A barge is a shoal-draft flat-bottomed boat, built mainly for river and canal transport of bulk goods. Originally barges were towed by draft horses on an adjacent towpath. Today, barges may be self-propelled, usually with a slow-revving diesel engine and a large-diameter fixed-pitch propeller. Otherwise, "dumb barges" must be towed by tugs, or pushed by pusher boats. Compared to a towed barge, a pusher system has improved handling and is more efficient, as the pushing tug becomes "part of the unit" and it contributes to the momentum of the whole.