Final answer:
The crankshaft converts the reciprocating motion of the piston into rotational motion, which then powers the propeller.
Step-by-step explanation:
The part that changes the reciprocating motion of the piston into the rotating motion for the propeller is the crankshaft. In a four-stroke internal combustion engine, the crankshaft translates the up-and-down movement of the piston—caused by the ignition of the air-fuel mixture—into rotational motion. This rotation is then transferred to the propeller, which provides the thrust for propulsion. The process involves several strokes: the intake stroke, the compression stroke, the power stroke, and the exhaust stroke. During these cycles, the piston's motion is converted into rotational movement by the crankshaft, which is essential for the functioning of the propeller.