The screws are arranged so that the top of each blade moves ______.

The screws are arranged so that the top of each blade moves ______.

• A. Inboard
• B. Outboard
• C. Upward
• D. Downward

Answer: (B)

The idea being tested is how propeller rotation creates forces on the boat and how these forces affect steering. A propeller blade sweeps a circle as it spins, and the portion of the blade at the top is where the water is being pushed most toward the outboard side or the inboard side, depending on the rotation. If the screws are arranged so that the top of each blade moves outboard (away from the boat’s centerline), the thrust produced by both screws tends to balance the steering moments on opposite sides of the hull. This makes the vessel track more straight and keeps the rudder effective, reducing unintended yaw when power is applied or changed. Moving the top inward would create an unbalanced yaw tendency, and the options describing vertical movement (upward or downward) don’t describe the horizontal steering effect propeller rotation has.

The idea being tested is how propeller rotation creates forces on the boat and how these forces affect steering. A propeller blade sweeps a circle as it spins, and the portion of the blade at the top is where the water is being pushed most toward the outboard side or the inboard side, depending on the rotation.

If the screws are arranged so that the top of each blade moves outboard (away from the boat’s centerline), the thrust produced by both screws tends to balance the steering moments on opposite sides of the hull. This makes the vessel track more straight and keeps the rudder effective, reducing unintended yaw when power is applied or changed.

Moving the top inward would create an unbalanced yaw tendency, and the options describing vertical movement (upward or downward) don’t describe the horizontal steering effect propeller rotation has.