van Manen and Oosterveld 



7o 



400 



300 



200 



10 



15 



20 25 30 



40 50 60 70 80 90 100 125 150 



Bn 



100 



200 



Fig. 4 - Comparison of optimum values for the contrarotating 

 propellers with B 4-70 screws 



At the top of Fig. 4 the different ship types are indicated for which the Bp 

 values are typical. The lightly loaded screws of fast ships are at the left, and 

 the heavily loaded propellers for towing vessels are at the right. This diagram 

 gives quick information which type of propeller will be the best with regard to 

 efficiency for a certain ship type. For fast ships (cargo liners) contrarotating 

 propellers appear to give a higher efficiency than conventional screws. It must 

 be noted, however, that by application of a conventional screw behind a ship, the 

 rudder partly removes the rotational velocity from the propeller jet and hence 

 improves the efficiency of the propulsion device. It is obvious that this im- 

 provement in efficiency will not be found by application of contrarotating pro- 

 pellers behind a ship. 



It can be seen from Fig, 4 that the optimum diameter of the contrarotating 

 propeller series is considerably smaller than the optimum diameter of the con- 

 ventional screw series. 



140 



