Model Tests on Contrarotating Propellers 



the tips of the two propellers lay on the same mean stream tube. This is not 

 sufficient to avoid the problem mentioned, and a further decrease is necessary. 



My second question is related to the figures Mr. Oosterveld gave on the 

 possible power gains with contrarotating propellers for the two different ship 

 projects. Do the figures given refer to the same rpm for the contrarotating and 

 the conventional propeller, or which assumption was adopted? 



DISCUSSION 



William B. Morgan 



Naval Ship Research and Development Center 



Washington, D.C. 



In the David Taylor Model Basin at the Naval Ship Research and Develop- 

 ment Center we have run a large number of experiments on contrarotating pro- 

 pellers. We have found that in general the hull efficiency, and relative rotating 

 efficiency of the ship model are different from that of the same model with a 

 single propeller. Also, we have shown that for best performance the design of 

 the propellers should be based on their particular application. For these two 

 reasons it is not clear to me how a series of open-water tests of contrarotating 

 propellers can be used to pick the best performing propellers. 



DISCUSSION 



C. A. Johnsson 



Swedish State Shipbuilding Experimental Tank 



Goteborg, Sweden 



I notice that the authors, when discussing the different propulsion coeffi- 

 cients, do not mention one of particular interest in this context: the relative 

 rotative efficiency. When we analyzed our self -propulsion data, we found that in 

 some cases most of the gain when using contrarotating propellers could be traced 

 to an increase in the relative rotative efficiency, for which values of 1.05 to 1.10 

 were obtained. Such figures of this coefficient always makes the practical de- 

 signer suspicious and often results in discussion of the accuracy of the instru- 

 mentation, etc. We were therefore glad to find that test results from NSRDC 

 showed similar figures, and we also got support from some theoretical calcula- 

 tions. I should like to ask the authors what their values of the relative rotative 

 efficiency look like. 



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