Prospects for Unconventional Marine Propulsion Devices 



Ducted propellers: ; -^ 



For ship propellers operating at high loading conditions, as in tugs when 

 towing or fishing vessels when trawling, the advantages of enclosing the 

 propeller in a duct which accelerates the inflow have been appreciated for 

 many years. However, the operating conditions of some large tankers and 

 bulk carriers are now in the range where propellers in such nozzles may 

 be useful. More recently it has appeared that there might be advantages in 

 enclosing a rotating impeller external to the hull in a duct or long shroud 

 ring in which the inlet flow is decelerated before reaching the impeller 

 blades. Operating conditions in which these two types of ducted propeller 

 are of advantage are indicated in Fig. 10, while a comparison between effi- 

 ciencies of an open unducted propeller, are one in an accelerating duct, is 

 shown in Fig. 11; this also indicates how unloading part of the total thrust 

 onto the duct makes it possible to reduce the diameter of the rotor com- 

 pared with that of an open screw. The performance and efficiency of ducted 

 propellers are sensitive to the clearance between the rotor tip and the 

 shroud ring, as indicated in Fig. 12, and the need for a small clearance 

 ratio can impose quite severe engineering and operational difficulties. Figs. 

 10-12 are taken from Ref. 8. 



1-2 



1-0 



4 - 



2 - o H 



REGION OF DUCT THRUST 



rj^ DUCTED > rjj UrsDUCTED 



(equal rotor area) 



REGION OF DUCT DRAG 



K)^ DUCTED < r)j UNDUCTED 



(equal rotor area) 



4 6 6 10 



TOTAL THRUST COEFFICIENT C_ 



14 



Fig. 10 - Ducted propellers: conditions for 

 accelerating and decelerating ducts 



■ 909 



