water tunnels; this is susceptible to much more detailed mathematical study than it has 

 yet been given. From this point of view it is possible that the effective length of a 

 marine propeller is much greater than its physical dimensions would suggest, and is 

 perhaps related to the axial dimension of the non-parallel induced flow and wake. 



I am a little disappointed that Prof. Lerbs did not say a little more about the 

 future of marine propulsion. Undoubtedly machinery powers are going to increase — 

 as we have been reminded earlier today — thus the powers to be transmitted by orthodox 

 propellers will also increase. Like Mr. Tachmindji, I believe that for this reason contra- 

 rotating propellers may have quite a lively marine future, particularly as recent devel- 

 opments have been made in their theory. While they may not be applied to spectacular 

 ocean liners, they may well be of value in future cross-channel type ships. It is also 

 clear that we need, and shall increasingly need, further developments in the theory 

 of fully cavitating propellers. Much work is at present going on in this field, of course, 

 and opportunities for merchant ship applications will probably soon occur. However, 

 there are many of us who believe that orthodox propellers will not be entirely adequate 

 for future really advanced conditions. The overlapping problems of heavier loadings, 

 the incidence and effects of cavitation, and propeller induced vibration may well force 

 us to consider rather less orthodox propulsion methods. Although Prof. Lerbs uses 

 the word 'propulsion' rather than 'propellers' in his title, he has in fact confined his 

 comments to orthodox propellers; some remarks on other propulsion devices would 

 have been a valuable addition. 



The equipment needed for model experiments with these advanced designs will 

 be considerably different from that used now. In this country and in Britain we are 

 attempting to meet this need by building much larger water tunnels than we have at 

 present, but I am perturbed that we are embarking on these expensive pieces of 

 equipment with such an inadequate knowledge of how propellers perform in them, 

 their limitations, and the relation between tunnel results and full-scale performance. 



Finally, I should like to register a mild disagreement with the apologies made 

 this morning for the naval architect's laggard approach to his problems compared with 

 the aeronautical engineer. I believe that the ship designer has not a great deal of which 

 to be ashamed, and I would suggest that Prof. Lerbs' exposition of the theory of broad 

 bladed propellers is an excellent example of one field in which the marine approach 

 has been at least as thorough as anything comparable in aircraft work. 



T. Y. Wu 



I wish to compliment Professor Lerbs on his interesting lecture which presents 

 some general and instructive viewpoints. As was described in his lecture, the vortex 

 theory for propeller problems has been developed to account for the flow conditions 

 where both the inflow and the bound vortices are arbitrary functions of the radius. 

 This contribution then narrows the gap between theory and application, for the marine 

 propellers in general operate with a nonuniform inflow. Concerning the theory itself, 

 I wonder whether a general expression has been obtained for the optimum configuration 

 of the trailing vortex sheets when the propeller operates at the highest efficiency with 

 a given inflow and loading distribution. I am also interested to know the feasibility 

 of generalizing the theory to include two additional factors: 1. the effect of heavy 

 loading due to the contraction of the slipstream and 2. the case of general nonuniform 

 inflow, such as a shear flow with no axial symmetry, as is typical for stern propellers. 



In the problem of the interaction between hull and propeller, some further 

 improvements perhaps can be visualized. In practice, most conventional ships are 

 likely too "fat" for the thin ship theory to remain a good approximation, especially 

 for those ships with blunt stern section for which the interaction problem is concerned. 

 The idea thus seems inviting that improvements could be obtained by using the slender 

 body theory to represent the hull with the added displacement thickness of the boundary 

 layer and by using a simplified vortex distribution for the propeller disc. In the case 

 where the boundary layer separates from the hull in front of the propeller, it would 



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