What is a hydrofoil?
A hydrofoil is a glass fibre or carbon built wing shaped fin that sits mounted on a mast underneath the board. As the board moves forward driving the foil through the water it begins to generate lift and as speed increases, so does the lift, that finally sees the board come up out of the water.
Wing foilers, kite foilers and downwind or surf foiling all work using the principles of Bernoullis Law, but the foils themselves vary in size and style, delivering different performance, speed and agility.
How does the foil work?
Based on Bernoullis Law, a foil generates lift as it travels forwards through the water.
Water pressure
As the foil drives forwards through the water, there is a difference in pressure on the top and bottom of the foil, due to the differing lengths of the surfaces. As the water has to travel faster over the upper surface compared to the bottom surface, the molecules of the water are effectively of a lower pressure, which means the higher pressure molecules on the bottom drive the foil up in the water.
The science behind foils and Fluid dynamics
Let’s delve into the theories of Bernoulli’s Equation on pressure and flow, The principle is derived from the conservation of energy. It says, that “in a steady flow, the sum of all forms of energy in a fluid is the same at all points that are free of viscous forces.” The sum of potential, kinetic and internal energy must remain constant.
Bernoulli’s Equation: Po = P1 + ½rv1² + rgy1 = P2 + ½rv2² + rgy2
| Variables | Units |
| Po Stagnation Pressure | [Pa] or [lbf/ft2] |
| P Pressure | [Pa] or [lbf/ft2] |
| r Density | [kg/m3] or [lbf/ft3] |
| V Velocity | [m/s] or [ft/s] |
| g Gravitational Constant | [m/s2] or [ft/s2] |
| y Height | [m] or [ft] |
 |
| Detail of Hydrofoil: a) Pressure Profile b) Momentum Transfer c) Circulation d) Streamlines |
This equation applies to flow along a streamline under the following assumptions: the flow is inviscid, incompressible, steady, irrotational, and influenced by conservative body forces. Additionally, the height difference across the foil (the distance from the lower to the upper section) is small enough that the term ρgy2−ρgy1\rho g y_2 – \rho g y_1 can be considered negligible compared to the differences in the other terms. What remains is that the pressure, plus half the density times the velocity squared, equals a constant (the stagnation pressure).
As the velocity along these streamlines increases, the pressure decreases—a concept that will be significant later. The fluid over the upper surface of the foil moves faster than the fluid below. This occurs partly due to viscous effects, which result in the formation of vortices at the foil’s trailing edge. To conserve angular momentum generated by the counterclockwise rotation of these vortices, there must be an equal and opposite momentum exchange at the trailing edge. This interaction creates circulation of the fluid around the foil.
The vector summation of velocities leads to a higher velocity on the upper surface and a lower velocity on the lower surface. According to Bernoulli’s principle, this velocity difference results in a pressure drop on the upper surface, which is essential for generating lift. As shown in the diagram, the net force, calculated as the product of pressure and area, acts upward.
Faster speeds
The faster the foil drives through the water, the greater the pressure difference, which helps create greater lift. at a certain speed, the lift forces are greated than the weight of the rider and board above. This is why it is so important to make sure you are using the right size foil and style of foil for the conditions and wind or wave speed.
A reduction in drag
Once the hull of the board is out of the water there is a significant reduction in drag which means the power that was used to gain lift can be reduced to maintain flight as the speed is already in place to maintain lift and as foilers, be it pump, wing , kite or surf, we can use foot pumping to increase the energy, with good effect and this is even more apparent with longer foils such as pump foils.
Benefits of using a hydrofoil
As we’re considering foils for watersports, such as kiteboarding, winging, surfing and downwinding we’d consider the main benefits as follows:
- Wind speed – compared to sports like windsurfing, wnig foiling can be done successfully in lower winds of around 15 knots. A foil will be able to lift and fly easily compared to a board that needs more wind to drive it onto the plane.
- The great thing with foiling is you can do it in places like harbours,were sports like kiteboarding are banned. We do a lot of winging down here in Chichester Harbour and other spots around West Wittering.
- Foils give a smoother ride, less knee jarring and more relaxed, so may be more comfortable if you have knee problems.
Conclusion
Now you understand how a hydrofoil works you can see that it has certain advantages over normal board sports on the water.
That said, there is a learning curve and risks as with any new sport and suffice to say you will need to put effort in to learn to use them properly.
We’d recommend taking a wing foiling course if you’re starting out as that will give you a good insight into winging. It will cover everything from wind to safety and first flights and will make more sense after reading this article on how foils work.
On the other hand, foiling is useless as is winging when the wing really drops, so it’s worth considering windsurfing if you’re after doing a windsport in low or high wind conditions.
