van Manen and Oosterveld 



130 



U.5 15 



155 16 165 17 



Speed in knots 



175 1 8 



2aooo 



16.000- 



mooo 



U.5 



15 



155 16 16.5 



Speed in knots 



17.5 



18 



Fig. 9 - Power and rpm curves for the tanker 



the tanker and the cargo liner. The extent of sheet cavitation on the back of the 

 aft propeller of the contrarotating propellers was for both ship types consider- 

 ably smaller than that on the forward propeller. Apparently the forward propel- 

 ler smoothes the peripheral irregularities in the flow behind the ship and con- 

 sequently in the inflow to the aft propeller. 



From the test results it can be seen that with regard to the strength of the 

 tip- vortex cavitation, the contrarotating propellers were slightly better than the 

 conventional screws. This was to be expected, since the nine blades of the con- 

 trarotating propellers were less loaded than the four blades of the individual 

 screws. 



148 



