Ducted and Contrarotating Propellers on Merchant Ships 



?0 

 0.7 



0.6 



O.S 



0.4 



10 



30 40 50 60 70 80 



Bp tSSPA 5.60} 



Fig. 2 - Propeller open-water efficiency. Trend curves. 



propellers. It is interesting to note that only a very few of these reports include 

 results based on self-propulsion experiments. 



In most cases only theoretical hypotheses are presented, and in some cases 

 open-water test results are included. Due to the complexity of the problem and 

 to the difficulty in defining and separating different propulsive factors, complete 

 self-propulsion experiments are, however, required for a final comparison be- 

 tween different propeller alternatives. 



2.1. Contrarotating Propellers 



Very extensive experiments with alternative propulsive arrangements have 

 been carried out with the American 106 000 TDW tanker project "Manhattan" 

 (2, 3). The investigation includes conventional single- and twin-screw propul- 

 sion, tandem propellers, and overlapping propellers. It also includes the only 

 complete experiments with contrarotating propellers hitherto published. These 

 self -propulsion test results have been condensed in Fig. 3, where the propeller 

 shaft power for the different propeller arrangements has been plotted against 

 propeller rate of revolutions. For reference, an approximate line representing 

 conventional single -screw propulsion at different number of revs, has been cal- 

 culated. The results indicate that for this project the contrarotating alternative 

 is the most favourable, whilst the extreme 9-bladed propeller required the high- 

 est shaft power. The gain in shaft power obtained with the contrarotating system 



1267 



