LCH (X) (2018) LCH(X) Landing Craft Hybrid Air Cushion Displacement
Marine vehicles are categorized as either displacement craft or dynamic-lift craft.The following are dynamic-lift: hydrofoil ships, air cushion vehicles (ACV), seaplanes, wing-in-ground effect (WIG) craft, planing hydrofoil ships, surface effect ships (SES) and ram wing craft. Displacement craft include: slender monohull ships, catamaran ships, SWATH (Small Waterplane Area Twin Hull) and displacement hydrofoil ships.
Displacement craft derive their lift from buoyancy (displacement). Dynamic-lift craft derive their lift dynamically, such as by hydrofoils or planing surfaces. The drag of displacement craft is primarily frictional and wavemaking. The drag of dynamic-lift craft is primarily frictional and induced (drag induced by lift). In high-speed craft of either type, frictional drag is normally more than half the total drag. It is important to reduce frictional drag, although all types of drag are reduced to achieve a large lift-to-drag (L/D) ratio.
The need for reduction in frictional drag has long existed. Means for reducing frictional drag include laminarization, air cavities and air films, riblets, magnetohydrodynamics, microbubble ejection, polymer ejection and moving walls. Vessels supported on air cushions are propulsively more efficient at high speeds than displacing and planning vessels.
Experience from previous air cushion vessels shows that, with a smaller cushion area and a moderate air pressure, it is possible to support the same proportion of the vessel's displacement as a hovercraft or an SES. This, combined with the fact that it is possible to divide the air cushion between two or more hulls, can be used to make each air cushion hull and air cushion chamber slimmer (greater length/width ratio) than for conventional air cushion vessels, so that the hulls have a reduced resistance, are more seakind and seaworthy, and create a safer vessel with greater comfort and less speed reduction in waves. The same conditions can be used to combine the air cushion hull with a more conventional, planing forebody, which has the combined job of confining the air cushion using a rigid construction, generating a limited dynamic lift in addition to the lift of the air cushion, containing reserve displacement for large vessel motion and generating a damping of this motion.
A hybrid craft can operate as a high speed SES in open water and deploy a retracted flexible skirt to operate as a hovercraft across beaches and overland. A downwardly extending boundary structure on a hull periphery reaches through the surface of water. The boundary structure has thin rigid side-walls extending downward from lateral sides, a forward seal wall extending downward from a forward portion, and an aft seal wall extending downward from an aft portion. Machinery creates a pressurized air cushion under the hull inside of the boundary structure to support and transport the hull. A flexible skirt system on said hull inwardly of and adjacent to the boundary structure can be retracted to a first position above the boundary structure and the water and extended to a second position below the boundary structure and the surface of the water to transit beaches and overland on the air cushion.
Two primary types of high-speed advanced marine vehicles are the surface effect ship (SES) (or surface effect vehicle (SEV)) and the fully skirted hovercraft. Both are considered air-cushioned vehicles (ACV) in that each rides on a pressurized cushion of air. The air cushion serves to separate the vehicle from the surface over which it operates, allowing freer movement while greatly decreasing the hydrodynamic drag. The lower drag allows much greater speeds to be achieved at lower drive power when compared to conventional displacement hulls. In each case powered lift fans are needed to maintain the flow of volumes of pressurized air under the vehicle due to the flow of air escaping from the cushion.
The air cushion of the hovercraft is contained around its perimeter by a flexible bag and finger skirt system which conforms to the irregular operating surface be it waves or uneven terrain, thereby greatly reducing the flow of air escaping from the cushion. The air cushion of the SES or SEV is contained on the sides by relatively thin, rigid, lateral walls contacting and protruding into the water, and at the front and rear by flexible or semi-flexible seals or skirts. Though the hovercraft skirt theoretically can have a lower drag due to the air gap which would give no interaction with the surface, in reality considerable drag can be experienced in rougher water due to wave interaction with the skirt's fingers. So, the SES will have considerably lower hydrodynamic drag in rougher water due to the thinness and rigidity of the side-walls as opposed to the unwieldy side portion of the skirt of the hovercraft. A further advantage of the SES is that since its side-walls protrude below the water surface there is practically no air loss out the sides. The reduced drag of the SES reduces the thrust required to achieve a given speed, and the reduced air loss reduces the power required for the lift fans. In addition, water-jet or other water propulsion means may be employed with the SES which are generally more efficient than air-propellers on hovercraft. Also, the operational costs of the side-walls on the SES are much lower when compared to the side portions of the skirt of the hovercraft. This is because the side portions of the skirt of the hovercraft are composed of a multitude of components made of flexible materials which are prone to wear and damage and, due to waves, are in constant contact with the water surface while underway. However, despite the advantages of the SES, its rigid side-walls mean that it is strictly a non-amphibious vehicle, and is limited to water-borne operations of suitable depth. In contrast to the SES, the hovercraft is a truly amphibious vehicle with is capable of traversing on dry land as well as open water, and it can also operate effectively on soft mud, marsh, or tidal flats that are impassible to any other vehicle.
The hybrid craft has a hull having a downwardly extending boundary structure on the periphery of the hull to reach through the surface of water. The boundary structure has thin rigid side-walls extending downward from lateral sides of the hull, a forward seal wall extending downward from a forward portion of the hull, and an aft seal wall extending downward from an aft portion of the hull. Means mounted on the hull creates a pressurized cushion of air under the hull inside of the boundary structure to support and transport the hull on the water on an air cushion. A flexible skirt system is mounted on the hull inwardly of and adjacent to the boundary structure to retract to a first position above the boundary structure the surface of the water and to extend to a second position below the boundary structure and the surface of the water to allow transit across beaches and overland.
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