Duport, Visoontij and Merle 



The authors show that his can be applied (while with 

 some precautions) to short-ducted, nacelle-type pro- 

 pellers. They describe an unconventional test-facility 

 which has been specially developed for the design- 

 study of high specific-speed impellers now applied to 

 a 50 knots water-jet propeller. Comparison of pro- 

 totype and predicted performances of this propeller 

 is in favour of the applicability of the "partially se- 

 parated" hydrodynamic approach. 



I. WHY IS WATER JET PROPULSION OF INTEREST FOR RAPID 

 SURFACE VESSELS 



The use of subcavitating free stream propeller in surface 

 vessels is practically limited beyond speeds of approximately 40 

 knots. The main cause of this limitation is that the propeller thrust 

 behind the propeller requires that flow speed is at least equal to 

 forward drive speed and, consequently, relative speed at the rotor is 

 considerably higher than forward drive speed. 



Thus even if a very low load coefficient is adopted, the 

 inception of cavitation is inevitable. 



If the subcavitating impellers are to be used in the high 

 velocity range then the drive flux must be separated from the main 

 flowstream i.e. water jets must be used. These comprise : 



an immersed water intake, 

 an internal hydraulic circuit, 

 a pump, 

 a discharge nozzle. 



With this arrangement the practical range of subcavitating 

 impellers may be extended, principally, for the following reasons : 



selected impeller approach and discharge speeds may be 

 adopted independently of forward drive speed (of the ship) 



. use may be made, if necessary, of impellers other than 

 the axial flow type which is suitable for free stream pro- 

 peller s(centrifugal pumps, mixed flow pumps, multistage 

 pumps, etc. ) 



