A service craft built to provide an independent source of propulsion for other vessels, specifically for those disabled by breakdown, those built as dumb vessels, and for sailing vessels becalmed by lack of wind or short of a crew. Although once propelled by oars or sweeps, steam and later diesel power were adopted at the earliest possible date on grounds of efficiency.
Tugs may be divided into those optimised for manoeuvrability within harbours and those incorporating power for long-distance towing. The former commonly formerly used paddles, but now regularly incorporate specialised power transfer systems (Schneider propellers and Kort nozzles) operating under automatic control. Both types may be fitted with firefighting equipment and/or helicopter platforms
