USCG Exam PracticeTwin-screw vessel handling
On a twin-screw vessel, a coxswain continues to increase the RPM differential between engines to sharpen a turn. At some point, propulsion efficiency begins to fall off. What phenomenon causes this degradation, and on which drive does it first occur?
- A. The pivot point migrates aft, causing the suction screw current to dominate on the higher-RPM drive.
- ✓ Cavitation or aeration sets in on at least one drive, reducing its thrust.Correct
- C. Frictional wake builds on the slower engine's shaft, stalling the propeller on the windward side.
- D. The discharge screw current from the faster engine blankets the rudder, reducing steering authority.
Why B is correct
Beyond a certain RPM differential — which depends on the boat, propulsion type, sea state, and speed — cavitation or aeration sets in and propulsion efficiency falls off on at least one drive. The source does not specify which drive fails first; it depends on the specific vessel and conditions. (USCG Boat Crew Seamanship Manual §10-D)