Low-drag hydrodynamic surfaces

Abstract

The invention relates to the use of gas cavities to reduce frictional drag on underwater surfaces such as hydrofoils, struts, fins, rudders, keels, propeller blades, ship hulls, underwater bodies, and wetted surfaces in general. Each gas-filled cavity is formed behind a discontinuity in the surface that causes the water boundary layer to separate from the surface. Gas is ejected into a region behind the discontinuity to fill the cavity; the gas can be air. If a cavity is open to the atmosphere, then air can typically fill the cavity naturally without air ejection. Cavities can either be closed or open. A low drag hydrofoil may have a closed cavity on one side, and an open cavity on the other side. For closed cavities, the underlying surface can be shaped to minimize cavity closure drag. Various ways to generate cavities, change hydrodynamic forces, and duct gas internally on hydrofoils and struts with cavities are covered. Different designs of hydrofoil boats, hydrofoil ships and ship hulls that are amenable to drag reduction are presented.

Description

[0001] This invention was made with Government support under DAAH01-96-C-R228, and DAAH01-98-C-R115 awarded by the Defense Advanced Research Projects Agency. The Government has certain rights in this invention.BACKGROUND OF THE INVENTION [0002] This invention applies to the field of hydrodynamics, and relates to the use of gas cavities to reduce the frictional drag of hydrofoil craft, ship hulls and underwater surfaces in general. [0003] Using cavities to reduce frictional drag is covered in U.S. Pat. No. 3,077,173 (1963) and U.S. Pat. No. 3,109,495 (1963) for base-vented and side-vented hydrofoils, U.S. Pat. No. 3,205,846 (1965) for torpedoes, and U.S. Pat. No. 6,167,829 (2001), together with a pending continuation of that patent, for submerged surfaces in general. [0004] The reduction of frictional drag provides basic benefits: power is reduced, and fuel consumption is reduced. These benefits reduce the weight of a vessel, which further reduces power and fuel consumption compared ...

AI Technical Summary

Benefits of technology

[0006] A primary objective of this invention is to reduce the drag of high-speed hydrofoil craft by forming a closed gas cavity on each side of each lifting hydrofoil, forming an open cavity on each side of each support strut, and by covering at least one side of each propulsor blade with a cavity.

[0007] A preferred design is a hydrofoil craft that has one highly-swept-back v-hydrofoil in planform, supported by three swept struts, powered by two superventilating propellers wherein each drive shaft is located within a strut, and wherein the hydrofoil sweep back eliminates cavitation and reduces craft motion in waves.

[0008] Another objective is to efficiently control the lift of hydrofoil cross sections having closed cavities by using trailing edge flaps, optional leading edge flaps, and optional means for controlling gas flow rates.

[0009] Other objectives are to efficiently initiate cavities by introducing discontinuities to separate the water boundary layer in various ways, efficiently distribute the gas to each cavity, control the cavities, and separate adjacent cavities with different kinds of fences.

[0010] Still another objective is to minimize cavity drag on underwater surfaces by closing the cavities as smoothly as possible by minimizing the contact angle between the cavity and the surface. Another objective is to further minimize cavity drag by adding parallel ridges in the vicinity of cavity closure to reduce forward splash and thereby minimize gas entrainment out of the cavity.

Problems solved by technology

The problem is how to design underwater surfaces to make full use of cavities to reduce drag.

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