First and foremost, a hydrofoil does not support the rider using buoyancy like a surfboard or a kickboard does. It actually flies through the water using the exact same physical force—lift—that allows airplanes to fly through the sky.
The front wing of a hydrofoil, submerged in the water, has a cross-sectional shape practically identical to an airplane’s main wing: curved on top and flat (or nearly flat) on the bottom. As it moves forward, a difference in speed is created between the water flowing over and under the wing, causing the pressure to drop above it and pulling the wing upward. This is lift. In other words, a foiler is less of a surfer and more of a “pilot flying underwater.”
How Does Such a Tiny Wing Lift an Adult? (Air vs. Water)
While an airplane’s wings are massive, a kitefoil’s wing is only about the size of a laptop. The main reason for this drastic difference lies in fluid density.
Water is approximately 830 times denser than air. If air were a porous, lightweight sponge, water would be a densely packed jelly. Both lift (the upward force) and drag (the resistance pulling back) are directly proportional to the density of the fluid. Therefore, a foil moving through water is in an environment where it is about 830 times easier to generate lift than in the air.
Moving your arms and legs in water feels heavy due to this high resistance, but by the same token, even a small amount of forward speed can generate immense upward force, allowing a tiny wing to easily lift a fully grown adult.
Comparing a Hydrofoil to a Jumbo Jet
Lift is proportional to “Fluid Density × Wing Area × Speed Squared.” To understand what it means for the physical equations to balance out perfectly, let’s run a thought experiment comparing a kitefoil with a Boeing 747 Jumbo Jet.
- Jumbo Jet (in Air): Weight: ~400 tons / Wing Area: ~510 ㎡ / Takeoff Speed: ~290 km/h
- Kitefoil (in Water): Weight: ~85 kg (Rider + Gear) / Wing Area: ~0.1 ㎡ (1000 ㎠)
1. Shrinking the Jumbo Jet’s Wing
Imagine tearing off the massive wings of a 400-ton jumbo jet and replacing them with a tiny 0.1 ㎡ kitefoil wing. The wing area drops to 1/5100th of its original size. Unsurprisingly, it lacks the lift to fly in the air.
2. Moving to Water and Dividing by Density
Next, let’s change the environment from air to water. Because water is about 830 times denser than air, this becomes a powerful advantage for generating lift. If we take the massive disadvantage in wing size (5100x smaller) and divide it by the advantage in fluid density (830x denser), the deficit in lift shrinks to only about 6.14 times (5100 ÷ 830). Since lift is proportional to the square of the speed, we just need to compensate for this deficit with speed (the square root of 6.14 is roughly 2.48).
Takeoff speed 290 km/h × 2.48 = approx. 720 km/h. Amazingly, even a 400-ton block of iron could fly using a single laptop-sized wing, provided it was dragged underwater at 720 km/h (close to an airliner’s cruising speed).
3. Adjusting Back to Human Weight
Finally, let’s adjust the weight from a 400-ton jet back to an 85 kg human. The weight is now about 1/4700th of the jet. With 4700 times less weight to lift, the required speed drops by the square root of 4700, which is about 1/68th. Divide our underwater 720 km/h by 68, and we get… approximately 10.5 km/h.
The fact that a foil effortlessly lifts off the water at the speed of a fast walk or light jog (around 10-15 km/h) is not magic or a random coincidence. It is the direct result of the exact same fluid dynamic equations that keep a jumbo jet in the sky balancing out perfectly.
The Triangle of Wing Size, Speed, and Drag
Foilers constantly change their front wing sizes based on wind conditions and their riding goals. The strict rules of fluid dynamics apply here as well.
Both lift and drag are proportional to “Wing Area” and “Speed Squared.” The golden rule is: if speed doubles, both lift and drag quadruple.
- Large Wings (e.g., 1500-2000 ㎠)
Due to their large surface area, they generate ample lift at low speeds, making it easy to fly in light wind. However, water drag is also high. If you try to go fast, it feels like hitting a wall of water, limiting top speed. Pushing the speed too far generates excessive lift, causing you to lose control and breach the surface. - Small Wings (e.g., 500-800 ㎠)
These offer minimal water resistance. Once up to speed, they slice through the water like a scalpel, generating explosive speeds (racing kitefoils can exceed 70 km/h). The trade-off is that they generate zero lift at low speeds. Getting up on the foil requires strong wind power or advanced pumping techniques.
Feeling these dynamics firsthand and controlling the trade-off between lift and drag to find the perfect setup for the day’s conditions is the true joy of foiling.
The Jetfoil: Lifting a Massive Ship
This foil principle has been scaled up to lift massive ships, most notably the “Jetfoil” (hydrofoil ship), used as high-speed passenger ferries in Japan and elsewhere. Coincidentally, this was also developed by the aerospace manufacturer Boeing.
Normally, as a ship goes faster, wave-making resistance increases exponentially until it hits a speed limit. However, a Jetfoil uses gas turbine engines to blast seawater backward (waterjet propulsion), rapidly accelerating to 80 km/h. This intense speed generates immense lift on the submerged foils, lifting the entire 160-ton hull completely out of the water.
Once airborne, the hull is immune to wave resistance, creating a magical ride that doesn’t rock even in rough seas. This, too, is a triumph of mathematical calculation using wing area and high-density fluids.
Does Water Temperature Affect Foiling?
To conclude, let’s look at a slightly geeky physics question: “Does the foil behave differently in the freezing winter sea compared to the warm summer ocean?”
The short answer is: “Theoretically, you get more lift in the cold, but drag also increases (making the water feel ‘heavier’).” There are two reasons for this:
- Density Change: Water becomes denser (heavier) as its temperature drops. Water at 10°C is about 0.4% denser than at 30°C, so theoretically, cold water provides a tiny bit more lift.
- Viscosity Change: This has a much larger impact. As water temperature drops, its kinematic viscosity increases. Water at 10°C is about 1.6 times more viscous than at 30°C. On a microscopic level, cold water is “stickier” and “slushier.” This increases the skin friction drag clinging to the wing’s surface, limiting top speed and making the water feel stiff or heavy.
When top racers say “the water is heavy in winter,” this change in viscosity is the physical reason why.
Around your tiny carbon wing, factors like fluid density, viscosity, wing area, and the square of your speed intertwine in real-time, continuously generating the beautiful lift that defies gravity every time you ride.
Search Aliases: Why Do Hydrofoils Float? Unraveling the Physics of Flying on Water, WhyDoHydrofoilsFloat?UnravelingthePhysicsofFlyingonWater