A tugboat is a small sturdy and powerful vessel designed to push or tow other vessels and barges
Tenacious
You will see them in every sizable port; smart, businesslike small ships, low in the water and surging out to a large inbound ship. Tugs represent power for pushing and pulling, an engine with just enough hull for adequate buoyancy. Thick fenders for close-quarters work, pushing a big ship alongside the quay against the wind, hauling her off at the end of a towing wire.
A steam-powered winch on a schooner barge. Note the various drums for drawing up anchor chain, towing hawsers, etc, and the anchor chains themselves going down through the deck into the chain locker below.
Prior to steam power, the only force available on a sailing ship to perform all the necessary work was the men on board. For some tasks, such as raising the anchor, it might be necessary to yoke the entire crew to a multi-deck manual capstan. On the largest vessels, even with every available man, this might take several hours to complete. With the advent of steam power, a "donkey engine" and a single engineer could do the work of many men, in less time, and these were soon installed in almost all vessels.