In addition to the thrust along the shaft axis, another effect of propeller rotation is ______.

In addition to the thrust along the shaft axis, another effect of propeller rotation is ______.

• A. TORQUE
• B. LIFT
• C. VORTEX
• D. SIDE FORCE

Answer: (D)

Propellers don’t just push water straight backward to make forward motion. As the blades spin, they impart swirl to the water, creating a circulating flow behind the propeller. That swirling water interacts with the hull and its stern, producing a lateral pressure distribution that pushes the vessel to the side. This side force is a separate effect from the forward thrust along the shaft axis and can influence steering and stability, especially during turns or when the boat’s loading or speed emphasizes the wake. While a swirling wake (vortex) is indeed created by the propeller, the practical effect described here is the sideways push on the vessel, which is why side force is the best answer.

Propellers don’t just push water straight backward to make forward motion. As the blades spin, they impart swirl to the water, creating a circulating flow behind the propeller. That swirling water interacts with the hull and its stern, producing a lateral pressure distribution that pushes the vessel to the side. This side force is a separate effect from the forward thrust along the shaft axis and can influence steering and stability, especially during turns or when the boat’s loading or speed emphasizes the wake. While a swirling wake (vortex) is indeed created by the propeller, the practical effect described here is the sideways push on the vessel, which is why side force is the best answer.