Lindgren, Johnsson and Dyne 



As pointed out by Mr. Oosterveld, the optimum diameter of the different 

 ducted propellers in our investigation must be different. Calculations carried 

 out show, however, that the deviation from the optimum diameter means a loss 

 in efficiency of less than about 2%. 



The authors agree with Mr. Oosterveld that the risk for flow separation on 

 the duct depends also on the thickness of the duct profile. The reason why ducted 

 propeller PI 31 5 D6, which has a higher efficiency than NSMB nozzle at the design 

 point, is less effective at lower loads is certainly its more slender duct profile. 



Since the wake behind a ship is nonuniform, it seems, at a first glance, to 

 be natural to use noncylindrical ducts. The question is, however, \ how to design 

 the duct shape in this case. A local increase of the low velocities in the upper 

 part of the screw disk will certainly make the inflow velocity more uniform, but 

 this does not necessarily mean an increased propulsive efficiency. However, 

 the NSMB tests with the noncylindrical ducts are very interesting, and we hope 

 that there will appear a more detailed description of the investigation in a near 

 future. 



Turning to Mr. Silovic's question, it is true that we use a simplified repre- 

 sentation of the 0.8 mean line for our lifting- surface calculations. As described 

 in Ref. (14), a constant distribution over 90% of the chord length is used, start- 

 ing from the leading edge. Comparison with results of calculations using the 

 true 0.8 mean-line distribution show very good agreement, and this is the rea- 

 son why we have not yet used our limited programmer staff for completing the 

 program in this respect. 



Mr. Emerson's summary of the Stone Manganese Marine investigation on 

 contrarotating propellers is valuable, and it is interesting to note that the im- 

 provements obtained with regard to propulsion efficiency and cavitation proper- 

 ties are in good agreement with the results presented by the authors. 



The authors are not convinced that the balancing of power between the for- 

 ward and aft propellers represents a critical problem. As a matter of fact, our 

 results indicate an astonishingly small influence of the relation of power be- 

 tween the two propellers, as could be seen, for instance, in Fig. 8. However, no 

 doubt the scale-effect problem including Reynolds -number effects is very diffi- 

 cult, and there are still many confusing problems to be solved. In connection 

 with our experiments with the tanker, we tested one pair of contrarotating pro- 

 pellers designed for the model case and one pair designed for the ship case. At 

 the conventional self -propulsion experiments the "model" pair was superior, but 

 when we tried to carry out experiments at a / value corrected for wake scale 

 effects the "full-scale" pair gave better results. The scale effects are, how- 

 ever, not known with sufficient accuracy and, furthermore, it is impossible to 

 copy the full-scale case on a model in a quite true way. 



As mentioned in our paper, we have no experience of our own with regard 

 to the application of tandem propellers. Furthermore, we do not know of any 

 measurements that include individual thrust and torque readings. 



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