Roll-on/roll-off (RORO or ro-ro) ships are designed to carry wheeled cargo such as automobiles, trailers or railway carriages. This is in contrast to lo-lo (lift on-lift off) vessels which use a crane to load and unload cargo.
RORO vessels have built-in ramps which allow the cargo to be efficiently "rolled on" and "rolled off" the vessel when in port. While smaller ferries that operate across rivers and other short distances still often have built-in ramps, the term RORO is generally reserved for larger ocean-going vessels.
Various types of RORO vessels include ferries, cruiseferries, cargo ships, and barges. A true RORO's ramps can serve all of the vessel's decks; otherwise it is a hybrid type. New automobiles that are transported by ship around the world are often moved on a large type of RORO called a Pure Car Carrier (PCC) or Pure Car Truck Carrier (PCTC).
Unlike elsewhere in the shipping industry where cargo is normally measured by the metric tonne, RORO cargo will typically be measured in the more convenient unit of lanes in meters (LIMs). This is calculated by multiplying cargo length in meters by its width in lanes (lane width differs from vessel to vessel and there are a number of industry standards). Aboard PCCs cargo capacity is often measured in RT or RT43 units which is based on a 1966 Toyota or by car equivalent units (CEU).
The largest RORO barges in the world operate between the United States and Puerto Rico carrying highway trailers, shipping containers on chassis, new and used cars, and oversized cargos on three decks. These barges are towed by ocean-going tugs and sail four times per week from Jacksonville, Florida to San Juan.
Since 1970 the market for exporting and importing cars has increased dramatically and the number and type of RO/ROs has increased also. In 1973, Japan's K Line built the European Highway, the first Pure Car Carrier, which carried 4,200 automobiles. Today's pure car carriers and their close cousins, the Pure Car/Truck Carrier are distinctive looking ships with a box-like superstructure running the entire length and breadth of the hull, fully enclosing and protecting the cargo. They typically have a stern ramp and a side ramp for dual loading of many thousands of vehicles, as well as extensive automatic fire control systems.
The PCTC has liftable decks to increase vertical clearance as well as heavier decks for "high and heavy" cargo. A 6500 unit car ship with 12 decks can have three decks which can take cargo up to 150 tons with liftable "panels" to increase clearance from 1.7 meters to 6.7 meters on some decks. Lifting decks to accommodate higher cargo reduces the total capacity. The largest PCC currently in service is the MV Mignon, which can carry up to 7,200 cars.