van Manen and Oosterveld 



The question of this efficiency is rather important. Our values seem to 

 show an increase in the relative rotative efficiency with decreasing thrust coef- 

 ficient K^. This indicates that the optimum diameter could be smaller than that 

 obtained from open water tests. 



REPLY TO DISCUSSION 



J. D. van Manen and M. W. C. Oosterveld 



It was a great privilege to obtain at the final moment the opportunity to pre- 

 sent the NSMB-results on contrarotating propellers at this symposium. 



The investigations performed at the NSMB were confined to efficiency, cavi- 

 tation, vibratory forces, and stopping abilities of a contrarotating propeller sys- 

 tem consisting of a four-bladed propeller forward and a five-bladed propeller 

 aft. Our final conclusion is that we see a future for application of contrarotating 

 propellers on fast cargo liners, since the power will increase so much that two 

 propellers are needed to absorb the required power. 



Our future research will be concentrated on propeller induced vibratory 

 forces for well-selected combinations of the blade numbers fore and aft. In our 

 opinion the problem of propeller induced vibratory forces will be the most criti- 

 cal one in future discussions about the application of contrarotating propellers. 

 The other qualities such as efficiency, cavitation, and stopping are no longer a 

 serious point to delay a possible application. 



With regard to Mr. Lindgren's remarks concerning the diameter reduction 

 of the aft propeller, it should be noted that for the tanker and the cargo-liner 

 contrarotating propeller sets these reductions were 12.4 and 6.5 percent respec- 

 tively. Especially of the cargo liner the tip vortex of the forward propeller in- 

 terfered with the blades of the aft propeller in the upper and lower part of the 

 aperture and caused unfavorable cavitation phenomena. 



Therefore it may be useful to reduce the diameter of the aft propeller 

 slightly more. In regard to Mr. Lindgren's second question it must be empha- 

 sized that the propeller designs for both ships were based on given DHP, rpm, 

 and speed and that for the determination of the optimum diameter of the contra- 

 rotating propeller sets and the conventional screws, use was made of open-water 

 test results with the contrarotating propellers and conventional screw series. If 

 the propeller designs were based on given DHP, speed, and diameter (for in- 

 stance the maximum allowable propeller diameter), and if the optimum rpm's 

 with regard to efficiency were chosen, then it can be expected that the reduction 

 in DHP due to contrarotating propeller application in comparison with conven- 

 tional screws will be larger than by comparing systems with equal rpm. 



164 



