Fully Cavitating Propeller for a Hydrofoil Ship 



Fig. 8 - Screw and wake simulator in tunnel 



For reasons of brevity it is not proposed to discuss the details of all the 

 screws that have been tested in the course of this project, but since the final 

 design has been built on the results of the earlier testing it is informative to 

 summarise the conclusions drawn from this work. A summary of the various 

 models that have been tested is given in Table 2. 



In the early design procedures the most important hydrodynamic features 

 of the screws, apart from diameter, i.e., the pitch and camber of the wetted 

 faces of the cylindrical sections, were designed using a mixture of momentum 

 theory, in which an appropriate allowance was made for the presence of the 

 cavities in the wake, and noncavitating propeller theory. The camber shape 

 adopted for the wetted faces of the sections was of circular-arc form, mainly 

 because of the availability of Wu's nonlinear theory for calculating the flow 

 around isolated fully cavitating foils at nonzero cavitation numbers (Ref. 10). 

 Also, it is possible to calculate accurately the position of the back cavity rela- 

 tive to the wetted faces— a factor that was considered highly important in the 

 early design stages for minimising section cavitation drag, i.e., ensuring that 

 the cavity runs clear of the back of the section. 



In retrospect, it would now appear that while the above approach gave a 

 satisfactory pitch distribution, the section cambers predicted by the method 

 were underestimated. This led to a propeller which, although highly efficient, 

 would not absorb the full engine power and hence did not produce sufficient 

 thrust at the high-speed design point or the takeoff condition. Modifications 

 were then made to the propeller geometry in order to increase the load-carrying 

 capacity. These consisted of increasing the amount of circular-arc cambers 



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