America's Cup foiling boats are racing sailboats that lift most of their hull out of the water and skim above the surface on underwater wings called hydrofoils. This matters because water drag is much larger than air drag, so raising the hull can make the boat dramatically faster. These boats look like they are flying, but their motion is controlled by the same physics ideas used in airplanes, ships, and submarines.
Forces, pressure, drag, and stability all work together during a high speed race.
Key Facts
- Hydrofoil lift increases with water density, foil area, speed squared, and lift coefficient: L = 1/2 rho v^2 A CL.
- Drag force can be modeled as D = 1/2 rho v^2 A CD, where CD is the drag coefficient.
- A boat begins foiling when upward hydrofoil lift is about equal to the boat's weight: L = mg.
- Power needed to overcome drag is P = Dv, so reducing drag saves large amounts of energy at high speed.
- Apparent wind is the wind felt by the moving boat and is a combination of true wind and the boat's own motion.
- Foiling sailboats can exceed true wind speed because the sail acts like an airfoil and the hydrofoils provide a low drag path through the water.
Vocabulary
- Hydrofoil
- An underwater wing that produces lift as water flows around it.
- Lift
- A force perpendicular to the flow that can raise a foil or support the weight of a boat.
- Drag
- A resistive force that acts opposite the motion of an object through a fluid.
- Apparent wind
- The wind direction and speed experienced on a moving boat due to the combination of true wind and boat motion.
- Righting moment
- The turning effect that helps keep a boat from tipping over when wind force pushes on the sail.
Common Mistakes to Avoid
- Thinking the boat is lifted by air under the hull is wrong because the main lift comes from water flowing around the submerged hydrofoils.
- Assuming faster always means more efficient is wrong because drag usually increases with v^2, so small speed increases can require much more force and power.
- Treating true wind and apparent wind as the same is wrong because a fast moving boat creates its own felt wind direction and speed.
- Forgetting weight in foil calculations is wrong because the boat foils only when hydrofoil lift is large enough to support most of the boat's weight.
Practice Questions
- 1 A foiling boat has a mass of 1200 kg. What upward lift is needed for the foils to support its full weight? Use g = 9.8 m/s^2.
- 2 A hydrofoil experiences 800 N of drag while the boat moves at 15 m/s. What power is needed to overcome this drag? Use P = Dv.
- 3 Explain why lifting the hull out of the water can help an America's Cup boat sail faster than the true wind.