USCG Exam PracticeSingle-screw maneuvering and prop walk
When a vessel is making sternway, why is rudder effectiveness significantly reduced compared to going ahead?
- A. The discharge screw current flows forward past the rudder, opposing the water flow generated by sternway
- ✓ The rudder lies in the suction screw current, which is weaker and less concentrated than the discharge current, and cannot direct propeller thrustCorrect
- C. Propeller side force acts in the same direction as rudder force, canceling the net turning moment
- D. The pivot point moves forward, reducing the lever arm of the rudder force
Why B is correct
Running astern, the rudder lies in the screw suction current, which is neither as strong nor as concentrated as the discharge current. Additionally, the rudder cannot direct any propeller thrust when backing, so steering astern depends almost entirely on water flowing past the rudder due to sternway.