قطمران
A catamaran (/ˌkætəməˈræn/) (informally, a "cat") is a multi-hulled watercraft featuring two parallel hulls of equal size. It is a geometry-stabilized craft, deriving its stability from its wide beam, rather than from a ballasted keel as with a monohull sailboat. Being ballast-free and therefore lighter than a monohull, catamarans often have a shallower draft (draught) than comparably-sized 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 wave-induced motion, as compared with a monohull, and can give reduced wakes.
Catamarans range in size from small (sailing or rowing vessels) to large (naval ships and car ferries). The structure connecting a catamaran's two hulls ranges from a simple frame strung with webbing to support the crew to a bridging superstructure incorporating extensive cabin and/or cargo space.
History
Catamarans from Polynesia and South Asia became the inspiration for modern catamarans.
Polynesian and Asian catamarans
Performance characteristics
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
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 cube 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. For powered catamarans, this implies smaller power plants (although two are typically required). For sailing catamarans, low forward resistance allows the sails to derive power from attached flow, their most efficient mode—analogous to a wing—leading to the use of wingsails in racing craft.
Stability
Catamarans rely primarily on form stability to resist heeling and capsize. Comparison of heeling stability of a rectangular-cross 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 monohull. Compared with a monohull, a cruising catamaran sailboat has a high initial resistance to heeling and capsize.—a fifty-footer requires four times the force to initiate a capsize than an equivalent monohull.
Tradeoffs
One measure of the trade-off between speed and carrying capacity is the ), compared with calm water transportation efficiency. FnV applies when the waterline length is too speed-dependent 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:
Sailing catamarans
Powered catamarans
A catamaran configuration fills a niche for powered vessels, where speed and sea-kindliness is favored over bulk capacity. In larger vessels, this niche favors car ferries and military vessels for patrol or operation in the littoral zone. Two advances over the traditional catamaran are the small-waterplane-area twin hull (SWATH) and the wave-piercing configuration—the latter having become a widely favored design.
SWATH
The small-waterplane-area twin hull (SWATH) configuration reduces wave-generating resistance by moving displacement volume below the waterline, using a pair of tubular, submarine-like hulls, connected by pylons to the bridge deck with a narrow waterline cross-section. The submerged hulls are minimally affected by waves. The SWATH form was invented by Canadian Frederick G. Creed, who presented his idea in 1938 and was later awarded a British patent for it in 1946. It was first used in the 1960s and 1970s as an evolution of catamaran design for use as oceanographic research vessels or submarine rescue ships. The SWATH. concept was further developed by Thomas G. Lang, ca. 1968. The US Navy commissioned the construction of a SWATH ship to test the configuration.
SWATH vessels compare with conventional powered catamarans of equivalent size, as follows:
- Larger wetted surface, which causes higher skin friction drag.
- Significant reduction in wave-induced drag, with the configuration of struts and submerged hull structures.
- Lower water plane area significantly reduces pitching and heaving in a seaway.
- No possibility of planing.
- Higher sensitivity to loading, which may bring the bridge structure closer to the water.
Wave-piercing
Wave-piercing catamarans employ a low-buoyancy bow on each hull that is pointed at the water line and rises aft, up to a level, to allow each hull to pierce waves, rather than ride over them. This allows higher speeds through waves than for a conventional catamaran. They are distinguished from SWATH catamarans, in that the buoyant part of the hull is not tubular. The spanning bridge deck may be configured with some of the characteristics of a normal V-hull, which allows it to penetrate the crests of waves.
Wave-piercing catamaran designs have been employed for yachts, passenger ferries, and military vessels,.
References
- ^ Garrett, Ross (January 1, 1996). . Sheridan House, Inc. p. 268.
- ^ Yang, C.; Löhner, R.; Soto, O. (Aug 22, 2001), "Optimization of a wave-cancellation multihull using CFD tools", in Wu, You-Sheng; Zhou, Guo-Jun Zhou, Practical Design of Ships and Other Floating Structures: Eighth International Symposium, Technology & Engineering, 1, China: Elsevier, pp. 1422
- ^ Weltner, Klaus (January 1987), "A comparison of explanations of the aerodynamic lifting force", Am. J. Phys. 55 (1): 52, doi:
- ^ Nielsen, Peter (May 14, 2014). "Have Wingsails Gone Mainstream?". Sail Magazine. Interlink Media. Retrieved 2015-01-24.
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للاستزادة
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