WATERJET PROPULSION 



Virgil E. Johnson, Jr. 

 Hydronautics, Inc. 

 Laurel, Maryland 



INTRODUCTION 



Although waterjet propulsion is certainly not a new type of propulsion, it 

 has become of increasing interest in the past decade. In the late 1950's, water- 

 jet propulsion received considerable publicity as the method of propulsion for 

 sports craft of the future— probably because the glamour of je^ -propelled air- 

 craft appealed to the small sports craft owner. Most of this type of appeal has 

 worn off now, and the rush to waterjet-propelled small sports craft has not yet 

 materialized. The facts are that at the present speed of most small craft (35 

 knots or less), the efficiency of the conventional screw propeller is considerably 

 higher than that achievable by waterjet systems. Consequently, except in those 

 cases where very shallow craft or a completely protected impeller is required 

 (regardless of efficiency), waterjet propulsion is not likely to take over in the 

 small sports craft field. 



At higher speeds, say, greater than 45 knots, the conventional screw pro- 

 peller suffers because of cavitation. The substitute for the conventional screw 

 propeller at these higher speeds is the "supercavitating" propeller. Such pro- 

 pellers have been successfully designed and built for numbers of planing and 

 hydrofoil craft in the speed range less than 60 knots. Much higher speeds are 

 achieved in racing craft using supercavitating propellers. 



The great increase in interest in waterjet propulsion in the past five years 

 has been brought about because of the need to propel high-speed craft (45-100 

 knots) such as hydrofoils and surface -effect ships. Although the supercavitating 

 propeller is a logical candidate for propulsion of these higher-speed craft, the 

 supercavitating propeller does experience serious performance degradation at 

 low advance ratios and thus has difficulty producing the large thrust required at 

 hump speed. In those configurations using a "Z" drive, the bevel gears required 

 in the pod to supply power to the supercavitating propeller tax the state of the 

 gear design art. 



Waterjet propulsion offers a substitute for supercavitating propellers at 

 high speeds. Waterjet propulsion systems eliminate the gearbox problem, and it 

 should be possible to design waterjet systems with "hump" thrust characteris- 

 tics superior to those of supercavitating propellers. Of course, it is generally 

 nature's way that solutions to existing problems introduce new problems— so it 

 is with waterjet propulsion. This paper presents no new information related to 

 waterjet propulsion, but rather highlights the present state of the art. For a 

 more comprehensive state-of-the-art report, including an extensive and up to 

 date bibliography, see Ref. (1). 



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