Design of "Z" Drives for Propulsion of Hydrofoil 



Byway of comparison the thrust deduction factor calculated by. 

 Eq. (2) is plotted on the same curve against propeller loading in the 

 absence of cavitation . The value Wpnom for the propulsion system 

 under study amounts to 0.0 45 .The agreement of t-values in the range 

 of moderate and high loads is considered to be quite satisfactory . 



The propeller mounted on "Z " drive operates in a radially 

 nonuniform axial flow . Besides this the propeller oncoming flow ex- 

 hibits circumferential nonuniformity due to the strut effect and some- 

 times due to the foils adjacent to the pod .For the purpose of adapting 

 the propeller to the wake the results of the experimental investigation 

 of the velocity field are used . The typical results of the measurements 

 for "Z " drive with length-to-diameter ratio — j— = 5 are given in Fig. 4 

 (a) and (b) . It is of interest to note that the nominal wake factors obtai- 

 ned by integrating the diagrams of velocity distributions are in good 

 agreement with the theoretical data and also with the design wake factor 

 defined in terms of propeller characteristics . 



In making calculations use is made of the wake values averaged 

 over the circumference ; however , after the propeller design is finished 

 it is necessary to verify the possibility of cavitation inception on the 

 face of propeller blade in the positions corresponding to the minimum 

 wake values . The most effective method of verification is the visual 

 observation during the tests in a cavitation tunnel . 



In choosing the optimum values of the propeller diameter and 

 the number of revolutions for the propulsion systems with "Z " drives 

 it should be taken into account that the increase in the number of revo- 

 lutions makes it possible to decrease the dimensions of the reduction 

 gear and consequently the propulsion pod dimensions and , as a 

 result , to decrease "Z " drive resistance . In this case the number 

 of revolutions may increase in comparison with the optimum number 

 specific for an isolated propeller . 



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