other. They are inefficient hydrofoils but do the job because of the force of the propeller discharge flowing around them. (Try steering a powerboat when the engine’s not running; good luck not hitting anything.)

Fins, Foils and Stabilizers

Nobody likes excessive rolling when trying to enjoy a day on the water, so active stabilizers are becoming almost standard equipment on larger, more expensive vessels. (Passive stabilizers, like bilge keels, are affixed to the hull to increase roll resistance; they don’t move, hence “passive.” Ships often carry long bilge keels.) Gyrostabilizers are self-contained within the hull, but fin stabilizers use—you guessed it—hydrofoils to quiet the boat’s motion. Most stabilizer fins are shaped in cross-section much like sailboat rudders, and work the same way: When deflected from the water flow, they develop lift opposite to the direction of the roll. A complex control system senses the vessel’s motion and deflects the fins appropriately, via hydraulic or electric motors. Like rudders, stabilizer fins rely on water flow to develop lift, so aren’t as effective at low speed or at anchor.

The shape of stabilizer fins hasn’t changed much in decades—they are basically straight rudders mounted near the chines or the turn of the bilge. But today there are fins with more complex shapes whose manufacturers claim do an all-around better job at stabilizing than straight fins, including at anchor. Many racing sailboats carry J-curved, retractable fins—sailors call them “foils”—that replace ballast in offsetting the heeling force of the sails, keeping the boats lighter and faster; the fins also provide enough lift to reduce wetted surface, sometimes enough to lift the boat completely clear of the water. The curvature of the foil directs the force in the most effective direction.

Sleipner is using sailboat-inspired fins to build a better active stabilizer for powerboats. The company’s curved Vector Fins—which splay outwards, like the flippers of a sea lion when it’s on land—direct the lifting force almost vertically where, the company says, it’s most effective at roll damping. A straight fin’s force runs roughly parallel to the boat’s bottom, oblique to both the vertical and horizontal. Sleipner claims the Vector Fins are 20- to 30-percent more effective than straight fins on hulls with typical deadrise angles; the advantage increases as the deadrise decreases.

Sleipner also says the complex curves worked into the surface of Vector Fins create lift as well as fight roll; they are also thinner in cross-section compared to typical straight fins, and asymmetric—all factors that reduce drag, which in turn improves fuel economy. What’s more, Vector Fins reduce roll up to 50 percent when the boat’s at anchor, says the builder. Most fin stabilizer systems are minimally effective when the boat’s stationary. Vector Fins are electric, not hydraulic, and engineered to be quiet and energy-efficient. If all of the above is true, I guess you could say that Sleipner hasn’t been foil-ing around when designing the Vector Fins.

If you want to experience a modern hydrofoil but don’t want to invest in an expensive racing sloop, pick up an electric hydrofoil surfboard. Driven by a battery-powered propeller, their airplane-shaped foil generates enough lift to let you zip around the harbor supported by hydrodynamics. Or try hanging ten without a motor on an actual hydrofoil-equipped surfboard—either way, shredders call them “foilboards.” If you really want to get crazy, buy a foiling powerboat or powercat; we have reviewed several here at Power & Motoryacht in the past few years and more are on the way. Check our website, and then go out and have fun with foils.

This article originally appeared in the February 2025 issue of Power & Motoryacht magazine.

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