USCG Exam PracticeTwin-screw vessel handling
When increasing RPM differential between the two engines of a twin-screw vessel to sharpen a turn, at what point does further increase in differential become counterproductive?
- A. When the rudder reaches its maximum deflection angle
- ✓ When cavitation or aeration sets in and propulsion efficiency falls off on at least one driveCorrect
- C. When the pivot point migrates forward of amidships
- D. When discharge screw current from the ahead engine begins to act on the astern engine's propeller
Why B is correct
§10-D states that beyond a certain point — dependent on the boat, propulsion type, sea state, and speed — cavitation or aeration sets in and propulsion efficiency falls off on at least one drive, so further RPM differential no longer produces a proportional increase in heading change.