Duport* Viscontij and Merle 



The critical conditions for the inception of cavitation in the 

 pump involve the pump characteristics : 



/ 



Q 



Cm = 



Sr ^/2gH 



in which 



in which 



Q is the rate of propulsive flow 

 Sr is the cross-section of the impeller 

 H is the head generated by the pump 

 H 



u is the peripheral velocity of the impeller. 



Finally for the given value of Cm and ^ the pump cavitation 

 limit is expressed by the Thomas parameter : 



NPSH 



H 



A complete examination of this question, which will not be given 

 here, reveals the following principal considerations : 



In propulsion units for high forward speeds : a diverging section 

 before reaching the pump inlet is, in every case, necessary : for 

 example at speeds of 50 knots pump intake speed has had to be 

 reduced by approximately 80% of the forward drive speed. 



As with the ZFJP scoop, increase in speed to slightly over 

 50 knots results in a slight increase of coefficient K and, beyond a 

 certain limit speed, super -cavitating external flow is required. 



Finally, it may be seen quite readily that available pump a 

 for a given w decreases as velocity increases, this may be compen- 

 sated by : 



_ either reducing w by discarding the optimum values result- 

 ing from compromise between theoretical efficiency and nacelle 

 drag, 



14 



