SHP 



^7 000 



46 000 



45 000 



44 000 



4 J 000 



42 000 _ 



Liindgren, Johnsson and Dyne 

 O 9'b/. single screw 



Contra-rot. prop. 



Twin screw 

 ^ (conv. stern) 

 rudder angle- 

 4" 



Q Twin screw 

 (conv. stern) 

 rudder angle: 

 O Twin screw ~ "^ 



(spec stern) (calculated) 



Project "Manhattan", 19.25 knots 

 106 000 TDW tanker 



100 



f05 



no 



115 



RPM 



120 



Fig. 3 - Condensed results from self-propulsion tests 

 with "Manhattan" (2) 



was about 5.5%, compared with a conventional single-screw arrangement at the 

 same number of revs. It should be mentioned that the load coefficient (Kj./j'*) i/"* 

 for the contrarotating case was about 1.0, which is a fairly low value for a 

 tanker project. 



In this connection it is necessary to state that it is at present impossible to 

 make any quite fair comparison between different propeller alternatives. Such a 

 comparison must be based in some cases on constant propeller diameter and in 

 some cases on constant number of revs, or something between. This will be 

 discussed in Sec. 6. The most important problem is, however, that a fair com- 

 parison requires that for all propellers the margin against different disadvan- 

 tages due to cavitation is the same and that in no case are dangerous vibratory 

 forces introduced. Lack of reliable criteria for cavitation erosion for different 

 kinds of propellers as well as criteria for dangerous vibratory forces makes the 

 comparisons questionable in most cases. 



Some of the self-propulsion test results with contrarotating propellers pre- 

 sented in Sec. 6 have been reported earlier (4). These preliminary tests indi- 

 cated that the contrarotating propeller arrangement in some cases was very 

 favourable from the point of view of efficiency, especially for slender types of 

 ships. 



I 



1268 



