USCG Exam PracticeSingle-screw maneuvering and prop walk
A coxswain on a single-screw vessel is attempting the compound maneuver in a crosswind. During the astern burst, the rudder provides minimal steering response and propeller side force dominates. During the ahead burst with the rudder hard over, the stern moves strongly. Which combination of principles from the source material best explains why the ahead burst is disproportionately more effective at moving the stern than the astern burst?
- A. Going ahead, the frictional wake accelerates flow across the rudder face; going astern, aeration of the descending blade reduces effective pitch and lowers thrust
- B. Going ahead, propeller side force reverses and reinforces the rudder; going astern, propeller side force and rudder force act in the same direction and partially cancel
- ✓ Going ahead, the discharge current — stronger and more concentrated than the suction current — increases flow past the rudder and directs approximately half the propeller's thrust to the rudder side; going astern, the rudder lies only in the weaker suction current and cannot direct any propeller thrustCorrect
- D. Going ahead, the pivot point moves aft, increasing the moment arm of the rudder force on the stern; going astern, the pivot point moves forward, reducing that moment arm
Why C is correct
USCG Boat Crew Seamanship Manual §10-B establishes that running ahead the discharge current — which §10-A identifies as the stronger and more concentrated flow — increases water speed past the rudder and putting the rudder over directs about half the propeller's thrust to that side. Running astern, the rudder lies in the suction current, which is weaker and less concentrated, and cannot redirect any propeller thrust. These two factors together explain the large asymmetry in stern-moving effectiveness between the ahead and astern bursts.