How do Hydrofoils work?

Introduction

I’ve always wanted to swim since I was a kid. I tried learning through summer camps but never made it as I was afraid of water back then. And also, surfing has been on my bucket list for a long time!

My experience using a hydrofoil will be different as I now know how it works. And so will be yours!

What is a Hydrofoil?

Figure 1: Hydrofoil with the board above the waterline

Hydrofoils are surfboards with an inverted T-shaped structure attached to the bottom, which helps the board fly above the waterline beyond a certain speed. Note: The T-shaped structure is submerged in the water as seen in Figure 1.

Why fly above the waterline?

There is lesser drag force and friction as the entire surface area of the board is not in contact with the water like in a regular surfboard. In this way, you can travel at higher speeds. This technology has been used in several passenger boats for decades.

How do they fly above the waterline?

Newton’s third law: Any force has an equal and opposite reaction.

Hydrofoils achieve lift similar to how an airplane does by traveling at a high speed (horizontal thrust). The foil wing at the bottom of the T-shaped structure is fully submerged in water most of the time. They are aerofoil-shaped similar to an aircraft wing.

Figure 2: T-shaped structure with its foil wings shaped like an aerofoil.

So when the hydrofoil is at a high speed with the surfboard pointed upwards at an angle of attack of around 3 to 4 degrees, the water is deflected downwards at an angle due to the aerofoil shape of the foil wing. This in turn causes an equal and opposite force thereby pushing the foil wing upward, effectively driving the board over the waterline. Lowering the angle of attack reduces the lift, which can be achieved by pressing the board down with your front leg.

Figure 3: Hydrofoil in action

Types of hydrofoils

Hydrofoils can be classified into two types.

Figure 4: Efoil with a fan at the base

1. E-foil: These are electric and have a propeller fan at the base like in some boats. The fan rotates with the help of a motor to apply a horizontal thrust.
2. Regular hydrofoil: You can get towed behind a boat for an initial push and keep catching the waves to not lose speed and continue surfing above the waterline. Or you can also pump the board using your legs to use them in flat water.

And this is exactly how hydrofoils work!

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