Tugboats are powerful, small watercraft used to berth large ocean-going ships, move barges, and tow disabled vessels. The first tugboats were paddlewheel-propelled and appeared about 1800, after the advent of steam power. Screws replaced paddlewheels about 1850 and by 1900 diesel engines had supplanted steam. Throughout the 20th century, the size of tugboats has remained constant, ranging from 70-210 feet (21-64 m), but their engines now are much more powerful. Tugboats used for berthing ships have resilient wooden hulls that prevent damage to both the tug and the ship during the docking operation. Tugs operating with barges in the United States normally push the barges, while those in Europe normally pull the barges by a towline. A single tugboat may power 10-20 barges.