Which of the following is NOT part of the definition of a propeller?

The correct answer identifies that a propeller is not primarily a device that controls steering. A propeller is fundamentally designed to generate thrust, which propels the boat forward through water. The components that directly control steering, such as the rudder, are separate from the function of the propeller.

A propeller indeed is a device that turns; its rotation creates lift in the water, analogous to how an airplane's wing generates lift in the air. It is also a device that powers the boat, as it translates engine power into movement. Describing it as a rotating device accurately captures its essential mechanics. However, steering control is a function managed by other systems on the boat, emphasizing that while a propeller contributes to the boat's overall movement, it does not directly manage the direction in which the boat travels.