Ducted and Contrarotating Propellers on Merchant Ships 



Contrarot. propellers (94RPM) 

 Forward prop No. PI 319 Aft prop No. P13A8 



Cf 



Single propeller (I39RPM) 

 Prop No. P103Z 



270 90 



180 



tao° 



270" 



Fig. Z4 - Container-vessel project. Results of 

 cavitation tests in irregular flow. 



8. CONCLUSIONS AND FURTHER INVESTIGATIONS 



From the results presented above it may be concluded that: 



(1) The contrarotating propellers of this investigation show slightly 

 lower values of open-water efficiency than the conventional propellers, when the 

 comparison is based on the same number of revs, and optimum propeller diam- 

 eter is assumed in both cases. (See Fig. 14.) In the case of a 150 000 TDW 

 tanker a corresponding comparison in behind condition resulted, however, in a 

 gain in propulsive efficiency of about 6% for the contrarotating propellers (see 

 Fig. 19). The main reason seems to be more favourable values of tj^ and 77^. 

 (See Table 7.) 



(2) If the diameter is limited for some reasons (see Fig. 18) but opti- 

 mum number of revs, can be used, the comparison between different types of 

 propellers must be based on the same value of Kj/j^. Under these conditions a 

 gain in open water efficiency of about 5-7% was obtained for contrarotating pro- 

 pellers, compared to conventional propellers (see Fig. 13). In the case of a 



12 000 TDW container vessel, a corresponding comparison in behind condition 



1297 



