On a steamboat, the main shaft carried the paddle wheel or wheels, not a flywheel. On a railroad locomotive, the connecting rods spun the main drive wheels. A true flywheel's purpose is to create sufficient momentum to smooth out the movement of the other parts. The linear motion tends to be jerky, and the flywheel minimizes the variations in speed. In such a case, the belt from the beltwheel to the pulley on the sawmill transfers the power of the engine to the saw.
Many flywheels function as beltwheels, for they can be belted to other machines.