Ducted and Contrarotating Propellers on Merchant Ships 



Advance ratio J 



Fig. 12 - Results of open-water tests with 

 contrarotating propeller family 



(2) Compared with a conventional propeller at the same diameter, 

 about 35% lower optimum number of revs, is obtained with the contrarotating 

 propeller. A gain in open- water efficiency of the order of 5-7% is obtained with 

 contrarotating propellers under these conditions. 



5.2. Ducted Propellers ; 



On merchant ships the duct is introduced primarily to increase the propel- 

 ler efficiency. To obtain this, the duct must be formed in such a way that the 

 axial velocity at the propeller disk is increased, which means that the duct is 

 taking over some part of the thrust from the propeller. If the duct vorticity, 

 which determines the duct-induced velocity, is increased too much, however, the 

 diffusor angle at the rear part of the duct internal surface becomes so large that 

 the flow separates and the efficiency decreases. Thus, for a given total thrust, 

 there exists a certain duct vorticity which gives maximum values of duct thrust 

 and propeller efficiency. 



In the experiments mentioned in Sec. 4.2., the total thrust was kept con- 

 stant, while the thrust of the duct was varied systematically. Maximum values 

 of duct thrust and propeller efficiency was obtained with duct D6 (see Figs. 9 

 and 10), which had a diffusor angle of about 8.5°. The calculated ratio between 

 the wake area 4^ and the disk area 4^ was AJA^ = 1.086. 



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