Water-Jet Propulsion for High-Speed Surface Ships 



c) Nacelle body and rejection nozzle 



_ conventionnal friction resistance computations were ap- 

 plied to predict the nacelle external friction drag. 



_ experimental study of a complete small scale model of 

 the propulsion unit (fig. 4) was carried out, having in view the 

 determination of cavitation limits of the external flow and the 

 lift coefficient of the propeller. 



During these tests the internal rate of flow was simulated with 

 help of a small internal pump similar to that of the prototype but no 

 shaft power measurement was made since the impeller Reynolds 

 number and bearing friction torque were not proper for that purpose. 



d) Integral test of the entire propulsion unit 



Since the effect of mutual interaction of external and internal 

 flows is of the same order of magnitude as the one of scale 

 effects, we decided to run integral accurate tests only on the 

 full-size prototype unit. As already mentionned this implies 

 that the hydraulic design of some parts of the propulsion units 

 (mainly the rejection nozzle) has to be finalised during the 

 prototype test. This is presently carried out as explained in 

 § 7. 



VI. TEST RIG FOR HIGH SPECIFIC SPEED PROPELLER-PUMP 

 DRIVE UNITS 



6. 1. Description of the test-rig 



If centrifugal or mixed flow pumps are involved, conventional 

 test rigs may be used to perfect the hydraulic design of pumps used 

 in water -jet drive units. The only significant difference between 

 these and normal pumps is the relatively higher velocity at the 

 volute outlet. 



However the test rigs normally used for propeller pumps are 

 not suitable for solving the problems associated with high specific 

 speed propeller pumps, used in the SFJP. 



Firstly the head generated by these pumps is often relatively 

 low in comparison with the kinetic head corresponding to approach 

 velocity and to discharge velocity so that accurate measurement of 

 the generated head is difficult. Moreover pump efficiency, as a pro- 

 peller, depends upon both generated head and also upon the transversal 



17 



