USCG Exam PracticeTwin-screw vessel handling
A twin-screw coxswain increases the RPM difference between the two engines to sharpen a turn. At what point does this technique begin to lose effectiveness?
- A. When sternway develops and the pivot point shifts aft, reversing the turning moment.
- B. When the rudder is placed hard over, which blocks the discharge current from the high-RPM engine.
- C. When the suction screw current from one engine interferes with the discharge current of the other.
- ✓ When cavitation or aeration sets in on at least one drive, reducing propulsion efficiency.Correct
Why D is correct
USCG Boat Crew Seamanship Manual §10-D states that beyond a certain point — depending on the boat, propulsion, sea state, and speed — cavitation or aeration sets in and propulsion efficiency falls off on at least one drive, limiting the benefit of further RPM difference.