On the nineteenth-century Erie Canal, mule power was the norm, with animals like "Sal" pulling boats along towpaths. A major innovation of the Erie's twentieth-century rebuilding was the use of mechanical motor power on boats, ending the need for towpaths and animal towing. Tugs, or "motor ships," then pulled or pushed the canal's freight, which had become much larger and heavier. The propeller was the most important part of these vessels, providing the force to move through the water, and its size and shape were matched to the characteristics of each vessel, from small buoy boats to large motor ships.