America's Cup: Characteristics and Physics Behind the 2013 AC72 Design
America's Cup is one of the largest sailing races in the world; each year, the moderators release the official design for the racing teams who can then work within the accepted range of measurements to find the best combinations to create the fastest ship possible. The ships are 72 feet long with masts as tall as 131 feet, and each boat weighs about 13,000 (which is pretty light for a sailboat that size). The 260-square-foot sails, made of rigid material instead of traditional canvas, can generate up to 7 tons of force when the wind conditions are right.
This year, the design (AC72) became a topic of debate due to its immense capacity for speed, making it extremely dangerous; the Emirates Team New Zealand sailboat managed to reach 50.8 mph using only 18 mph winds. How is this possible? The main part of it has to do with apparent and true wind. Keeping the true wind at an angle to the sails allows it to push the boat, while the apparent wind created by this motion adds to that movement even more. If you sail perpendicular to the wind, only true wind will move the ship and you can sail ONLY as fast as the wind is blowing; sailing at an angle, however, allows the ship to use the apparent wind to move even faster, a trait clearly demonstrated by the AC72.
Other features of this ship that allow for its incredible speeds are the hulls, sails, and daggerfoils (keels for catamarans or trimarans). The hulls are extremely long and thin, which decrease the drag felt by the hulls from the water, as well as allows the hulls to break through the waves as opposed to slowing the ship when they crash into waves head on. For catamarans, small hulls can be dangerous because their width and weight usually help to counteract the force of the sail acting towards the water and seeking to capsize the ship. Eliminating this can be dangerous as it makes the ship more prone to capsize; this problem, however, is fixed by increasing the distance (and therefore the leverage against capsizing) between the two separate hulls.
The sails, as mentioned before, are not made of canvas like most sailboats. In the AC72 design, the sails are rigid, which allows the shape to be more easily controlled when under sail. The sails are built more like airplane wings than traditional sails, and the round leading edge of the sails decreases drag from the air and allows the ship to go faster. Lastly, a canvas sail is extremely sensitive to luffing, or the flapping you see in a sail when air currents in isolated areas of the sail change. The sails used by the AC72, because they are so rigid compared to canvas, are generally less susceptible to these same air currents so sailing is more efficient.
This year, the design (AC72) became a topic of debate due to its immense capacity for speed, making it extremely dangerous; the Emirates Team New Zealand sailboat managed to reach 50.8 mph using only 18 mph winds. How is this possible? The main part of it has to do with apparent and true wind. Keeping the true wind at an angle to the sails allows it to push the boat, while the apparent wind created by this motion adds to that movement even more. If you sail perpendicular to the wind, only true wind will move the ship and you can sail ONLY as fast as the wind is blowing; sailing at an angle, however, allows the ship to use the apparent wind to move even faster, a trait clearly demonstrated by the AC72.
Other features of this ship that allow for its incredible speeds are the hulls, sails, and daggerfoils (keels for catamarans or trimarans). The hulls are extremely long and thin, which decrease the drag felt by the hulls from the water, as well as allows the hulls to break through the waves as opposed to slowing the ship when they crash into waves head on. For catamarans, small hulls can be dangerous because their width and weight usually help to counteract the force of the sail acting towards the water and seeking to capsize the ship. Eliminating this can be dangerous as it makes the ship more prone to capsize; this problem, however, is fixed by increasing the distance (and therefore the leverage against capsizing) between the two separate hulls.
The sails, as mentioned before, are not made of canvas like most sailboats. In the AC72 design, the sails are rigid, which allows the shape to be more easily controlled when under sail. The sails are built more like airplane wings than traditional sails, and the round leading edge of the sails decreases drag from the air and allows the ship to go faster. Lastly, a canvas sail is extremely sensitive to luffing, or the flapping you see in a sail when air currents in isolated areas of the sail change. The sails used by the AC72, because they are so rigid compared to canvas, are generally less susceptible to these same air currents so sailing is more efficient.
|
The last main difference in the AC72 that allows for much faster sailing is from the specially designed daggerboards or hydrofoils. These are slightly modified keels that are used on ships with multiple hulls, and each hull has at least one, if not two, hydrofoils. The ones used in the AC72 design, and with many other racing sailboats, are specially designed to provide balance to the entire sailboat while actually pushing the boat out of the water. Having the ship leave the water is beneficial for speed because drag caused by the movement on the water is replaced by drag from the much less hindering air. You can see from the picture to the right that the hydrofoils have an almost horizontal component to them, which is what helps to push the boat out of the water. While sails act similar to vertical airplane wings, the hydrofoils act exactly like an airplane wing, with the faster water on the top forcing the 'wing' upwards and helping to push the boat out of the water. You can also watch the video below to get a better idea of what foiling is.
|
pointwise.com
|