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How Hydrofoils Work and What They Do

  • by: Maxime Robert
  • On: 19, Nov 2018
3 min read

As discussed in our previous post on the basic principles of hull design, this initial factor can highly influence all subsequent design ideas and the ultimate performance of the boat itself. But Marine engineers consider other aspects when a racing yacht is in motion: they must work hard to enhance performance and make the boat efficient in different conditions.

What Is A Hydrofoil?

The lower part of the racing yacht should have foils to ensure that yachts are stable and do not fall. The faster the boats sails, the more efficient these foils must be. A foil consists of two parts: a tip and a shaft, they limit the amount of heel. The tips provide lift to the boat and reduce the wetted surface, reducing drag. The foils work similarly to those of an aircraft but must be optimised for water instead of air.
Foils also help the canting keel, which takes the weight off the hull. The thrust acting in the opposite direction increases with speed. Foils achieve their maximum efficiency at these conditions where the wind blows faster. In light wind conditions and when sailing upwind, foils have low efficiency.

How Does a Hydrofoil Work?

Foils also reduce the pitching of the boat. When sailing, force from the wind tries to push the boat sideways, so the opposite side of the board must resist that force to avoid collapsing. The design of the foil varies from a wide/narrow shaft/tip to a large/short shaft/tip depending on the conditions, type of boat and results of the technical studies prior construction.

There are three factors that determine the speed of a boat:
· Ideally the boat should be as stable as possible
· Crew reaction times must be managed properly, and
· The system that connects the crew with the boat must be tightly controlled and monitored.

Dynamic Challenges to Hydrofoils


Different forces also contribute to boat speed. When sailing, the boat experiences what is known as a righting moment. This is the result of the centre of gravity in the boat working against the centre of buoyancy when the wind pushes the boat to one side. When weight such as keels is added to the lower part of the boat, right lever increases, therefore righting moment is gained, and the boat is more stable.

However, there are two ways to increase performance, and foils work in both cases. Yacht performance is increased by lifting the boat out of the water, reducing drag and increasing speed. The second option to increase yacht performance is to increase the power of the foil and maintaining the drag at a consistent level. Additionally, small angles of attack are used on foils to optimise the lift to drag ratio.

Energy and Foil Configuration

From the energy point of view, the crew could balance the power generated by the foils, but the wind also provides energy potential. When downwind, more energy is received into the sails which directly increases the speed.

A T foil configuration has been used in the past to great success. However, recently a J configuration of foils has been used in the America’s Cup. The difference is that the foil shapes like a V, allowing the boat to be lifted at different heights as V changes with the boat’s speed.

When racing, teams must balance the height lifted because if the boat is too high and only a small part of the foil is immersed in the water, the boat can go sideways. Teams must ensure that the boat is stable and goes as fast as possible. Additionally, a good control system is needed to regulate the foils. A good system will automatically identify when the boat is outside certain parameters and articulate the angle of attack to maintain stability and determine the optimum boat flying height.

Materials Used in Hydrofoil Construction

When designing a hydrofoil it’s important to select the best materials. Hydrofoil materials should provide resistance, a light weight, a good shape and hardness. Different materials have been used in the past but those used most recently are: a core material that guarantees the desired shape, for example a core of foam, centre accessories such as aluminium to provide a solid connection and a lay up material (this could alternatively be carbon fibre which will wrap around the core).

Many America’s Cup teams use carbon fibre for their hydrofoil designs. Good technology is a key part of a team’s strategy to achieve positive results and increase performance. Foils have definitely helped boats to race better and have made the different between success and failure in many yacht races.

Discover the Basic Principles of Hull Design or Ways to Address the Marine Skills Shortage.

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