Tankers carry petroleum products or other liquid cargo. Dry bulk carriers carry coal, grain, ore and other similar products in loose form. Multi-purpose vessels, as the name suggests, carry different classes of cargo – e.g. liquid and general cargo – at the same time. A Reefer, Reefer ships (or Refrigerated) ship is specifically designed and used for shipping perishable commodities which require temperature-controlled, mostly fruits, meat, fish, vegetables, dairy products and other foodstuffs. Roll-on/roll-off (RORO or ro-ro) ships are designed to carry wheeled cargo, such as cars, trucks, semi-trailer trucks, trailers, and railroad cars, that are driven on and off the ship on their own wheels. Specialized cargo ship types Specialized types of cargo vessels include container ships and bulk carriers (technically tankers of all sizes are cargo ships, although they are routinely thought of as a separate category). Cargo ships fall into two further categories that reflect the services they offer to industry: liner and tramp services. Those on a fixed published schedule and fixed tariff rates are cargo liners. Tramp ships do not have fixed schedules. Users charter them to haul loads. Generally, the smaller shipping companies and private individuals operate tramp ships. Cargo liners run on fixed schedules published by the shipping companies. Each trip a liner takes is called a voyage. Liners mostly carry general cargo. However, some cargo liners may carry passengers also. A cargo liner that carries 12 or more passengers is called a combination or passenger-run-cargo line. Size categories Cargo ships are categorized partly by cargo or shipping capacity (tonnage), partly by weight (deadweight tonnage DWT), and partly by dimensions. Maximum dimensions such as length and width (beam) limit the canal locks a ship can fit in, water depth (draft) is a limitation for canals, shallow straits or harbors and height is a limitation in order to pass under bridges. Common categories include: Dry cargo Small Handy size, carriers of 20,000– Seawaymax, the largest vessel that can traverse the St Lawrence Seaway. These are vessels less than in length, wide, and have a draft less than and a height above the waterline no more than . Handy size, carriers of 28,000– Handymax, carriers of 40,000– Panamax, the largest size that can traverse the original locks of the Panama Canal, a length, a width, and a draft as well as a height limit of . Limited to loaded, empty. Neopanamax, upgraded Panama locks with