Duportj Visoontt , and Merle 



NPSH 



a - 



Ve 2 /2g 



NPSH being the net positive suction head at the elbow intake, 



Ve is the mean velocity in the intake section of the elbow. 



To the best of our knowledge, even with an extremely good 

 vane profile, o must be at least 0, 35. 



If this condition is to be respected then a diverging section is 

 required at high speed so that the velocity at the elbow intake is 

 lower than forward speed. 



This limit has a direct effect upon the central cross-section of 

 the scoop and upon strut thickness. 



Scoop drag (friction and wave resistance) depends upon : 



the shape of the cross-section of the scoop and the intake 

 pipe, 



the fairing and dimensions of the scoop and the strut, 



dimensions. 



In order to calculate simply the incidence of the non- cavitation 

 condition of the elbow upon propulsion unit performance, we will 

 suppose that the external centre section for optimised shapes of 

 scoop and strut is proportional to the cross- section of the elbow in- 

 take. 



Scoop and strut drag D may thus be expressed as : 



D = -^r-K C P Se V 2 



2 x 



where 



K is the form factor 



C is the coefficient of drag 

 x 6 



P is water density 



Se is the elbow cross-section 



V is forward speed 



12 



