A hydrofoil is a wing-like structure mounted beneath a boat that lifts the hull out of the water as speed increases. Once airborne above the surface, the vessel experiences dramatically less drag, allowing it to travel faster and more efficiently than a conventional boat. The same word refers to both the underwater wing itself and any watercraft equipped with one.
How Hydrofoils Generate Lift
Hydrofoils work on the same principle as airplane wings, just in water instead of air. The foil has a curved shape that forces water flowing over its top surface to move faster than the water passing beneath it. Faster-moving fluid exerts less pressure, so the pressure difference between the top and bottom of the foil creates an upward force: lift. As the boat accelerates, that lift increases until it’s strong enough to raise the entire hull clear of the water.
This is where the real advantage kicks in. A traditional boat sits in the water, and the faster it goes, the more drag it fights against. Water is roughly 800 times denser than air, so pushing a hull through it at high speed takes enormous energy. A hydrofoil flips this relationship. As speed increases, the hull rises above the surface, and only the thin foils and their support struts remain submerged. The boat’s contact with water shrinks to a fraction of what it was, cutting drag significantly and letting the same engine push the vessel faster on less fuel.
Surface-Piercing vs. Fully Submerged Foils
There are two main hydrofoil designs, and they handle stability very differently.
Surface-piercing foils extend through the water’s surface at an angle, like a V or U shape. As the boat rises, less of the foil stays submerged, which naturally reduces lift and prevents the craft from climbing too high. If the boat tilts to one side, the deeper foil generates more lift and pushes that side back up. This self-correcting behavior means surface-piercing designs don’t need electronic control systems. They’re mechanically simple and inherently stable in both pitch (nose up/down) and roll (side to side).
Fully submerged foils stay entirely underwater at all times. The struts connecting them to the hull are just structural supports and don’t contribute lift. Because the foils never break the surface, they can’t self-adjust the way surface-piercing designs do. Instead, they rely on sensors and actuators that constantly change the foils’ angle to respond to waves, speed changes, and shifting weight. This added complexity pays off in rough water, where submerged foils can maintain a smoother ride by adjusting dozens of times per second beneath the chop.
Fuel Savings and Performance
The core promise of a hydrofoil is efficiency. By lifting the hull free of the water, the vessel burns less fuel to maintain the same speed, or it can go faster on the same amount of fuel. The exact savings depend on the vessel’s size, hull design, sea conditions, and speed. Some newer commercial designs combining hydrofoil technology with cleaner engines have demonstrated fuel savings of around 12% compared to conventional setups, with additional gains possible depending on wave conditions.
Speed was always part of the appeal. In 1919, Alexander Graham Bell and his collaborator Casey Baldwin set a world water speed record of 114 km/h (about 71 mph) in their HD-4 hydrofoil on Bras d’Or Lake in Nova Scotia. That record stood for over 40 years. While today’s unlimited water speed records belong to jet-powered craft (the current mark is 511 km/h, set in 1978), hydrofoils remain among the fastest practical vessels on the water, particularly for passenger ferries and military patrol boats.
A Brief History
Italian inventor Enrico Forlanini built the first hydrofoil boat in 1906. His design used a “ladder” configuration with multiple horizontal wings stacked between vertical struts beneath the hull. Powered by a 60-horsepower engine driving two counter-rotating propellers, it reached 42.5 mph during testing. Bell and Baldwin refined Forlanini’s ladder concept into the HD-4, which set the speed record mentioned above.
By the 1960s, military interest surged. Multiple countries developed hydrofoil warships, and the U.S. Navy partnered with Boeing to create the Patrol Hydrofoil Missileship. Around the same time, aeronautical engineer Walter Woodward built the first water-ski hydrofoil, a towed recreational version. Frazer Sinclair was the first person to ride it, marking the start of towed hydrofoiling as a sport.
Where Hydrofoils Are Used Today
Hydrofoil technology shows up in three main areas: commercial ferries, competitive sailing, and personal watercraft.
Passenger ferries remain the most visible application. In the Mediterranean, Liberty Lines operates an extensive hydrofoil ferry network connecting Sicily, the Aeolian Islands, the Egadi Islands, Calabria, and ports along the Istrian coast including stops in Croatia, Slovenia, and Italy. Similar services run in parts of Asia and South America. For island routes where speed matters and distances are moderate, hydrofoils offer faster crossings than conventional ferries.
In competitive sailing, hydrofoils have transformed the sport. America’s Cup boats now fly above the water on foils, reaching speeds that would have seemed impossible a generation ago. Olympic sailing classes have adopted foiling dinghies and catamarans. The visual effect is striking: a sailboat appearing to hover above the surface with only its foils touching the water.
The newest consumer application is the eFoil, an electric-powered surfboard with a hydrofoil mounted underneath. Riders stand on the board and control speed with a handheld remote. Most eFoils cruise at 25 to 35 km/h, with premium racing models topping 50 km/h. Battery life ranges from about 60 to 120 minutes depending on how hard you push it. At a relaxed 20 to 25 km/h, two hours of ride time is realistic. At full throttle, expect under an hour. Wing size matters too: beginner wings keep speeds in the 20 to 30 km/h range for stability, while smaller race wings allow 40 to 55 km/h for experienced riders.
Limitations and Trade-Offs
Hydrofoils aren’t ideal for every situation. They need a minimum speed to generate enough lift, so at low speeds they behave like any other boat, just heavier because of the extra hardware beneath the hull. In very shallow water, the foils risk striking the bottom. And while submerged foils handle rough seas well, extremely large waves can still overwhelm the system or force the vessel to slow down and settle back onto its hull.
Maintenance costs run higher than conventional boats because the foils, struts, and (in submerged designs) control systems add mechanical complexity. Foils can also accumulate marine growth or sustain damage from debris, both of which degrade performance. For smaller recreational craft, the learning curve is real. Riding an eFoil or sailing a foiling dinghy requires adjusting to a completely different feel, since small weight shifts produce large changes in height and attitude when you’re balancing on a wing underwater.