What is the function of a propeller on a boat?

The function of a propeller on a boat is to rotate and power the boat forward or backward. Propellers operate by converting rotational energy from the boat's engine into thrust, which propels the vessel through water. This action is essential for moving the boat in the desired direction, whether that be forward or in reverse.

The design of the propeller, usually consisting of blades that extend outward from a central hub, allows it to push water behind it, resulting in a forward motion due to Newton's third law of motion (for every action, there is an equal and opposite reaction). In practical terms, when the propeller turns, it creates a pressure difference on either side of the blades, leading to movement.

In contrast, the other options do not accurately represent the function of a propeller. For instance, while direction control is crucial for navigation, it is primarily managed by the rudder rather than the propeller itself. Fastening ropes securely pertains to anchoring or tying up the boat, which does not involve the propeller's role. Finally, the definition of the hull’s shape is a separate aspect of boat design that concerns hydrodynamics and stability but does not relate to the propelling function of a propeller.