Panel Discussion 



BAR 



100 150 20U 



NUMBER OF RP.M 



260 



Fig. 2 - Effect of rpm on screw diameter 

 and efficiency of a single screw cargo liner 

 having an engine power of 30,000 DHP and 

 a speed of 25 knots 



conventional propellers. These percentages make it desirable to calculate the 

 comparative costs for the two types of propulsion system. So far our remarks 

 concern contrarotating propellers at equal rpm (identical propulsion plant). 



Our design considerations will lead to quite different conclusions if we 

 start our propeller design for these fast cargo liners from an equal maximum 

 allowable propeller diameter for the propeller types in question. At a diam- 

 eter of 6.30 m the rpm of the conventional four-bladed screw amounts to 130 

 and for contrarotating propellers to 90. The improvement in efficiency at equal 

 diameter is in this case for the contrarotating propellers more than 10% (at 

 equal rpm this increase in efficiency is, as mentioned, 4 - 5%). For the up-to- 

 date fast cargo liners it is very desirable to make calculations for the 

 two described alternatives, i.e., a ship with the conventional ship screw and a 

 ship with contrarotating propellers of the same diameter as the conventional 

 screw propeller. The consequences of the difference in rpm of the propulsion 

 plants have, of course, to be taken into account. 



In Figs. 3a and 3b, examples have been given of the results of systematic 

 cavitation tests with the Wageningen B- screw series. On base of the screw 

 load Ct, 



1577 



