Ducted and Contrarotating Propellers on Merchant Ships 



On basis of these experimental results, a series of "optimum" ducted pro- 

 pellers has been calculated for loads which can be of interest for merchant 

 ships. At the optimum advance ratio J opti the total thrust coefficient was found 

 to be the same, Kjj. = 0.246 for all ducted propellers. The ducts had a length 

 L/D =0.5 and a maximum thickness tj^/L = 0.14. The thickness distribution 

 was similar to NACA 0015. The ratio AJA^ was 1.10, which means a diffusor 

 angle of about 8.5°. The results of the calculations are given in Table 5. 



Table 5 



o-D and fj^ are defined in Fig. 15. 



0.J5 



■O ^ co/^o =1.10: varying load > 5^^^^ ^^^^ 



« ^Kj^lJ^' =1.76. varying duct thrust] ^"''■°"<=^ 



• Ducts tested by van Manen [6] 



Fig. 15 - Different relations between camber and geometrical 

 angle of attack for duct profiles 



For the same number of revolutions, the optimum propeller diameter was 

 7% less, and the maximum diameter of the duct 7-15% greater than the optimum 

 diameter of a conventional propeller. 



The calculated efficiency of ducted propellers with AJA^ = 1.10 ("optimum") 

 and AJA^ = 1.00 (smaller diffusor angle) is given in Fig. 16, where a comparison 



1285 



