USCG Exam PracticeSingle-screw maneuvering and prop walk
When a vessel is making sternway, rudder effectiveness is reduced primarily because the rudder lies in which flow?
- A. Aerated water produced by cavitation on the ahead side of the propeller
- B. The discharge screw current, which is strong and concentrated
- C. The frictional wake layer generated by the hull moving astern
- ✓ The suction screw current, which is neither as strong nor as concentrated as the discharge currentCorrect
Why D is correct
USCG Boat Crew Seamanship Manual §10-B explains that when running astern the rudder lies in the screw suction current, which is neither as strong nor as concentrated as the discharge current, and the rudder cannot direct any propeller thrust. Steering astern therefore depends mostly on water flowing past the rudder due to sternway.