The lightermen were a vital component of the Port of London before the enclosed docks were built during the 19th and 20th centuries. Ships anchored in the middle of the Thames or near bridge arches transferred their goods aboard or in respect of a few exports from lighters. Lightermen rode the river's currents — westward, when the tide was coming in, eastward on the ebb tide — to transfer the goods to quay-sides. They also transferred goods up and down the river from quays to riverside factories and vice versa. This was an extremely skilled job, requiring an intimate knowledge of the river's currents and tides. It also demanded a lot of muscle power, as the lighters were unpowered; they relied on the current for motive force and on long oars, or "paddles", for steering.