Tug-Barge (ATB/ITB)

A propulsion unit in which a tug is mechanically coupled to a barge to work as one ship, in two forms. An articulated tug-barge (ATB) couples the tug into a stern notch through pins that lock yaw and surge but allow the tug to pitch relative to the barge, so the two hulls articulate in waves. An integrated tug-barge (ITB) rigidly fixes the tug into the barge so the combination behaves as a single rigid hull. Both let one engine room push a large barge; US regulators document and crew the tug and barge separately, which affects manning and tonnage.