ship Maneuvering in Deep and Confined Waters 



A 



N/ 



Normal twin - 

 screw propulsion 



Starboard screw 

 idling 



Fig, 19. Force fields on twin-screw tanker on straight steady- 

 course. 



The diagrams in Fig, 19 illustrate the symmetric force field 

 around a twin-screw tanker in normal straight course conditions, 

 and the steady state situation when running with starboard propeller 

 idling. The non-symmetric suction force on the port quarter is 

 balanced by the forces due to drift and checking rudders. The drift 

 angle is a fraction of a degree only, and some 90 per cent of the 

 compensation force is due to the rudders, set at some 5 to 7 degrees, 

 With the twin rudder arrangement it should be possible to maintain 

 75 per cent of the speed in this condition. The induced resistance 

 due to rudder lift would be larger in case of a single rudder between 

 the propellers, but the main cause of speed loss of a ship propelled 

 by one of its screws only is the additional drag from the idling 



855 



