If only the starboard screw is used in reverse (astern) and the port screw is stopped, the stern will swing toward which side?

If only the starboard screw is used in reverse (astern) and the port screw is stopped, the stern will swing toward which side?

• A. Port
• B. Starboard
• C. Forward
• D. Aft

Answer: (A)

Differential thrust from the two screws creates a yaw moment about the boat’s center. When only the starboard propeller is reversed and the port propeller is stopped, the backward thrust acts at the starboard side. That unopposed force twists the hull so the stern moves toward the port side (the left when facing forward). So, the stern swings to port. If you reversed the port prop instead while the starboard was stopped, the stern would swing to starboard.

Differential thrust from the two screws creates a yaw moment about the boat’s center. When only the starboard propeller is reversed and the port propeller is stopped, the backward thrust acts at the starboard side. That unopposed force twists the hull so the stern moves toward the port side (the left when facing forward). So, the stern swings to port. If you reversed the port prop instead while the starboard was stopped, the stern would swing to starboard.