Ducted and Contrarotating Propellers on Merchant Ships 



Table 3 

 Important Design and Test Data for Ducted Propellers 



Common design data 



Total thrust coefficient 



Advance ratio 



Number of blades 



Hub diameter 



Mean line of blade sections 



Thickness distribution of blade section 



Model propeller diameter 



Cavitation margin 



Ducted propeller No. 



Duct thrust K ■j.jy/K J.J. . . . 

 Blade area ratio Ap/A^ . . 

 Pitch ratio P/D at x = 0.7 

 Camber f /i of the blade 



g 



sections at x = 0.7 . . . . 



P1313 D4 



0.01 

 0.68 

 0.92 



0.033 



P1314D5 



0.15 

 0.65 

 0.96 



0.024 



NACA a 

 NACA 16 

 D 

 c 



0.278 



0.412 



5 



0.186 



0.8 



0.186 m 

 30% 



P1315 D6 



0.30 

 0.64 

 1.04 



0.019 



P1316 D7 



0.45 

 0.69 

 1.18 



0.014 



Hence the design method seems to function satisfactorily under the condi- 

 tions tested, as long as no separation phenomena occur. 



The most extreme ducted propeller, which theoretically should have given a 

 duct thrust of Kj-^/Kj^ = 0.45, suffered from flow separation inside the rear part 

 of the duct, which decreased Kj^/Kj-^ to 0.29. The separation was detected in a 

 series of flow visualization studies, which were carried out using a quartz lamp 

 illuminating small air bubbles in the flow through a narrow slit. 



To investigate the sensitivity of the co-operation between duct and propel- 

 ler, some of the ducts were also tested together with propellers originally de- 

 signed for other ducts. At the design Ktt/J^ both the duct thrust and the propel- 

 ler efficiency were decreased considerably, if the pitch ratio of the propeller 

 was lower and the camber of the blade section higher than the design values, see 

 Fig. 10. In one case of two this was also true when the pitch ratio was higher 

 and the camber lower than the design values. The flow visualization studies in- 

 dicated that the probable reason was flow separation inside the rear part of the 

 duct. 



Flow separation outside the ducts was recorded only at values of Kjj/J'^ 

 lower than the design value. 



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