Liindgren, Johnsson and Dyne 



DISCUSSION 



A. Emerson 



University of Newcastle-on-Tyne 



- .. ._ . i.-.- Newcastle, England 



I would like to add to the list of references on contrarotating-propeller re- 

 sults and then make one or two comments. 



For the high-speed cargo ship, particularly the container ship, the change 

 from single- to twin-screw propulsion leads to considerable loss of cargo space 

 and requires a different, bigger ship. For this reason, Stone Manganese Marine 

 investigated the engineering and hydrodynamic problems of contrarotating- 

 propeller drive. The result for a ship similar to that described by the authors 

 was given before a meeting of the Institute of Marine Engineers in January of 

 this year (L. Sinclair and A. Emerson, "The design and development of propel- 

 lers for high-powered merchant ships"). The model self -propulsion results 

 were given in the discussion by Mr. C.A. Lister of Vickers St. Albans Tank, be- 

 cause they designed the excellent model instrumentation for S.M.M. and carried 

 out the experiments. Briefly, a set of contrarotating propellers replaced the 

 single propeller on a recently completed 22.5-knot cargo vessel, changing only 

 the size of aperture. They showed a 10% increase in propulsive efficiency above 

 the excellent result obtained with the single propellers. The diameter of the 

 forward propellers was maintained the same as for the single, and the design 

 used optimum rpm. The contrarotating pair was designed using a method de- 

 vised by Glover; the results showed the calculated improvement of 12% in effi- 

 ciency of the propeller, but the increased aperture size caused a small increase 

 in thrust-deduction fraction. Cavitation-tunnel observations suggested that the 

 area of the propeller could be reduced; there are other alterations that are be- 

 ing investigated. 



Turning now to the comments: We are used to the scaling differences be- 

 tween the propeller results from the towing tank at relatively low Reynolds 

 number and the results in the cavitation tunnel. But for contrarotating propel- 

 lers with critical balancing of power between the forward and aft propellers, the 

 question of designing for the model experiments and then redesigning for the 

 ship becomes a very real one. 



A second observation from tunnel experiments is designing for a particular 

 condition. 



Thirdly, in related experiments with tandem propellers on a tanker model, 

 we have obtained consistent values when the total result is used, but the individ- 

 ual thrust and torque results show peculiarities. Have the authors any com- 

 ments ? 



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