PRINCIPLES OF NAVAL ENGINEERING 



The primary advantages of the hydrofoil form 

 are high speed and superior seakeeping abilities. 

 The high speed is attainable because the resis- 

 tances encountered by other shipforms are sub- 

 stantially reduced in the hydrofoil. When the hy- 

 drofoil is flying, the hull is entirely above the 

 surface of the water and is therefore not impeded 

 by frictional resistance or wave- making resis- 

 tance and is not disturbed by waves, swells, or 

 choppy surfaces that slow down other craft. 



At present, the hydrofoil form is not con- 

 sidered feasible for very large vessels. How- 

 ever, the future development of propulsion plants 

 with smaller specific weights^ could well extend 

 the tonnage range of the hydrofoil form. In fact, 

 a "hydrofoil destroyer" has even been proposed. 



Several types of hydrofoil systems have been 

 developed. The Navy hydrofoil program has con- 

 centrated largely on fully submerged subsurface 

 wings or foils. Control surfaces on the fully sub- 

 merged foils act like aircraft ailerons and con- 

 trol the course of the craft through the water; 

 the foils are controlled by a height- sensing sys- 

 tem which maintains level flight. In another type 

 of control (incidence control) the entire foil is 

 moved instead of flaps. Other hydrofoil systems 

 include skids or other planing devices and sur- 

 face-piercing foils. Surface-piercing foils have 

 a lifting area that is proportional to the amount 

 of foil immersed. 



A number of hull forms have been used for 

 hydrofoils, but most of them are basically adap- 

 tations of conventional hull forms. The hull is 

 relatively long and narrow, with the length of the 

 craft being eight to ten times the beam. It has 

 been suggested that the catamaran'* design may 

 have certain specific advantages for hydrofoils; 

 however, this type of design is not currently used 

 in Navy hydrofoils except on an experimental 

 basis. 



Although a wide variety of power plants have 

 been considered for hydrofoil craft, the gas tur- 

 bine and the diesel engine are the types primarily 

 installed in these craft at present. Some hydro- 

 foils are equipped with both gas turbines and 

 diesel engines. 



"^The specific weight of a propulsion plant is the 

 number of pounds of propulsion machinery required 

 per shaft horsepower. 

 4 

 The catamaran or twin-hull design consists of two 



slender hulls which are joined together above the 

 waterline. 



A major problem in the development of hydro- 

 foils has centered around the transmission of 

 power. When an underwater propeller is used, 

 power must be transmitted downward from the 

 prime mover in the hull to the propeller, which 

 is at a deeper level. Various solutions to this 

 problem have been tried. One solution is to use 

 an inclined shaft and an inclined prime mover 

 which drives into a V-gear. Another solution is 

 to use double right-angled gearing. To date, no 

 solution has been found that is entirely satisfac- 

 tory. 



Propellers have also been a continuing pro- 

 blem with hydrofoils. The cavitation encountered 

 at high speeds has led to numerous propeller 

 failures. One solution to this problem is the su- 

 percavitating (SC) propeller discussed later in 

 this chapter. Because of the propeller problem, 

 other propulsion devices such as airscrews and 

 air jet or water jet propulsion systems have been 

 considered for use with hydrofoils. 



The Navy's first operational hydrofoil, USS 

 High Point , PCH 1, is shown in figures 25-1 and 

 25-2. In flight, this hydrofoil reaches speeds of 

 more than 45 knots. 



Hydroskimmers 



The hydroskimmer belongs to the general 

 category of craft designed to ride on a bubble or 

 a cushion of air. Vehicles and craft in this cate- 

 gory are called ground effect machines (GEM), 

 air cushion vehicles (ACV), or surface effect 

 ships. In general, any GEM that is designed to 

 operate 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 be 

 grasped by considering an inverted washtub with 

 a fan mounted inside the tub. When the fan is 

 turned on, the tub begins to rise off the ground 

 as soon as the air pressure inside the tub be- 

 comes sufficiently great. This, in essence, is 

 the principle of the hydroskimmer. 



In a real hydroskimmer or other GEM, the air 

 escapes uniformly around the bottom edges of the 

 craft, thus providing a cushion of air which lifts 

 the craft evenly above the ground or the water. 

 The air cushion is developed and maintained in 

 various ways, depending upon the basic design of 

 the vehicle. 



SErwin A. Sharp, JOC. USN, "Sailing on a Bubble of 

 Air." All Hands . December 1960, pp. 8-11. 



630 



