Dupovt, Visconti,and Mevle 



Other circuit arrangements are, a priori, feasible, for 

 example those with a lateral water intake to provide a " T " shape, 

 or those ( ft shape) in which the discharge nozzle would be lowered 

 to surface level, so as to reduce head generated by the pump. 



However neither of these arrangements has been adopted 

 for rapid surface vessels. 



As regards the former ( T ), the cavitation limit character- 

 istics of the lateral water intake are considerably less advantageous 

 that those of the frontal intake, so prohibiting its use at high speeds 

 for surface vessels. 



The latter arrangement ( ft ) is of no practical interest 

 since the additional weight and the head loss in the "return sector" 

 more than offset the reduction of geometric head. 



The Z flow propeller practically always includes a relativ- 

 ely long internal hydraulic circuit, part of which is situated above 

 the free stream surface, where as the straight flow propeller gener- 

 ally has a short hydraulic circuit and is often called a pumpjet and 

 in some ways, it is related to ducted propellers (ducted propellers 

 with a decelerating nozzle). 



The internal losses in the Z flow propeller circuit can be 

 relatively high so that the head produced by the pump results not 

 only from the kinetic energy added to the jet but also from the inter- 

 nal head loss and the geometric head. For this reason and also due 

 to weight consideration the optimization of this type of propeller 

 generally leads to a relatively high ratio of jet speed versus forward 

 drive speed and lower efficiency than that of a "straight flow" pro- 

 peller which can accept a relatively low ratio. 



III. PERFORMANCE AND QUALITY PARAMETERS CHARACTER- 

 ISING A PROPELLER 



We have indicated in § 2 that efficency is neither the only 

 one nor the most important parameter to take into account when 

 selecting the type or optimising the characteristics of a propeller 

 to be adopted for high-speed surface ship. 



Restricting ourselves to hydrodynamic considerations 

 concerning both the propeller and the ship, we propose to define 

 some simple parameters, which might express the main preoccupa- 

 tions of a fast-ship designer. 



