Panel Discussion 



J I I I L 



00 01 02 0,3 04 05 06 07 08 09 10 



LEADING FRACTION OF THE CHORD TRAILING 



EDGE EDGE 



Fig. 5 - Selected results on the drag of rotationally symmetric bodies 

 (no corrections applied) 



been referred to on this problem, but he knew of at least five. In 1955 there was 

 a Symposium held in Russia on ship hydrodynamics at which Bavin contributed 

 two articles. These are quite extensive, one giving the theoretical treatment of 

 the problem, the other the experimental. These were both very good pieces of 

 work. Also, research has been done at the Naval Ship Research and Develop- 

 ment Center by Beveridge on thrust deduction with a fully cavitating propeller, 

 and by Nelson at Naval Weapons Center. It seems very clear with the fully 

 cavitating propeller, at least, that the apparent change in thrust deduction is 

 due to the increase in cavity size. It acts like a thickness effect, and there is 

 some retarding of the flow ahead of the propeller. Dr. Morgan found it difficult 

 to believe that the small amount of cavitation probably present on a large tanker 

 would have such a big effect as stated by Dr. van Manen. 



Dr. Morgan next referred to scale effect on propeller efficiency. The usual 

 procedure for making powering predictions is to assume that the propeller ef- 

 ficiency does not change between model and full scale. This is obviously incor- 

 rect and does affect the magnitude of the correlation factors as well as giving a 

 scale effect on rpm. The statement is made without regard to whether or not 

 the flow is turbulent. For the blade sections normally used on propellers and 

 for the conditions for which they normally operate, the viscous drag is for all 

 practical purposes all frictional drag, i.e., no form drag. It would be expected 

 for the usual condition that the frictional drag of the full-scale propeller would 

 be about one-half that of the model which would lead to efficiency changes of be- 

 tween 4 and 8%, depending on the particular conditions. This difference in drag 

 could be greater if the Reynolds number is low and if the flow remains turbulent. 

 It would seem reasonable to include the changes in propeller performance, be- 

 cause of frictional drag, in our prediction procedures. Theoretical calculation 

 methods should be sufficient for this purpose, although confirming experimental 

 results are needed. 



In regard to thrust deduction. Dr. Morgan felt that the discussion of any 

 substantial scale effect on thrust deduction should be dismissed in the light of 

 recent work by Beveridge at the Naval Ship Research and Development Center. 

 In this regard, it is necessary to remember that thrust deduction is the differ- 

 ence between two large numbers and subject to considerable error. It has been 

 possible to calculate the thrust deduction quite accurately on a body of revolu- 

 tion and a slow- speed cargo ship (the Simon Bolivar). The results of the calcu- 

 lations are shown in the following table. 



1657 



