Munk and Prohaska 

 THE AUXILIARY PROPELLER ARRANGEMENT 



Another propeller arrangement which may reduce the risk of cavitation 

 and vibrations was developed at the Hydro- and Aerodynamics Laboratory 

 (HyA) at Lyngby, Denmark. It consists of a normal single- screw propeller and 

 a small auxiliary propeller placed in the high- wake zone between the upper 

 part of the main propeller and the stern. The auxiliary propeller will acceler- 

 ate the water behind the stern and thereby smooth out the wake at the position 

 of the main propeller. The primary effect is the higher safety against cavita- 

 tion and vibrations, but a minor increase in propeller efficiency should also 

 be expected. 



PRACTICAL POSSIBILITIES 



In practice, both systems may be adopted in ships without any technical 

 difficulties. In the case of interlocking propellers the two shafts are geared 

 together and driven by a single propulsion unit. The cost of the gear and the 

 extra shaft will be modest in comparison to the gain in total efficiency of the 

 system, and the gear will permit an optimum number of revolutions to be 

 chosen. 



Separate drives of the two shafts may be adopted if the two overlapping 

 propellers are placed clear of each other in the longitudinal direction. This 

 arrangement corresponds to a twin-screw ship with an abnormally low distance 

 between the two shafts at the tail end. 



In the auxiliary propeller system, the small propeller may be driven in any 

 number of ways: by gear, chain-drive, or electric motor. The power required 

 will only be about 10 percent of the total power. 



Even a combination of the two systems might be advantageous, e.g., a three- 

 propeller system with two interlocking and one auxiliary propeller, all driven 

 by a single propulsion unit. 



TESTING OF THE SYSTEMS 



At HyA the results from Ref . 1 were found to be of the greatest interest, and 

 as no further treatment of interlocking propeller arrangements was available, it 

 was decided to carry out supplementary tests with this system in order to con- 

 firm the results of Ref. 1, and to obtain more knowledge of the interaction be- 

 tween propellers. It was considered of special interest to know how the vibration- 

 generating variation of the forces on a propeller blade would compare with the 

 variation on a normal single screw, and how the wake of one propeller is influ- 

 enced by the induced velocities from the other propeller, since this would be of 

 importance in the design of interlocking propellers. 



The problems connected with the auxiliary propeller system are similar, 

 and it was therefore decided also to carry out tests with this arrangement. 



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