America's Cup: how the yachts go faster than the wind
The victory by Sir Ben Ainslie and his Oracle Team USA in the America's Cup is being ranked as one of the most astonishing comebacks of all time. Oracle Team USA, with Sir Ben Ainslie on board as tactician, completed one of the most remarkable sporting comebacks of all time on Wednesday night, trouncing Emirates Team New Zealand by 44 seconds in their winner-takes-all finale in San Francisco.
Since this moment, technological advances have been at the heart of the America's Cup, with teams using cutting edge materials and innovative designs to get an edge. Multihull boats such as catamarans and trimarans were among those innovations that have transformed the America's Cup racing and pushed back the boundaries of what is possible.
These catamarans use innovative wing sail designs and hydrofoils that were initially expected to achieve speeds of up to 1.6 times the speed of the wind when sailing downwind.
However, the yachts have achieved almost 2.79 times the wind speed and reached speeds of up to 47 knots, or 55 miles per hour.
How is it possible to sail faster than the wind?
At first glance this appears to defy logic – how can a yacht travel faster than the wind that is propelling it? However, the boats in the America's Cup use rigid wing sails rather than traditional cloth and mast mail sails. These fixed wings use the same principals of lift force that enables aircraft to fly to drive the boat forward. The speed produced also lifts the catamarans out of the water. When combined with reduced drag through the water, the catamarans essentially fly above the surface of the water.
What is a wing sail?
The AC72 catamarans have rigid sails that are the same size as the wing from a Jumbo Jet Boeing 747 passenger airliner.
Measuring 2,800 square feet, these enormous sails catch huge amounts of wind. They are also shaped just like an aircraft wing, with a wide, rigid front edge and a thin trailing edge. In the same way as an aircraft wing, the sails take advantage of the Bernoulli principle, which a difference in pressure on either side of the sail will create lift, or in this case forward motion through the water.
On these boats the wing sail is built in two separate elements, producing an asymmetric wing where the curved surface over which the air flows can be altered by changing the angle between these elements. The wing sail works because the air on the rear, or leeward, side of the sail travels faster than the air on the front, or windward, side.
This difference in air speed creates low pressure on the leeward side of the sail and high pressure on the windward side, essentially lifting the sail forward just like an aircraft wing generates upward lift. By adjusting the angle between the two elements of the wing, the sailors on board can control the amount of "forward lift" they get from the sail. The more the flaps bend, the more power is generated. However, if the sailors bend the wings too much, then they can lose control or the sail will stall.
What are the wings made of?
The leading edge of the wing is made from carbon fibre and forms a rigid structure a little like a mast. At the rear of the wing, there are soft trailing edges while the rest is made from a thin, lightweight composite shell. The two elements of the wing are fixed close together to ensure there is as small a gap between them as possible. If the gap is too big, air can leak between them, creating drag. The teams have worked closely with the aerospace industry to develop their wings, creating small tweaks and alterations in the curve to maximise lift while giving the crews as much control as possible.
How are the wings controlled?
A series of lines and ropes are attached to the wings to allow the crew to alter the angle of the camber, while the flaps are controlled using hydraulic cables. The crew also carry wireless electronic devices that provide them with updates from sensors carried on the wings to help them keep them in optimal shape.
What about the drag of the water?
In general boats are large, heavy objects that must force their way through the water. Without power they quickly slow down due to the pressure of the water pushing against their hulls.
Catamarans generally have lower drag through the water than single hull boats due to the relatively small amount of the boat that is actually in the water at any one time.
All the same, the AC72s weigh seven tonnes each and that is a lot of boat to force through the water.
What is different about the AC72 boats is that they can sail without any of the hull in the water. They glide along on foils that extend down from the bottom of the hulls instead - something crews have described as being like "flying on water".
Once they reach speeds of around 20 knots the boats pop up out of the water so the weight of the boat is carried on the foils on the rear of the vessel.
Forward winglets are lowered into the water using hydraulic daggerboards.
These allow the yachts to almost glide through the water, cutting through the waves so power is not lost by having to force the boat through oncoming waves.
What is so special about the hydrofoils?
These surfboard sized foils are made from carbon fibre yet are able to withstand up to 15 tonnes of pressure when performing a turn. The entire weight of the boat is lifted out of the water on these foils, and the reduction in drag makes the boat go between 10-15 per cent faster. Crews have discovered that they can push the boundaries of this further and squeeze even more speed out of the boats by maintaining a stable flight height about 1 metre above the water. They do this by controlling the angle of the front foils to ensure the boats remain stable.
Are there risks?
In a word, yes. These boats are travelling at speeds of up to 55 miles per hour above the water while supported on tiny blades of carbon fibre.
The crews must constantly fight to keep the catamarans from pitching forward, causing the bow of the hulls to hit the water and causing the vessel to somersault, known as a pitchpole.
In May this year British Olympic medallist Andrew Simpson was killed while training for the America's Cup when the Swedish Artemis Racing team yacht he was aboard flipped and broke into pieces.
Simpson was trapped under the hull and could not be revived. Crews wear protective armour and helmets. Since Simpson's death they are also required to wear portable air canisters in case they are trapped under water during a capsize.
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