A service vessel that can rescue a ship in distress by towing or pushing it to facilitate berthing and unberthing. Tugs are characterised by a very powerful propulsion system, a bulky silhouette, with superstructures gathered toward the bow, leaving the stern of the vessel free for the winch or tow-hook of the tug. A distinction is made between harbour or coastal tugs, ocean-going tugs, and salvage tugs, equipped for firefighting and other accidents at sea.