Propeller-Hull Interaction 



It was his contention that since the thrust deduction can be calculated with 

 such good accuracy there can be little scale effect on thrust deduction per se. 

 Since most of the thrust deduction comes from the potential part of the problem, 

 the scale effect must be small as only the frictional part can be affected. This 

 is assuming, of course, that the propeller loading on the model is the same as 

 the full-scale loading. 



Apparent scale effect on thrust deduction could arise if the radial loading 

 of the propeller is radically different between model and full scale or if the 

 propeller action changes the separation point at the ship stern or the ship trim. 

 The loading effect could come about by differences in the radial wake distribu- 

 tions. Calculations show that the radial load on the propeller can affect the 

 thrust deduction to some extent. However, this effect is not large and in any 

 case, it is not a true scale effect on thrust deduction. Also, changes in ship re- 

 sistance by change in separation point or trim, even though the effect could be 

 large, should be considered more rationally and not lumped in as part of the 

 thrust deduction. 



The chairman asked if these calculated values of t were done for the model 

 size. Dr. Morgan said they were all model values. Similar calculations could 

 be done for the full scale if it were possible to measure the full scale wake. 

 What are called scale effects are really indications that there is something 

 going on that is not understood. 



Dr. M. Schmiechen (Berlin Towing Tank, West Berlin) said he had actually 

 touched on this problem that morning and had given a set of equations for the 

 thrust deduction in terms of thrust loading, wake ratio, which is the ratio of the 

 total wake to the frictional wake, and the known uniformity of the propeller jet. 

 This set of equations was derived upon the assumption that the frictional thrust 

 deduction is zero. The work is based on that of Dickman, which is apparently 

 completely forgotten (and is not even mentioned in the new Principles of Naval 

 Architecture). In 1939 he presented a paper on this subject, but in his work 

 there was a slip, and Dr. Schmiechen had tried to find out what the error was 

 and so come up with the proper set of equations by putting the frictional com- 

 ponent equal to zero, and saying that the thrust deduction is a function of the 

 thrust loading, of the wake ratio and the known uniform thrust of the propeller 

 jet. 



L. A. Van Gunsteren (Lips Propeller Works, Holland) referred to Lind- 

 gren's earlier remarks, in which he said that the Swedish tank was building a 

 test facility for models of 8 m in length. Van Gunsteren pointed out that that 

 was about the length of the models employed at the NSMB. For the large tankers 



1658 



