Ducted and Contrarotating Propellers on Merchant Ships 



Fig. Dl - Open water test results of k 4-55 screw series in nozzle No. 19A 



thickness ratio, and thickness distribution of the nozzle profile. The optimum 

 relationship between the efficiency Tjp and the thrust coefficient (^r/7'')^''' is 

 shown in Fig. 2 for the N.S.M.B. nozzle no. 19A and the SSPA duct D6. In addi- 

 tion the nozzle shapes are presented in this diagram. From Fig. D2 it can be 

 seen that the standard nozzle no. 19A of the N.S.M.B. has better characteristics 

 at low screw loads than the SSPA duct, in spite of a larger angle a^. 



For a propeller operating in the wake of a ship, the intake velocity will be 

 lower in the upper part of the screw disk than in the lower part. Consequently, 

 the propeller is relatively more heavily loaded in the upper part of the screw 

 disk. The inflow velocity can be made more constant over the screw disk by 

 surrounding the propeller by a noncylindrical nozzle which is adapted to the 

 wake distribution and the flow direction as occur behind the ship. A view of the 

 stern of a tanker equipped with a noncylindrical ducted propeller is given in 

 Fig. D3. The noncylindrical nozzle is at the inside of the nozzle still cylindrical 



1305 



