Hydrodynamic Problems Solved by Rheoelectric Analogies 



Supercavitating Propellers — The boundary conditions and the analog equip- 

 ment necessary for this study have already been taken into account in Eq. (15) 

 and in the subsection on Supercavitating Cascades. The boundary conditions will 

 now be imposed in the intersection of the straight cylinders ^ = Ct , for the 

 helicoids. 



Notice, however, the advantages of the lifting surface method as applied to 

 supercavitating propellers when it is compared with the approximate calculation 

 methods in use at the present time. First, the blade and cavity contours are 

 correcting represented, which allows, for a given blade form, the study of the 

 influence of the cavity form on the performance of the propeller. Second, the 

 cascade phenomenon and that of the interaction of the cavities are taken into 

 account during the calculation without having to introduce corrective terms. 



Various propellers have been designed by this method. The first propeller 

 calculated was tested in the cavitation tunnel of the Bassin des Carenes, Paris. 

 The results obtained did not confirm the theoretical estimates. This discordance 

 does not seem to be due to a fault in the theory, verified in the subcavitating 

 case, but to an unrealistic choice of speed coefficient. Three propellers have 

 recently been calculated and one of them should be tested very soon at the U.S. 

 Naval Ship Research and Development Center. Figure 25 shows one of these 

 propellers, designed for an optimal span circulation distribution and pressure 

 distribution on each section of the blade such that, at the leading edge a very 

 localized infinite pressure encourages the starting of cavitation (behavior of the 

 flat-plate foil), and the most heavily loaded part of the foil is near the trailing 

 edge (high lift-drag ratio criteria in the two-dimensional case). The charac- 

 teristics of these three propellers are summed up in Table 2, not taking into ac- 

 count the friction resistance. The figures in the table are for an advance coef- 

 ficient \ = Vq/wR = 0.261, a cavitation number a= 2(Po -p^)/pVq 2 = o.4, and 

 various blade and cavity forms. 



Table 2 

 Characteristics of Three Supercavitating Pro- 

 pellers Calculated by the Lifting Surface Method 



Ducted Propellers— The advantages of ducted propellers over ordinary pro- 

 pellers, for certain speed coefficients, have long been known. However, very 

 little study has been devoted to improvement of the functioning of the nozzle and 

 to increasing the efficiency of the propeller- nozzle system. The analog method 

 (11,45) offers calculation possibilities for this type of device which promise a 



403 



