Vlater-Jet Propulsion for High-Speed Surface Ships 



Moreover the gross thrust of the propulsion unit may be expres' 

 sed as : 



T PQV (w-1) 



g 



where 



V j 

 w = — and V is jet velocity. 



The critical cavitation condition may be used to calculate Se : 



Se = Q 



V- 



2g (NPSH) 



where <r e is the "critical" value of a 



From these equations it is possible to calculate the non-dimensional 



parameter : 



= 2 (w-l)_L ' ' 2g NPSH 



w 



D KCx V 



This equation shows that, for a given forward speed (and thus 

 a given NPSH) Tq/D increases with w , which is evident since an 

 increase in w for a given thrust leads to a decrease in the rate of 

 propulsive flow. This effect though beneficial upon scoop drag, reduC' 



es the theoretical drive efficiency which is equal to — =— . This 



will not be expanded in this discussion since the optimisation of w 

 also involves head losses in the circuit and the weight balance. 



f 

 It should also be noted that j f" decreases when the forward 



speed of the ship increases. 



Moreover the non- cavitation condition of external flow and the 

 external streamlining of the scoop will increase coefficients K and 

 Cx. Beyond a speed of approximately 50 knot , sub-cavitational 

 flow can not be maintained around the scoop and super cavitational 

 conditions of external flow would lead to an increase in KCx. 



4.2. SFJP 



The non -cavitation condition for the external circuit may not, 

 in this case, be expressed so simply as for the scoop elbow of the 

 ZFJP. 



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