Water-Jet Propulsion for High-Speed Surface Ships 



Naturally the question must be raised as to the viability of extend- 

 ing the use of subcavitating impellers. 



A discussion of this question would involve a comparative 

 analysis of all systems which have been, or which could be, used 

 for the propulsion of rapid surface ships. This would exceed the 

 scope of the present subject and we present graph fig. 1, extracted 

 from Mr Myers paper at the SNAME Hydrofoil Symposium 1965, to 

 which has been added a curve for the "straight flow jet propeller" 

 which we will discuss later. 



The existence of viable field of use for jet propellers (i. e. 

 Z flow or "water-jets" or straight flow) is even more evident if 

 reference is made to available thrust at the propeller at intermediate 

 speeds (and not simply efficiency at cruising speed as shown in 

 figure 1). In fact all rapid ships where the hull is designed to lift at 

 cruising speed suffer from relatively high resistance at intermediate 

 speeds and from this aspect, the performance of jet propellers is of 

 particular interest, as will be demonstrated. 



II. TYPES OF WATER JET PROPELLERS FOR RAPID SURFACE 

 VESSELS 



To the best of our knowledge, two main types of water jet 

 have been developed, or proposed, for fast surface vessels and they 

 may be differentiated by the general arrangement of the internal 

 drive circuit. 



_ the type which we propose to call "Z flow jet propeller" 

 (ZFJP) comprising an inlet component (scoop) the forward 

 part of which is open, and a discharge nozzle at the rear 

 the axis of which is offset in height compared to that of the 

 water inlet axis. Between these two components, the cir- 

 cuit proper is rectilinear, or practically so. One of the 

 two elbows of the circuit may be formed by the pump volute. 

 This arrangement (specifically used by Boeing for hydrofoil 

 drive, is essentially based upon the advantage of fitting the 

 pump within the hull of the ship with in-line or parallel 

 shaft drive). 



_ the type which we propose to refer to as "Straight flow 

 jet propeller (SFJP) the inlet component and the discharge 

 nozzle being aligned, implying that the whole of the propel- 

 ler is submerged. 



