Super cavitating Propeller Theory 



theory for moderately loaded noncavitating propellers and as such would make 

 use of adequate singularity distributions, to represent not only blade loading 

 but also cavity thickness. 



The author's paper must therefore be regarded as a most welcome step 

 towards producing the basis for a more rational approach to the design prob- 

 lem of propellers which are to operate under fully cavitating conditions. I am 

 particularly looking forward to the numerical results that eventually will be 

 obtained by this method. With the fairly comprehensive model-test data, that 

 nowadays exist for fully cavitating propellers, one should easily be in a position 

 to judge the relative merits of the various assumptions that have been made by 

 the author in the process of deriving the expressions for the induced velocities. 



It is in this context that I would like to put a question to the author: Having 

 carried out cavitation-tunnel tests for a number of fully cavitating propellers, 

 which were based on the so-called Newton-Rader series, I feel that the assump- 

 tion of a basically cylindrical propeller race may well be justified under operat- 

 ing conditions when the angle of attack of the propeller blade sections is just 

 high enough to ensure absence of face cavitation. However, at low advance 

 ratios the expansion of the cavity-filled propeller race and the retarded inflow 

 to the propeller disc are well-known features. Does the author expect that his 

 theoretical work can also be used for analyzing the off-design performance of 

 fully cavitating propellers, as encountered at the take-off point in hydrofoils 

 or at the shallow-water hump in svirface- effect ships? In asking this question 

 it is, of course, realized that propeller lifting surface theory for fully wetted 

 propellers can nowadays be regarded as an adequate method for analyzing the 

 off-design performance of noncavitating propellers. 



REPLY TO DISCUSSION 



Geoffrey G. Cox 



The author is grateful for the encouraging remarks of Mr. Bavin, Dr. 

 English and Professor Kruppa. Their common desire to see numerical design 

 data, based on the presented theory, is shared by the author. Work is presently 

 being carried out at NSRDC to achieve this goal. 



It is interesting to hear from Mr. Bavin that a similar approach to the 

 supercavitating propeller design problem is being developed in the U.S.SJl. 

 His suggestion regarding the use of two-dimensional supercavitating hydrofoil 

 thickness data, as an approximation, appears to be plausible for the design 

 condition. For this condition, the cavities will be relatively long and thin, and 

 precision regarding cavity shape, especially for the trailing portion of the cavi- 

 ties, is not necessary. In any event strip-theory methods are necessary when 

 allowing for friction and cavity pressure drag effects. 



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