Faster than the wind

In the recent America’s Cup races,  the boats were frequently sailing at more than twice the wind speed. For instance, in the final race the wind speed was given as about 20 knots, while the boats were travelling at well above 40 knots. How is this possible?

■  A sailing boat is not a mere wind-blown object, like a leaf or hot-air balloon. The sail is cut so that it assumes a shape similar to an aircraft’s wing. The wind travels faster along the convex side, creating lift. It is fastest with the wind blowing across the boat’s side. But this force also generates an opposite reaction, which acts as drag on the boat’s keel – the finshaped part extending down into the water from the hull that provides stability. In addition, as a boat sails, its own speed adds to the wind’s speed. This is similar to running in still air when you can feel the “wind” on your face. This “apparent wind” creates further lift from the sail and  the speed of the boat increases. Boats with keels have a limited speed due to the drag this feature creates. Ice yachts don’t have  this constraint and can sail  much faster, until aerodynamic drag limits their speed. The disputed world speed record for an ice boat is about 230 kilometres an hour, but  over 88 kilometres an hour is commonplace. John Davies Lancaster, UK

■  The sails of a boat are curved in a similar way to an aircraft wing. They are adjusted by the crew to ensure that the air flows evenly over them. This maximises the extraction of energy from the wind, which is used to drive the boat forwards. As well as the true wind, a boat experiences an “apparent wind” caused by its own motion, and it is this effect that enables boats to sail faster than the true wind. In the America’s Cup example, simple vector analysis shows that “The bigger the sail area, the more energy can be captured, but sails have to be as vertical as possible” a boat travelling at 74 kilometres an hour, broadside on to a 37 km/h wind, will actually be sailing in an 83 km/h apparent wind. The bigger the sail area, the more energy can be captured, but the sails have to be kept as vertical as possible. Large sails create a huge turning momentum that can rapidly capsize the boat. To counter this tendency, some boats use a large weighted keel; dinghies and catamarans rely on the weight of the crew. But increasing weight tends to lower the boat in the water, and this increases drag. Most boats have a limitation on their speed, known as the hull speed, which is roughly 1.34 times the square root of their length in feet. Increasing the power in such cases, through engines or sail area, only results in a bigger bow wave and no increase in speed. However, clever design coupled with sufficient power can induce a phenomenon known as planing. In this case the hull can sit higher in the water and even sometimes briefly come clear of the surface  as the boat skims along, much faster than its hull speed. Racing power boats often exhibit this phenomenon. A further development of this is the hydrofoil, where the whole hull lifts out of the water with even greater drag reduction.  Long, thin, wave-piercing hulls also reduce drag but are not very stable. If you place two of these in parallel and separate them, with the mast and sails in between,  you have created a stable and fast catamaran. Now just add the biggest sails you dare, foils that lift the hull out of the water, plus Ben Ainslie on the helm and you have designed a winning entry  for the next America’s Cup. James Stafford London, UK