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Catamaran

The hull of virtually all conventional ships and boats has a so-called monohull-configuration. The term "monohull" denotes a hull which is constructed of a single water displacing body. A monohull typically narrows in cross-section toward the front thereof to define a pointed bow which facilitates the ship's ability to cut efficiently through the water. But a monohull is otherwise relatively wide and a good portion thereof remains submerged at all times below the water surface. This enables a monohull to withstand and to remain more stable in rough seas. However, because a large portion of the monohull is submerged at all times, a monohull produces greater drag at high speeds, i.e. resistance to motion, which results in a ship whose top speed is limited and/or which requires more powerful engines.

A catamaran is a type of a ship which has a different hull structure which has been known for a long time to reduce drag and to result in a faster ship or boat. The hull of a catamaran consists of a pair of hulls each of which is comparatively narrow and long. The catamaran hulls are laterally spaced and typically held together by the deck or by the superstructure of the ship. By definition, a catamaran is an oar-, sail-, and/or motor-propelled boat consisting of two identical parallel hulls joined by means of cross-beams, fabric, netting, a floor, a cabin, or a combination of these various components. The advantages of catamarans over conventional hulls are well known.

Catamarans offer the advantages of reduced drag which permit catamarans to attain speeds not possible with a monohull and/or the option of being equipped with less powerful and therefore less expensive engines. Because of their relatively great length in relation to their width, a catamaran is able to travel at high speed and has excellent lateral stability.

Among modern high speed vessels, the catamarans have since the 1980s gained a dominating market position over monohulls, particularly of size less than 100 meters. This type of vessel is characterized by its simplicity of operation, high stability and relatively high speed- and seakeeping capabilities, particularly in the speed regime of 30-35 knots. The marked, however, seems to continue putting increased demands to speed performance, and several catamarans making 45 knots, and exceptionally above 50 knots, have recently become a reality. Seakeeping performance has also become a major issue in modem high- speed marine transportation. These demands have resulted in larger propulsion power plant installations and the introduction of active motion damping systems, like T-foils located in the bow region and trim-tabs or interceptors located aft, for improvement of ride comfort. The introduction of T-foils, which basically are non permanent lift generating devices, however, is associated by a notable drag that reduces the speed with approximately 2-3 knots on a 40-45 knots catamaran.

Parallel with the increased speed demands on certain routes, most fast ferry operators are still reluctant to join this trend of development because of the associated sky-rocking fuel consumption. It is very likely that the catamaran technology, initially commercially developed during the early seventies, today have reached its optimum stage of development from a hydrodynamic point of view. Further reduction of drag is severely limited by the fact that the major drag component is related to hydrodynamic skin friction. To overcome this, either wetted surface area has to be reduced, or the skin friction has to be reduced by application of new technology, like air lubrication. Recognizing the lack of proven means to solve these technological challenges, it indicates that the catamaran concept, as we know it today, is no longer particularly suited to fully comply with the future marked needs in all respects. This view is supported by the increased attention concerning environmental issues paid by the public and authorities, which is likely to force through the development of novel concepts that performs better in this respect. Also the environment effects of the wave-making tendency of high speed crafts has become a growing regional concern.

Catamaran-hulled boats, which have two parallel pontoons joined by a thin deck and an open space between them, offer greater lateral stability than deep-V hulled boats. That is, boats having these hulls have less tendency to roll side to side, especially when resting. Also, catamaran-type hulls tend to create a smaller wake, and operate more efficiently. However, catamaran-type boats offer less storage space and engine space. The performance of a catamaran-hulled boat is affected by how much load is being carried to a greater extent than mono-hulled boats. The amount of water displaced is limited by the volume of the pontoons of a catamaran-type boat making the boat sink further in the water than a mono-hulled boat with the same load. Catamaran-hulled boats tend to ride with the bow higher than the stern but ride more level than a deep-V hulled boat.

However, as is well known to those skilled in the marine arts, catamarans suffer from the serious disadvantage that they are considerably less stable in rough seas and from a propensity for submarining their forward hulls into large head seas which can result in the forward end of the hull "digging in" and the vessel flipping end for end. By its very nature, a planing type catamaran has the disadvantage that it is significantly affected by waves and is not always comfortable to drive. A displacement type hull, on the other hand, cannot run at high speeds even though it is stable because of the greater water resistance.

Motor powered twin sponson or catamaran type boats are known for high speed and are often referred to as "cigarette boats" due to their slim design and high speed. "Cigarette boats", known for speed, were designed by Don Aronow, a well known boat designer, and such boats include a central tunnel between the sponsons to permit the flow of air therethrough and a sharp, pointed, tapered bow for aerodynamic purposes, and include strakes on either side for stability.

Catamaran vessels have good speed characteristics due to optimum hydrodynamic shape of the hull, geometrical dimensions relationship, and most importantly, the length of the hull where L=length between perpendiculars, B=beam athwart ship of a hull on constructive waterline (CWL). Maximum speed of these vessels in still water corresponds to relative speed where V=speed, knots; V=volume displacement. Relative length of high speed catamaran vessels is .lambda.>10-12. Hulls are separate with a distance between them not less than the beam of the hull, which makes it possible to avoid negative interference of hulls' wave systems.

However, such vessels have low seaworthiness in rough seas due to considerable amplitudes of pitching and vertical rocking, causing shock overloads. Increase in seaworthiness is possible by using constructive peculiarities of catamarans, placing between their hulls hydrofoil-stabilizers, the most simple and effective ones of which are shallow-submergence hydrofoils.

The total resistance to forward motion of a boat is basically the sum of the skin friction (that is obtained by integrating the tangential stress over all the hull surface in the direction of the motion), the viscous drag (that is connected to the energy dissipated owing to the viscous effects) and the residual resistance. The residual resistance includes to a great extent the wave resistance, that is connected to the energy dissipated by the hull in making gravitational waves.

A hull moving forward generates a global wave formation, that is constituted in turn by two distinct but interacting wave systems: a diverging wave system and a transverse wave system. The global wave formation is contained inside two lines, that are called boundary lines of the diverging wave system. Each boundary line forms an angle of 19.5 degrees with the longitudinal symmetry plane of the hull. The crest lines of the transverse waves are perpendicular to the direction of the hull motion near the hull and turn backward as the transverse waves approach the diverging waves until they join the same diverging wave system. In front of the bow of the ship there is a high pressure area that generates a prominent wave front as a part of the transverse and diverging wave system. Further wave systems form near the bow and stern sides of the hull.

A resulting wave system may be often considered as formed by four wave systems: a bow wave system owing to the high pressure area that forms near the bow during the forward motion of the hull; a wave system forward from the bow side portion owing to a low pressure area that forms near such a side portion; a wave system that forms along the stern side portion owing to a low pressure area existing in such a part of hull; a stern wave system owing to a high pressure area that forms in the stern area.

It is very difficult to foresee the exact position of the crest of both the bow wave and stern systems. It is likewise difficult to foresee the position of the troughs of the wave systems formed in the bow and stern side portions of the hull owing to high pressure peaks that are generated near said bow and stern side portions. Said four wave systems that form the global wave system can interfere with each other in a more or less favorable manner for the resistance to forward motion of the hull. However, since the wave resistance contributes to the total resistance to a great extent, one should act just on the wave resistance by taking measures intended to reduce said wave resistance so that the propulsive power installed on a boat may be decreased, with the speed reached by the boat being the same.



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