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A catamaran (/ˌkætəməˈræn/) (informally, a "cat") is a multihulled watercraft featuring two parallel hulls of equal size. It is a geometrystabilized craft, deriving its stability from its wide beam, rather than from a ballasted keel as with a monohull sailboat. Being ballastfree and therefore lighter than a monohull, catamarans often have a shallower draft (draught) than comparablysized monohulls. The two hulls combined also often have a smaller hydrodynamic resistance than comparable monohulls, requiring less propulsive power from either sails or motors. The catamaran's wider stance on the water can reduce both heeling and waveinduced motion, as compared with a monohull.
Performance characteristics[edit]
Catamarans have two distinct primary performance characteristics that distinguish them from displacement monohull vessels: lower resistance to passage through the water and greater stability (initial resistance to capsize). Choosing between a monohull and catamaran configuration includes considerations of carrying capacity, speed, and efficiency.
Resistance[edit]
At low to moderate speeds, a lightweight catamaran hull experiences resistance to passage through water that is approximately proportional to the square of its speed. A displacement monohull, by comparison experiences resistance that is at least the third power of its speed. This means that a catamaran would require four times the power in order to double its speed, whereas a monohull would require eight times the power to double its speed, starting at a slow speed.[17] For powered catamarans, this implies smaller power plants (although two are typically required). For sailing catamarans, low forward resistance[18] allows the sails to derive power from attached flow,[19] their most efficient mode—analogous to a wing—leading to the use of wingsails in racing craft.[20]
Stability[edit]
Catamarans rely primarily on form stability to resist heeling and capsize.[17] Comparison of heeling stability of a rectangularcross section monohull of beam, B, compared with two catamaran hulls of width B/2, separated by a distance, 2×B, determines that the catamaran has an initial resistance to heeling that is seven times that of the monhull.[21] Compared with a monohull, a cruising catamaran sailboat has a high initial resistance to heeling and capsize.—a fiftyfooter requires four times the force to initiate a capsize than an equivalent monohull.[22]
Tradeoffs[edit]
One measure of the tradeoff between speed and carrying capacity is the displacement Froude number (FnV),[23] compared with calm water transportation efficiency.[24] FnV applies when the waterline length is too speeddependent to be meaningful. It uses a reference length, the cubic root of the volumetric displacement of the hull, V, where u is the relative flow velocity between the sea and ship, and g is acceleration due to gravity:
F n V = u g V 1 / 3 {\displaystyle \mathrm {Fn_{V}} ={\frac {u}{\sqrt {gV^{1/3}}}}} {\displaystyle \mathrm {Fn_{V}} ={\frac {u}{\sqrt {gV^{1/3}}}}}
Calm water transportation efficiency of a vessel is proportional to the fullload displacement and the maximum calmwater speed, divided by the corresponding power required.[25]
Large merchant vessels have a FnV between one and zero, whereas higherperformance powered catamarans may approach 2.5—denoting a higher speed per unit volume for catamarans. Each type of vessel has a corresponding calm water transportation efficiency, with large transport ships being in the range of 100–1,000, compared with 1114 for transport catamarans—denoting a higher efficiency per unit of payload for monohulls.[24]
https://en.wikipedia.org/wiki/Catamaran
