A cargo ship or freighter is any sort of ship or vessel that carries cargo, goods, and materials from one port to another. Thousands of cargo carriers ply the world's seas and oceans each year; they handle the bulk of international trade. Cargo ships are usually specially designed for the task, often being equipped with cranes and other mechanisms to load and unload, and come in all sizes. Today, they are almost always built of welded steel, and with some exceptions generally have a life expectancy of 25 to 30 years before being scrapped.
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-cum-cargo line.
The earliest records of waterborne activity mention the carriage of items for trade; the evidence of history and archaeology shows the practice to be widespread by the beginning of the 1st millennium BC, and as early as the 14th and 15th centuries BC small Mediterranean cargo ships like those of the 50 foot long (15 - 16 metre) Uluburun ship were carrying 20 tons of exotic cargo; 11 tons of raw copper, jars, glass, ivory, gold, spices, and treasures from Canaan, Greece, Egypt, and Africa. The desire to operate trade routes over longer distances, and throughout more seasons of the year, motivated improvements in ship design during the Middle Ages.
Before the middle of the 19th century, the incidence of piracy resulted in most cargo ships being armed, sometimes quite heavily, as in the case of the Manila galleons and East Indiamen. They were also sometimes escorted by warships.
A Delmas container ship unloading at the Zanzibar port in Tanzania
The words cargo and freight have become interchangeable in casual usage. Technically, "cargo" refers to the goods carried aboard the ship for hire, while "freight" refers to the compensation the ship or charterer receives for carrying the cargo.
Generally, the modern ocean shipping business is divided into two classes:
Liner business: typically (but not exclusively) container vessels (wherein "general cargo" is carried in 20 or 40-foot containers), operating as "common carriers", calling a regularly published schedule of ports. A common carrier refers to a regulated service where any member of the public may book cargo for shipment, according to long-established and internationally agreed rules.
Tramp-tanker business: generally this is private business arranged between the shipper and receiver and facilitated by the vessel owners or operators, who offer their vessels for hire to carry bulk (dry or liquid) or break bulk (cargoes with individually handled pieces) to any suitable port(s) in the world, according to a specifically drawn contract, called a charter party.
Larger cargo ships are generally operated by shipping lines: companies that specialize in the handling of cargo in general. Smaller vessels, such as coasters, are often owned by their operators.
A category designation appears before the vessel's name. A few examples of prefixes for naval ships are "USS" (United States Ship), "HMS" (Her/His Majesty's Ship), "HMCS" (Her/His majesty's Canadian Ship) and "HTMS" (His Thai Majesty's Ship), while a few examples for prefixes for merchant ships are "RMS" (Royal Mail Ship, usually a passenger liner), "MV" (Motor Vessel, powered by Diesel), "MT" (Motor Tanker, powered vessel carrying liquids only) "FV" Fishing Vessel and "SS" (Steam Ship, now seldom seen, powered by steam). "TS", sometimes found in first position before a merchant ship's prefix, denotes that it is a Turbine Steamer. (For further discussion, see Ship prefixes.)
Famous cargo ships
Famous cargo ships include the Liberty ships of World War II, partly based on a British design. Liberty ship sections were prefabricated in locations across the USA and then assembled by shipbuilders in an average of six weeks, with the record being just over four days. These ships allowed the Allies to replace sunken cargo vessels at a rate greater than the Kriegsmarine's U-boats could sink them, and contributed significantly to the war effort, the delivery of supplies, and eventual victory over the Axis powers.
Lake freighters built for the Great Lakes in North America differ in design from "salties" because of the difference in wave size and frequency in the lakes. A number of these boats are so large that they cannot leave the lakes because they do not fit into the locks on the Saint Lawrence Seaway.
Cargo ships are categorized partly by capacity, partly by weight, and partly by dimensions (often with reference to the various canals and canal locks they fit through). Common categories include:
ULCC (Ultra Large Crude Carrier), enormous supertankers between 320,000 and 550,000 DWT
Due to its low cost, most large cargo vessels are powered by bunker fuel also known as Heavy Fuel Oil which contains higher sulphur levels than diesel. This level of pollution is accelerating: with bunker fuel consumption at 278 million tonnes per year in 2001, it is projected to be at 500 million tonnes per year in 2020. International standards to dramatically reduce sulphur content in marine fuels and nitrogen oxide emissions have been put in place. Among some of the solutions offered is changing over the fuel intake to clean diesel or marine gas oil, while in restricted waters and Cold Ironing the ship while it is in port. If the various requirements are enforced, the International Maritime Organization's marine fuel requirement will mean a 90% reduction in sulphur oxide emissions; whilst the European Union is planning stricter controls on emissions.
Assessment and Standards Division, Office of Transportation and Air Quality (May 2009), Global Trade and Fuels Assessment— Additional ECA Modeling Scenarios (pdf), United States Environmental Protection Agency, EPA-420-R-09-009, retrieved June 2012