Panel Discussion 



nozzle for a single- screw ship. There was also a reduction in thrust deduction 

 fraction with the propeller operating in the wake adapted nozzle. 



Dr. J. W. English (National Physical Laboratory, England) referred to the 

 remarks of H. Lackenby and the comments on them by Professor Kinoshita and 

 Professor Prohaska. An NPL tanker model has confirmed Professor Kino- 

 shita' s remarks about directional stability and at the same time refutes the re- 

 marks of Professor Prohaska regarding the British method of analysis. This 

 particular tanker had its LCB well forward and it was directionally unstable, 

 but it is believed that there is a physical explanation of why a large hull effi- 

 ciency was obtained in this case. Like all modern tankers, it had a very large 

 beam/draft ratio and at the stern the flow outside the boundary layer was pre- 

 dominantly upwards. This had the effect of turning the inner boundary layer 

 upwards at the outside, and in fact, as has been pointed out at the last ITTC, 

 swirling areas of flow can be seen. It is probably incorrect to call them 

 vortices. Rather, they are a collection of the boundary -layer material from 

 further upstream which is passed through the stern in the vicinity of the pro- 

 peller disk. As a consequence of this, the wake is abnormally high and, one 

 ends up with a high hull efficiency. The flow has now been studied by five-hole 

 Warden tubes and flow visualization tuft techniques, etc., and the flow clearly 

 has a downward component near the center line and an upward one further out. 



Dr. Todd reminded Dr. English that he wished to comment upon Professor 

 Prohaska' s remarks about the method of analysis. Dr. English said that Pro- 

 fessor Prohaska had implied that the methods of analysis used in British tanks 

 gave different results, but he did not believe this to be true. There is a large 

 wake fraction, there is a large hull efficiency, what Lackenby has shown is 

 correct, and there is a physical explanation for them. 



H. P. Rader (Hamburg Model Basin, Germany) described some work being 

 done for the ITTC Cavitation Committee. The wake scale effect has not only 

 a great bearing on the scaling of the shaft speed, but also on the local wake dis- 

 tribution for determining the cavitation patterns and the effect of the latter on 

 thrust and torque variations in a nonuniform velocity field. Now, there are two 

 ways of studying this effect. One is to consider it as a boundary-layer problem, 

 and the other is to consider it as a wake problem far behind the ship. In be- 

 tween, one can interpolate (or make some guesses) with a polynomial of high 

 order. It can be assumed that an essential part of the wake of model and ship 

 consists of boundary-layer material which has been subjected to a pressure in- 

 crease. Velocity distributions in accelerated and retarded turbulent flows can 

 be described to a good approximation by the universal relation 



r (!)""■ a) 



where 



u = the local velocity inside the boundary layer, 



U = the undisturbed velocity outside the boundary layer, 



y = the distance from the wall, 



1653 



