Vlater-Jet Propulsion for High-Speed Surface Ships 



_ or by adopting a 2 stage pump. 



V. HYDRODYNAMIC STUDIES OF JET PROPELLERS 



5. 1. Advantages and limitations of separating the study of internal 

 and external circuits 



For long circuit water-jet propulsion units of the 2 flow- 

 type, separate examination of the hydraulics of the external and 

 internal circuits, and particularly the pump, is clearly well-founded 

 and advantageous. The internal and external flux have to be examined 

 conjointly only in the study of the immersed scoop. 



In the design studies of nacelle type propulsion units of the 

 "straight flow" type we considered that the same separate theoretic- 

 al and experimental approach was also of great interest for the fol- 

 lowing reasons : 



theoretical analysis of the internal flow and circuit design is 

 much simplified if it is considered separately from the extern- 

 al circuit. This is particularly significant when applied to the 

 pump design. 



experimental approach also is very much facilitated. For 

 example the test rig allocated for internal circuit study may be 

 used for measuring directly all characteristics of the internal 

 flux (rate of flow, momentum, thrust, cavitation limits, etc. ) 

 without any interference of the external flow. 

 High enough Reynolds number and a proper cavitation simul- 

 ation can be obtained with a reasonably small test rig as the 

 one described below. If equivalent limits were to be attained 

 in a hydrodynamic tunnel then the vein size would have to be 

 at least 1, 2 metres in diameter with a vacuum of 0, 5 m 

 absolute, a flow speed of 12m/s and discharge of 14m 3 /s so 

 that the facility would be considerably larger than that describ- 

 ed below whose discharge is limited to 0, 5 m /s. Moreover, 

 in such a tunnel facility, cavitation around the nacelle would 

 limit investigation of the cavitation limits of the internal cir- 

 cuit. Nevertheless separate study of the two flux gives rise to 

 certain difficulties since the internal flow is influenced by the 

 external flow as is clearly shown, for instance in theories 

 concerning ducted propellers. 



However in the case of "straight flow" propulsion units of the 

 type which we have developped for rapid surface ships, this effect of 



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