van Manen and Oosterveld 



; Confra-rotating 



propellers. 



Conventional screw 



6000 



5000 



4000 



3000 



2000 



1000 



500 750 1000 



Time in seconds 



1250 



1500 



17S0 



Fig. 19 - Comparison of the stopping abilities of 

 conventional and contrarotating propellers for a 

 steam turbine tanker 



• Contrarotating propellers offer a means of improving the propulsive effi- 

 ciency of ships. The reduction in DHP due to application of the contrarotating 

 propellers for a tanker was about 4.5 percent in the loaded condition and 8 per- 

 cent in the ballast condition of the ship if compared with the ship with conven- 

 tional screw. The contrarotating propellers behind the cargo liner compared 

 with the conventional screw requires about 6.5 percent less DHP. 



• Conventional screws and the forward propeller of the contrarotating pro- 

 pellers are quite comparable as far as blade cavitation is concerned. The ex- 

 tent of sheet cavitation on the back of the aft propeller of the contrarotating 

 propellers is relatively small. With regard to the strength of tip-vortex cavita- 

 tion the contra-rotating propellers were slightly better than the conventional 

 screws. 



• With regard to the propeller induced vibratory forces the thrust and 

 torque variations as well as the thrust eccentricity of conventional screws and 

 contrarotating propellers did not differ very much, although the average thrust 

 and torque of each of the contrarotating propellers are about half of that of a 

 comparable conventional screw. The application of contrarotating propellers 

 causes a considerable reduction in transverse forces compared to a conventional 



158 



