Lindgren, Johnsson and Dyne 



?0 

 0.6 



0.7 



0.6 



0.5 



OA . 



\ 



\ 



Conventional propellers 



Ducted propellers Ka-4.70; vanManen [6] 



-•— Ducted propeller PUIS D6. l/D'CL5;A^/Aq^Q64;z^5; SSPA [17] 



-o— Ducted propellers A^IAq=1.05\ calculated 



-^e— Ducted propellers aJ/Aq =f.00\ LID- QJS.Af^lA^ 0.60; z-5 



1.0 



1.5 



20 



2.5 



Fig. 16 - Comparison between optimum efficiencies of 

 ducted and conventional propellers 



with conventional propellers can be made. The experimental efficiencies of 

 SSPA ducted propeller P1315 D6 (see (17)), and of ducted propellers tested by 

 van Manen (8) are also given. From Fig. 16 the following conclusions can be 

 drawn: 



(1) It seems to be possible to design ducted propellers with higher ef- 

 ficiency than conventional propellers, also at low loads. For the "optimum" 

 ducted propellers the lower limit for efficiency gain is 



^^ 



r/r 



1.1 



which means Bu ^ 16 or Sp = 20. 



(2) The gain in efficiency due to the duct increases with increasing 

 load. Unfortunately it is not yet possible to make a fair comparison based upon 

 equal strength, vibration, and cavitation characteristics between different pro- 

 pulsion systems. However, a comparison has been made with the conventional 

 propeller series SSPA 5.60 and NSMB B.4.70 in Fig. 16. 



1286 



