Tug (harbor, ASD, ATD, RotorTug)

Harbor and escort tugs are classified by their propulsion and the resulting handling. A conventional tug has fixed propellers astern; an Azimuth Stern Drive tug has two steerable Z-drive units aft for high maneuverability and indirect escort towing; a tractor tug carries its drives forward; and the RotorTug carries three azimuth drives for redundancy and all-round thrust. Bollard pull, the static towing force the tug can develop, is the headline rating used to match a tug to the size of ship it must assist or escort.