Steamboats could be driven by screws like most modern ships, and many of them were. The most classic image of a steamship, however, is the riverboat paddle-wheeler. These ships came in two varieties: the sternwheeler, which had a single wheel at the stern of the boat, and the sidewheeler, with one wheel at either side. The wheel was large and fitted with paddle blades along the outside. Power to the boat was produced by pushing these blades through the water. Sidewheelers could also use their paddles to turn by powering one wheel and stopping or reversing the other.