SailGP yacht racing is an exciting sport combining top-level athletes with high-speed hydrofoil sailing technology and, believe it or not, these boats boil water!
SailGP F50 racing catamarans are the result of years of development in high-performance, foiling sailboat racing. The F50 was the first boat to achieve 57.5 miles per hour, that’s 92.6 km/h or 50 knots.
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So how do they go so fast? Well, one of the reasons is their hydrofoils. The latest foils used on F50 racing boats have been designed for another level of high-speed performance. These foils are incredible pieces of hydrodynamic and aerodynamic design and engineering. They enable an F50 catamaran to rapidly lift itself out of the water, producing minimum drag and maximum speed when racing. Foiling boats are now breaking sailing speed records on a regular basis.
Hydrofoils and fluid dynamics
All boats push water around and under their hull from the front to the back of the boat. This produces waves around the boat which form in two patterns, one at the front and one at the back. These are known as Kelvin Wave patterns which move at the same speed as the boat.
These waves create drag on the boat known as the wave-making drag. As the boat accelerates these waves get higher and longer, increasing the overall drag on the vessel. On top of the wave-making drag, the friction of the water further slows down the boat.
The term 'Hull speed' is the fastest the boat can travel through water. The speed a boat hull can travel through water is affected by wave-making drag. The hull speed of single-hull conventional sailing boats is very slow. For example, a similar length yacht to an F50 would typically only be able to achieve 12 mph.
However, foiling boats have been designed with small underwater wings, called hydrofoils. Hydrofoils reduce friction and wave-making drag meaning it's possible to produce much higher speeds. Hydrofoils are very similar to aircraft wings, they create lift through water in the same way an aircraft wing creates lift through air. By lifting the boat hull clear of the water there is significantly less drag and less friction slowing the boat down.
Water is much denser than air which means that boats only need small hydrofoils to produce enough lift to move the hull up and clear of the water. This is why foiling yachts are able to sail much faster than conventional yachts.
What is cavitation?
If you’re wondering what cavitation is, it’s the formation of bubbles in a liquid (water). Typically this occurs by the movement of a propeller through water but, in this case, by a hydrofoil. The foil slicing through the water causes a reduction of water pressure which makes the water around it boil, yes, boil! Cavitation creates air bubbles like in a kettle which increases drag and reduces the ability of the foil to lift the boat.
Here's what cavitation looks like. This still is taken from the lecture on Hydrofoil Cavitation, by Nicolaos Charalambous, referenced at the bottom of this page.
In this video, SailGP explains how cavitation can affect F50 racing.
SailGP F50 catamarans have high-performance hydrofoils that are designed to reduce the onset of cavitation. These improvements all work towards the boat moving significantly faster, as drag caused by cavitation is delayed until much higher speeds.
The crews of the SailGP F50s are constantly pushing their yachts to their operating limits in competition, pushing their hydrofoils to boiling point!
The technology of hydrofoils
If you're wondering how are sailing foils made, here's how F50 hydrofoils are manufactured.
Core Builders Composites in New Zealand designed and built the hydrofoils for the SailGP F50 catamaran. Core Builders design process is to create the foil shapes by computer, and then use a CNC engineering machine to carve the foil out of a block of tooling compound. Tooling compound is similar to a big block of plastic. The resulting carved 3-dimensional shape of a foil is a is called a ‘plug’.
They then make a mould from the plug using carbon fibre that is heat-treated and cured. When complete, the mould is used to create the foil.
The foils are constructed with stiffer (higher modulus) carbon fibre, this means the foils can be made thinner and have less resistance to the air (aerodynamic) and water (hydrodynamic). This enables the boats to reach much higher speeds.
During the manufacturing process, they layer carbon fibre into the mould to create a hydrofoil structure that is 250 layers thick. To make sure the carbon fibre solidifies correctly the foils are ‘cooked’ in their own heat-treatment stages. The foils are then machined, polished, and hand-finished to a mirror finish. The completed hydrofoil is then ready to be used on an F50.
The science behind how hydrofoils work is fascinating, and you can learn more in this article by Jonathan Ridley, Head of Engineering at Solent University. You can also find more references about hydrofoil science in the sources listed below.
Learn more about hydrofoil cavitation:
Modulus of Elasticity. Science Direct. Available at: Sciencedirect.com
Marine Rudders, Hydrofoils, and Control Surfaces. Anthony F. Molland Stephen R. Turnock 2022. Available at: sciencedirect.com
High-Speed Sailing. European Journal of Physics. 2018. Wolfgang Püschl. Faculty of Physics, University of Vienna. Available at: opscience.iop.org
Kelvin Wave Pattern. Carmen Lazzarotto. Available at: personal.math.ubc.caFluid Mechanics – Two Dimensional Laminar Boundaries. 2003. MIT. K. P. Burr, T. R. Akylas & C. C. Mei. Available at: web.mit.edu
Yacht Racing Life. 2018. SailGP F50 v America’s Cup AC50 catamarans. Available at: yachtracing.life What is a CNC machine and what role does it play in modern manufacturing? Goodwin University. 2018. Available at: www.goodwin.edu
Density effects, lift and drag, NASA
If you see anything in this article that’s incorrect or you want to add to it please get in touch, we’ll be happy to update it.