Johnson, Jr. 



Fig. 3 - Factors influencing optimum propulsor selection 



LARGE SHIP G.T. WATERJETS 

 B.L. INGESTION 



40 60 



VELOCITY - KNOTS 



Fig. 4 - Design propulsor 

 comparisons (Ref. 4) 



INLETS 



The two most important problem areas in the design of water -jet propulsion 

 systems are the inlet and the pump. 



The inlet must be designed to operate cavitation-free at cruise speed. Such 

 cavitation-free operation in uniform, zero -incidence operation should be straight- 

 forward, but the inflow is generally nonuniform and there is always some inci- 

 dence as the craft moves through rough water or maneuvers even in smooth 

 water. Furthermore, the optimum flow rate through the system varies with the 

 speed and thrust requirements of the boat, so that the inlet must be capable of 

 operating over a wide range of inlet velocity /forward speed ratios or must be 

 designed with special features, such as variable area. There is, at present, a 

 great lack of experimental data concerned with the cavitation characteristics 

 and efficiency of head recovery of inlets suitable for water jet propulsion sys- 

 tems. Reference (5) does present a procedure for the design of two-dimensional, 

 base-vented inlets for various inlet velocity ratios. 



1048 



