Ducted and Contrarotating Propellers on Merchant Ships 

 0.60 



0.55 



0.50 



0.45 



o Duct and propeller designed together 



X Duct and propeller not designed together 



PI3I3 



PI314 



PI3I5 



PI 316 



Propeller No- 



Fig. 10 - Efficiency of the different ducted 

 propellers at the design value of K^j/J . 

 For geometrical data of propellers and 

 ducts, see Table 3 and Fig. 9- 



CALCULATION OF OPTIMUM PROPELLER 

 EFFICIENCIES AND DIAMETERS 



5.1. Contrarotating Propellers 



Below diagrams showing the optimum relations between speed, number of 

 revolutions, thrust, and diameter will be given for contrarotating propellers. It 

 might, however, be of interest to look first into the reasons sometimes given 

 why better efficiency should be expected with a set of contrarotating propellers 

 than with the corresponding single propeller of the same diameter, developing 

 the same thrust at the same number of revs. The main reasons for an improve- 

 ment in efficiency should be: 



(1) The load is distributed over two propellers instead of one. 



(2) The rotational losses from the forward propeller are more or less 

 nullified by the aft propeller (tangential interference). 



These conditions are illustrated in Fig. 11. This figure shows the calculated 

 values of the propeller efficiency at the design advance ratio for the four sets of 

 propellers mentioned in Sec. 3. From the diagram in Fig. 11 it is evident that 



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