Lindgren, Johns son and Dyne 



te" 



30^ 



[20 _ 



•5 



I 



fO 



oL 



4-bladed 



5-bladed 



;»-• 6-bladed 



Measuring points 



Fig. 25 - Pressure amplitudes. 

 Influence of number of blades. 



resulted in a gain in propulsive efficiency of about 12%, in spite of a slightly 

 lower value of T)fj . (See Table 8.) 



(3) The ducted propellers generally had higher open-water efficiency 

 than the conventional propellers at the same number of revs. The gain in effi- 

 ciency increased with increasing thrust coefficient (see Fig. 16). In the case of 

 a 150,000 TDW tanker a corresponding comparison in behind condition resulted, 

 however, in a gain in propulsive efficiency of only 7.5%, which is less than could 

 be expected from the open- water tests. The main reason seems to be a lower 

 hull efficiency r]n . 



(4) There is no doubt that further work on the afterbody lines, in order 

 to improve the propeller -hull interaction, is of the utmost importance, particu- 

 larly for ducted propeller arrangements. Thus, tests carried out by van Manen 

 with ducted propellers in combination with different types of Hogner sterns have 

 given promising results (5). 



1298 



