Dutch river tugboat "Mascotte II"
German harbour-tug and DDR quick-freighter Karl Marx
Seagoing tugs (deep-sea tugs or ocean tugboats) fall into four basic categories:
- The standard seagoing tug with model bow that tows its "payload" on a
- The "notch tug" which can be secured in a notch at the stern of a specially designed barge, effectively making the combination a ship. This configuration is dangerous to use with a barge which is "in ballast" (no cargo) or in a head- or following sea. Therefore, "notch tugs" are usually built with a towing winch. With this configuration, the barge being pushed might approach the size of a small ship, with interaction of the water flow allowing a higher speed with a minimal increase in power required or fuel consumption.
- The "integral unit", or "integrated tug and barge" (ITB), comprises specially designed
vessels that lock together in such a rigid and strong method as to be certified as such by authorities (classification societies) such as the
American Bureau of Shipping,
Lloyd's Register of Shipping,
Indian Register of Shipping,
Det Norske Veritas or several others. These units stay combined under virtually any sea conditions and the tugs usually have poor sea-keeping designs for navigation without their barges attached. Vessels in this category are legally considered to be ships rather than tugboats and barges must be staffed accordingly. These vessels must show navigation lights compliant with those required of ships rather than those required of tugboats and vessels under tow.
- "Articulated tug and barge" (ATB) units also utilize mechanical means to connect to their barges. The tug slips into a notch in the stern and is attached by a hinged connection. ATBs generally utilize Intercon and
Bludworth connecting systems. ATBs are generally staffed as a large tugboat, with between seven and nine crew members. The typical American ATB operating on the east coast customarily displays navigational lights of a towing vessel pushing ahead, as described in the 1972
San Francisco harbor tractor tug "Delta Deanna"
Compared to seagoing tugboats,
harbour tugboats are generally smaller and their width-to-length ratio is often higher, due to the need for a lower draught. In smaller harbours these are often also termed lunch bucket boats, because they are only manned when needed and only at a minimum (captain and deckhand), thus the crew will bring their own lunch with them.
 The number of tugboats in a harbour varies with the harbour infrastructure and the types of tugboats. Things to take into consideration includes ships with/without
bow thrusters and forces like wind, current and waves and types of ship (e.g. in some countries there is a requirement for certain numbers and sizes of tugboats for port operations with
River tugs are also referred to as
towboats or pushboats. Their hull designs would make open ocean operation dangerous. River tugs usually do not have any significant hawser or winch. Their hulls feature a flat front or bow to line up with the rectangular stern of the barge, often with large pushing knees.