The America’s Cup is one of the most prestigious and captivating sailing competitions in the world. It brings together the best sailors, engineers, and designers to pursue perfection on and off the water. The boats competing in this race are more than just sailboats; they are flying machines that test the limits of aerodynamics and hydrodynamics.
Consider a boat weighing 6.9 tonnes, measuring 21 meters long and 5 meters wide, and capable of reaching speeds of 100 kilometres per hour. This may sound like science fiction, but it is a reality thanks to the AC75 boats used in the America’s Cup. These boats can rise more than a meter above the water, powered solely by the wind. How is that possible? The secret is in the incredible foils that allow these boats to “fly” over the water’s surface.
The Science of Foiling
The key to understanding how AC75 boats fly is to examine their foils, which are essentially wings beneath the water. A foil is a movable attachment that extends from the boat into the water. When a boat reaches a certain speed, usually around 18 knots (or 40 km/h), the foils generate enough lift to raise the hull out of the water. This reduces drag, allowing the boat to move even faster.
Foils are designed to resemble aeroplane wings. The difference in pressure between the water flowing over the top and bottom of the foil creates a lifting force, known as lift, which propels the boat upward. This principle is the same as how aeroplanes generate lift to fly through the air, but the AC75 uses water, which is much denser than air, allowing even a small foil to lift a heavy boat.
The physics behind foiling can be explained using the lift equation:
L = (1/2) × ρ × v² × A × CL,where L is the lift force, ρ is the density of water, v is the velocity of the boat, A is the area of the foil, and CL is the lift coefficient. This equation shows that lift increases with the square of velocity, explaining why foiling becomes more effective as speed increases. Similarly, drag forces on the boat can be calculated using the drag equation:
D = (1/2) × ρ × v² × A × CDwhere CD is the drag coefficient. By optimising the shape of the foils and hull, designers aim to maximise the lift-to-drag ratio, allowing the boat to achieve higher speeds with less resistance.
But what makes these foils so unique? First, their shape is carefully crafted to maximise lift while minimising drag. The angle of incidence, or the angle at which the foil meets the water flow, can be adjusted using flaps, similar to the control surfaces on an aeroplane. This allows the crew to adjust the amount of lift and keep the boat stable while flying above the water.
Speed and Sailing of America’s Cup Boats
You might be wondering how these boats can reach such incredible speeds. To answer this question, we must first understand the function of the sail. The sails of an AC75 are not like those of a traditional sailboat. They’re more like vertical wings. Instead of simply catching the wind, these sails generate lift, similar to the foils under water. The sails are made of carbon fibre and other advanced materials, allowing them to retain their aerodynamic shape even under extreme conditions.
The AC75 has two sails, which work together like the two sides of an aeroplane wing. The wind blows over the sails, producing lift and propelling the boat forward. What makes these boats unique is their ability to sail faster than the wind, sometimes reaching speeds three times faster. This is due to the concept of apparent wind, which refers to the wind felt by a moving object. For example, when you run, you feel the wind on your face even though the air around you is still. The same thing happens with these boats: they generate their own wind as they move.
The concept of apparent wind is critical to understanding the incredible speeds reached by AC75 boats. Let’s break it down by numbers. If a boat is sailing at 40 knots (74 km/h) in a true wind speed of 20 knots (37 km/h) at a 90-degree angle, the apparent wind speed will be around 45 knots (83 km/h) coming from a 66-degree angle. This increase in wind speed and change in direction enables the boat to generate more lift from its sails, potentially reaching speeds of 50 knots (93 km/h) or higher. This phenomenon explains why these boats can sail faster than the actual wind speed, which appears to defy intuition.
Balance and Control
Maintaining balance is a critical challenge when sailing an AC75. The enormous forces exerted by the wind on the sails would normally cause the boat to tip over or capsize. In traditional sailboats, this is mitigated by a keel, a heavy structure under the boat that adds stability. However, AC75 boats are distinguished by the absence of a keel. Instead, they rely on the foils to stay balanced.
When the boat is in water, the foil acts as a keel. Even at high speeds, the crew can keep the boat level by adjusting the foils and sail angle. This requires extreme precision, as even minor errors can cause the boat to lose balance and crash into the water. The crew must constantly monitor the wind, water conditions, and boat performance, making real-time changes to the sails and foils.
The rudder also helps to maintain balance and steering. It is more than just a steering tool; it actively manages the boat’s flight. Modern AC75 boats feature a twin rudder system, which uses both rudders to keep the boat stable and prevent it from tipping over.
The Role of the Crew in America’s Cup Boats
The AC75 is a technological masterpiece, but it requires a highly skilled crew to operate. Unlike older America’s Cup boats, which required the crew to manually adjust the sails and other controls, modern AC75 boats use a combination of hydraulics and electronics to fine-tune their performance. The crew is small—typically around 8 people—but each has a specific role.
There are cyclists on the boat, and their job is to pedal and generate the hydraulic power required to move the foils and sails. This system replaces traditional grinders who manually turned winches to control the sails. The energy produced by the cyclists is stored in hydraulic accumulators and then used to power the boat’s systems.
Trimmers are responsible for adjusting the sails. They work in tandem with the helm, who steers the boat. Every adjustment to the sails and foils must be precise, as any error could result in a loss of speed and control. Communication is critical, as the crew must collaborate to respond to changing wind conditions and other external factors.
The precision required to sail an AC75 is astounding. For example, the foil arms can be adjusted by up to 40 degrees, and even a one-degree change can have a significant impact on performance. The sailors must also maintain the boat’s ride height, which is typically between 0.5 and 1.5 metres above the water surface. Maintaining this balance necessitates frequent adjustments, often several times per second. The hydraulic systems on board can generate pressures of up to 5,000 psi (pounds per square inch), providing the power required for such rapid and precise movements.
The Technological Marvel of America’s Cup Boats
What truly distinguishes the AC75 from traditional sailboats is the level of technology used. These boats are essentially mechatronic systems, which combine mechanical engineering, advanced electronics, and software. The boat’s performance is tracked in real time via sensors and computers, which provide data to both the crew on board and the engineers on shore.
This data contains information about the boat’s speed, wind direction, foil angle, and much more. Engineers can use this information to make adjustments and improve performance. The shore crew is critical to the team’s success, as they use this information to fine-tune the boat before races. Every detail is important, from the aerodynamics of the hull to the materials used in the sails.
The level of optimisation in AC75 boats is truly impressive. For example, carbon fibre used in construction can have a strength-to-weight ratio that is up to five times that of steel. The sails, made of advanced composites, can generate up to 5 tonnes of force when fully powered up. The boat’s electronics system processes data from over 100 sensors at a rate of 50 times per second, enabling real-time analysis and adjustments. Even sailors’ suits are designed to reduce drag, with specially textured fabrics that can cut air resistance by up to 15% when compared to smooth materials.
Thank you for the clear explanation about how the America’s cup boats fly and sail. It was easy to read and the level of detail very satisfactory for me.
Naturally, I have further questions!
Even after watching 20 hours of America cup racing and boat launching, your article is the first mention I have come across about twin rudders. How do those rudders work, why do they help and where are they both (since I did not notice them when the AC75s were in the air)?
Cheers
Dave