What is the purpose of a propeller on a boat?

The propeller on a boat is designed to rotate and create thrust, which is essential for powering the boat either backwards or forwards. When the engine turns the propeller, the blades push water backwards, which in turn propels the boat in the opposite direction, harnessing the principles of Newton's Third Law of Motion. This functionality is crucial for maneuvering the vessel effectively in various water conditions.

The other choices do not accurately describe the role of the propeller. While some may help with other aspects of boating, such as indicating direction or measuring hull width, those functions do not pertain to the primary purpose of the propeller in propulsion.