Duportj Visconti ,and Mevle 



V is the rated cruising speed. 

 (-< 



To define T p and Vp let us consider the trust versus speed dia- 

 gram(fig. 2). Two main limiting curves can be drawn on this diagram: 

 the one wich corresponds to a constant shaft power equal to that 

 required at cruising conditions, the one which corresponds to the 

 cavitation limit of the internal circuit and pump. Both lines intersect 

 at a point where the thrust is the highest one obtainable within the 

 limits of the cruise condition shaft power and in view of cavitation 

 limits. The coordinates of this point are Tp and Vp respectively. 



Comparing two propeller types which might lead to equivalent 

 performances at cruise conditions, one may be induced to select the 

 one with the highest peak thrust ratio. 



There is no precise general indication about the desirable 

 "peak thrust relative speed". However it must be noted that a high 

 peak thrust ratio cannot occur at high relative speed, but more at 

 relative speed between, say, 0. 3 to 0. 6. 



A "low peak thrust relative speed" would mean that the effi- 

 ciency decreases rapidly with the ship speed and that the available 

 thrust at medium speed may be not sufficiently high. 



3.4. Following examples of performances can be proposed, accord- 

 ing to the information we have in hands. 



Straight-flow jet propeller 



The 800 Kw, 50 kn prototype of SFJP which we have developped 

 performs the following characteristics. 



The negative RWB figure is derived from a hydrofoil projected 

 application where, due to optimised lift of the propeller, the lift 

 exceeds the weight of the propulsion unit and its transmission 



10 



