Water-Jet Propulsion for High-Speed Surfaoe Ships 



3. 1. Efficiency, the most usual concept, needs some clarification, 

 which cannot be achieved by just transferring to jet propellers the 

 considerations which have been developped for conventional free- 

 stream propellers. 



We think that there is no ambiguity about the definition of 

 "input power", which should be in any case the shaft power of the 

 impeller, and which we designate by Ps. This implies, however that 

 the power transmission efficiency should be taken into account 

 through another way in the selection and optimisation procedure. 



"Usefull Power" on the contrary, as it is the case for convent- 

 ional propeller may be more ambiguous, as long as the "thrust" 

 concept has not been clarified. 



In case of jet propeller it is easy to define the "gross thrust" 

 Tg of the propulsion unit : this is the trust which results from the 

 jet reaction. In case of a drowned jet with a uniform distribution of 

 velocity, this being parallel to the drive speed, it writes : 



Tg = M (Vj - V) 



Tg = gross thrust 



M = rate of mass -flow 



Vj = jet velocity 



V = ship speed 



This gross-thrust is equal to the longitudinal component of 

 hydrodynamic forces transmitted to the ship by the propelling flux ; 

 these forces are transmitted not only to the surface of the internal 

 circuit but also partially to the ship hull if the intake mouth of the 

 propeller circuit is located close to it, (to this extent, gross thrust 

 of a jet propeller is not to be compared to the thrust of a convent- 

 ional "open-water" propeller). 



It is not enough to consider gross thrust of a jet propeller. 

 This is because : 



_ the wetted structures of the propulsion unit are the source 

 of a supplementary resistance which is not taken into account 

 in the gross-thrust. This supplementary resistance is due to 

 the external flow surrounding the wetted structures ; it can be 

 for instance : the friction and wave resistance of struts con- 

 necting the propulsion unit (or the intake scoop) to the ship, or 

 the external resistance of the nacelle of a "Straight Flow" 

 propulsion unit, etc. 



