Lindgren, Johnsson and Dyne 



DISCUSSION 



M.W.C. Oosterveld 



Netherlands Ship Model Basin 

 Wageningen, Netherlands 



The investigations presented in the paper under discussion are a valuable 

 contribution to the attempts of providing merchant ships with superior propul- 

 sion devices. I would like to make a comparison between the ducted propeller 

 work given in the paper and performed at the N.S.M.B.. 



Extensive investigations performed by van Manen at the N.S.M.B. concern- 

 ing ducted propellers with nozzles of the accelerating-flow type have led to the 

 development of a standard nozzle (nozzle no. 19A) which meets a number of 

 practical requirements. It has an axial cylindrical part on the inner side of the 

 nozzle at the location of the screw, the outside of the nozzle is straight, and the 

 nozzle has a relatively thick trailing edge. In addition, screw series especially 

 for use in this nozzle, were developed (the K^ screw series). The screws of 

 this series have wide blade tips (attractive with regard to cavitation), uniform 

 pitch, and flat face sections. Experimental investigations have shown that this 

 screw type is as good as theoretically calculated screws with regard to effi- 

 ciency and cavitation. Besides, they have reasonable stopping qualities. 



The open- water test results of nozzle no. 19A in combination with the K^ 

 4-55 screw series are given in Fig. Dl. It can be seen from this diagram that 

 at a design coefficient Cj- = 4.17 (corresponding with the design loading of the 

 ducted propellers given in the paper), the optimum ducted propeller with regard 

 to efficiency has an efficiency Vp = 0.53, an impeller thrust total thrust ratio 

 X = 0.72, while the impeller has a pitch ratio P/D = 0.88. The values of rj and 

 T are of the same magnitude as found for the best nozzle of the series given in 

 the paper under discussion. 



Not clearly given in the paper is the way in which for given speed and thrust 

 the optimum impeller diameter or rpm with regard to efficiency of the ducted 

 propellers were determined. For instance, if for a systematic series of nozzles, 

 the diameters of the impellers are chosen equal, then the optimum rotation 

 speeds with regard to efficiency will certainly not be equal. In the paper, all the 

 ducted propeller systems have equal impeller diameters and rpm's, and, conse- 

 quently, these systems will not be optimum. This fact has also been shown in 

 Fig. 10 of the paper. From this diagram it can be seen that only duct D6 and 

 screw P1316 form more or less an optimum combination from the standpoint of 

 efficiency. 



The conclusion in Sec. 5.2 of the paper that the N.S.M.B. nozzles suffered 

 from flow separation at the exterior surface of the nozzle at low screw loads, 

 due to the relatively large angle a^ (between nose -tail line of the nozzle profile 

 and the propeller shaft), must be considered with caution. The risk of flow 

 separation on the nozzle depends on the complete shape of the nozzle profile de- 

 termined by ttp, the camber ratio, the location of the maximum camber, and the 



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