Principles of naval engineering - Historic Naval Ships Association

PRINCIPLES OF NAVAL ENGINEERINGThe primary advantages of the hydrofoil formare high speed and superior seakeeping abilities.The high speed is attainable because the resistancesencountered by other shipforms are substantiallyreduced in the hydrofoil. When the hydrofoilis flying, the hull is entirely above thesurface of the water and is therefore not impededby frictional resistance or wave- making resistanceand is not disturbed by waves, swells, orchoppy surfaces that slow down other craft.At present, the hydrofoil form is not consideredfeasible for very large vessels. However,the future development of propulsion plantswith smaller specific weights^ could well extendthe tonnage range of the hydrofoil form. In fact,a "hydrofoil destroyer" has even been proposed.Several types of hydrofoil systems have beendeveloped. The Navy hydrofoil program has concentratedlargely on fully submerged subsurfacewings or foils. Control surfaces on the fully submergedfoils act like aircraft ailerons and controlthe course of the craft through the water;the foils are controlled by a height- sensing systemwhich maintains level flight. In another typeof control (incidence control) the entire foil ismoved instead of flaps. Other hydrofoil systemsinclude skids or other planing devices and surface-piercingfoils. Surface-piercing foils havea lifting area that is proportional to the amountof foil immersed.A number of hull forms have been used forhydrofoils, but most of them are basically adaptationsof conventional hull forms. The hull isrelatively long and narrow, with the length of thecraft being eight to ten times the beam. It hasbeen suggested that the catamaran'* design mayhave certain specific advantages for hydrofoils;however, this type of design is not currently usedin Navy hydrofoils except on an experimentalbasis.Although a wide variety of power plants havebeen considered for hydrofoil craft, the gas turbineand the diesel engine are the types primarilyinstalled in these craft at present. Some hydrofoilsare equipped with both gas turbines anddiesel engines."^The specific weight of a propulsion plant is thenumber of pounds of propulsion machinery requiredper shaft horsepower.4 The catamaran or twin-hull design consists of twoslender hulls which are joined together above thewaterline.A major problem in the development of hydrofoilshas centered around the transmission ofpower. When an underwater propeller is used,power must be transmitted downward from theprime mover in the hull to the propeller, whichis at a deeper level. Various solutions to thisproblem have been tried. One solution is to usean inclined shaft and an inclined prime moverwhich drives into a V-gear. Another solution isto use double right-angled gearing. To date, nosolution has been found that is entirely satisfactory.Propellers have also been a continuing problemwith hydrofoils. The cavitation encounteredat high speeds has led to numerous propellerfailures. One solution to this problem is the supercavitating(SC) propeller discussed later inthis chapter. Because of the propeller problem,other propulsion devices such as airscrews andairjet or waterjet propulsion systems have beenconsidered for use with hydrofoils.The Navy's first operational hydrofoil, USSHigh Point , PCH 1, is shown in figures 25-1 and25-2. In flight, this hydrofoil reaches speeds ofmore than 45 knots.HydroskimmersThe hydroskimmer belongs to the generalcategory of craft designed to ride on a bubble ora cushion of air. Vehicles and craft in this categoryare called ground effect machines (GEM),air cushion vehicles (ACV), or surface effectships. In general, any GEM that is designed tooperate over water is called a hydroskimmer.The hydroskimmer has been referred to as a"flying washtub,"5 and the comparison is apt.The basic principles of the hydroskimmer (and,indeed, of all ground effect machines) may begrasped by considering an inverted washtub witha fan mounted inside the tub. When the fan isturned on, the tub begins to rise off the groundas soon as the air pressure inside the tub becomessufficiently great. This, in essence, isthe principle of the hydroskimmer.In a real hydroskimmer or other GEM, the airescapes uniformly around the bottom edges of thecraft, thus providing a cushion of air which liftsthe craft evenly above the ground or the water.The air cushion is developed and maintained invarious ways, depending upon the basic design ofthe vehicle.SErwin A. Sharp, JOC. USN, "Sailing on a Bubble ofAir." All Hands . December 1960, pp. 8-11.630