Lindgren, Johnsson and Dyne 



0.8 



h 



0.6 

 O.A 

 0.2 _ 

 _ 



-^— ^— ^ Contra- rol props comp)ete cole (tang, ortd axial Merf.j 

 ^^" ""^ ^^ 2 %inalm oroos. Soma Ae^ IA^ »ach No interference 



2 tingle props. Some AqJAq each No interference 



2 single props Tangential interference 



2 single props. Tangential interference. Cq-O 



1 single prop Same ^ol^O 



03 



Fig. 11 



O.A 



Q5 



06 



0.7 



03 0.9 1.0 



Advance ratio J 



Different approximations when calculating the propeller 

 efficiency for contrarotating propellers 



the factor (1) above should be the most important reason for an improvement. 

 From Fig. 11 it is, however, also evident that, when the propellers are placed 

 behind each other, the beneficial influence of the factors mentioned above are to 

 a large extent compensated by the axial interference between the propellers. 

 Thus, the open-water efficiency of the contrarotating propeller set is only 

 slightly better than that of the corresponding single propeller. 



In Fig. 12 open water test results of the four sets of contrarotating propel- 

 lers, described in Sec. 3, are given. Based on this material, curves giving the 

 open water-efficiency and advance ratio for optimum propellers are presented 

 in Figs. 13 and 14, together with the corresponding values for conventional pro- 

 pellers. The results for the conventional propellers have been reproduced from 

 (1). In Fig. 13 the comparison is based on iKj/J^ , i.e., equal diameter for the 

 same load, while Fig. 14 gives the same comparison on the basis of {'*\lKj7j^), 

 i.e., at the equal number of revs, for the same load. From the diagrams the 

 following conclusions can be drawn: 



(1) Compared with a conventional propeller at the same number of 

 revs., about 20% smaller optimum diameter is obtained with the contrarotating 

 propeller set. The open-water efficiency is about the same in the two cases, or 

 slightly lower for the contrarotating case. 



1282 



