20-09-2012, 11:54 AM
Hydrofoil Ships
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INTRODUCTION
A Hydrofoil is a specially designed hydrodynamic surface that creates lift significantly exceeding drag. The main function of the hydrofoil is to lift the ships hull out side the water. At low speeds the ships hull sits on the water and the hydrofoils are totally submerged in water, but as the speed increases the hydrofoils create lift, bringing the hull outside the water surface.
The basic principle of the hydrofoil concept is simply to lift a ship's hull out of the water and support it dynamically on wing-like lifting surfaces, i.e. hydrofoils, to reduce the effect of waves on the ship and to reduce the power required to attain modestly high speeds. Engineers and naval architects have been intrigued with the possibilities of this concept for many years. A United States patent for a hydrofoil was defined in the late 1880s, about the same time as the early airplane and airfoil patents. The earliest record of a successful hydrofoil flight is 1894 when the Meacham brothers demonstrated their 14 foot test craft at Chicago, Illinois. This compares with the Wright brothers' first airplane flight in 1903. The early attempts to exploit the hydrofoil concept were frustrated by lack of suitable structural materials and power plants. However, advancement in these areas, much of it stemming from aircraft developments, has permitted development over the past 30 to 40 years of the technology necessary to achieve and demonstrate reliable and effective hydrofoil ships for both military and commercial below.
HYDROFOIL BASICS
Many people are familiar with airfoils. Foil is simply another word for the wing (such as the wing on an airplane). A hydrofoil is a wing that 'flies' in water. Hydrofoil is also used to refer to the boat to which the water wings are attached. A hydrofoil boat has two modes of operation:
(1) as a normal boat with a hull that displaces water and
(2) with the hull completely out of the water and only the foils submerged.
Hydrofoils let a boat go faster by getting the hull out of the water. When a normal boat moves forward, most of the energy expended goes into moving the water in front of the boat out of the way (by pushing the hull through it). Hydrofoils lift the hull out of the water so that you only have to overcome the drag on the foils instead of all of the drag on the hull.
The foils on a hydrofoil boat are much smaller than the wings (foils) on an airplane. This is because water is about 1000 times as dense as air. The higher density also means that the foils do not have to move anywhere near as fast as a plane before they generate enough lift to push the boat out of the water.
PRINCIPLE
This equation applies to flows along a streamline which can be modeled as: in viscid, incompressible, steady, and irrotational for which the body forces are conservative. Also the difference on the height of the foil (the distance from the bottom section to the upper one) is small enough so that the difference pgy2 - pgy1 is negligible compared to the difference of the rest of the terms. What is left is that the pressure plus one half the density times the velocity squared equals a constant (the stagnation pressure).
As the speed along these streamlines increases, the pressure drops (this will become important shortly). The fluid that moves over the upper surface of the foil moves faster than the fluid on the bottom. This is due in part to viscous effects, which lead to formation of vertices at the end of the foil. In order to conserve angular momentum caused by the counter-clockwise rotation of the vortices, there has to be an equal but opposite momentum exchange to the vortex at the trailing edge of the foil. This leads to circulation of the fluid around the foil. The vector summation of the velocities results on a higher speed on the top surface and a lower speed on the bottom surface. Applying this to Bernoulli’s it is observed that, as the foil cuts through fluid, the change in velocity produces the pressure drop needed for the lift. As it is presented in the diagram, the resulting or net force (force= (pressure)(area)) is upward.This explanation can be enriched with the Principle of Conservation of Momentum. (Momentum = (mass)(velocity)) If the velocity of a particle with an initial momentum is increased, then there isa reactant momentum equal in magnitude and opposite in direction to the difference of the momentums.
CONFIGURATIONS
Hydrofoil configurations can be divided into two general classifications, surface piercing and fully submerged which describe how the lifting surfaces are arranged and operate (see Figure 9.1). In the surface piercing concept, portions of the foils are designed to extend through the air/sea interface when foil borne . Struts connect the foils to the hull of the ship with sufficient length to support the hull free of the water surface when operating at design speeds. As speed is increased, the lifting force generated by the water flow over the submerged portion of the foils increases causing the ship to rise and the submerged area of the foils to decrease. For a given speed the ship will rise until the lifting force equals the weight carried by the foils. As indicated by the terminology, the foils of the fully submerged concept are designed...
WEIGHT LIMITATIONS
Like the airplane designer, the hydrofoil designer must, at all times, be extremely conscious of weight. The hydrofoil type of craft is weight critical, and every pound of weight saved in structure, outfit, or machinery means weight available for payload and fuel.
The structural engineer, in designing hydrofoils to conserve weight, uses aircraft techniques. Relative to conventional ships, hydrofoil craft are subject to very high loadings, as caused by high operating speeds. Likewise, lightweight, high strength materials are used. He also must contend with fatigue and problems of hydroelasiticity, including both divergence and flutter.
HULL CONSIDERATIONS
The development of a satisfactory hull form for hydrofoil application represents a significant challenge to the designer. The hull should perform well in the hull borne mode but also during takeoff and during foil borne operation where impacts with waves are involved. In addition, the hull configuration of a hydrofoil ship must satisfy all of the requirements for strength, freeboard, and intact and damaged stability for any other ship.
Relatively high power requirements for high-speed operation, in common with other high performance systems, pay a high performance dividend for achieving a minimum weight structure. Therefore, hydrofoil ship hulls are generally constructed using high-grade aluminum alloys, 5000 series weldable alloy being typical. Structurally, the hull must have the strength to resist wave impact at high speed as well as distribute the concentrated load at the strut attachment points. Although hydrofoil hulls may appear quite conventional, the required compromises are more complex than for a monohull because of the many operating modes of the ship. An efficient hull formfor a lower speed operation requires a narrow beam. However, a righting moment large enough to satisfy the stability criteria of reference [6] with the foils retracted generally dictates a wide beam. Cresting the tops of waves while foilborne points toward the use of a deep vee forward and high deadrise.