The earliest records of waterborne activity mention the carriage of items for trade; the evidence of history and archeology shows the practice to be widespread by the beginning of the 1st millennium BC. The desire to operate trade routes over longer distances and at 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.
Piracy is still quite common in some waters, particularly around Asia, most notably in the Malacca Straits, a narrow channel between Indonesia and Singapore / Malaysia. In 2004, the governments of those three nations agreed to provide better protection for the ships passing through the Straits. Also piracy prone are the waters off Somalia and Nigeria, while smaller vessels are also in danger along parts of the South American coasts.
While the definitions have become "cross-pollinated" over the years, "cargo" technically 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 "boxes"), 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.
Vessel prefixes: Before the vessel's name will be found a category designation. Naval ships, for example, will have "USS" (United States Ship), "HMS" (Her/His Majesty's Ship), "HTMS" (His Thai Majesty's Ship). Merchant ships may have "RMS (Royal Mail Ship, usually a passenger liner), "MV" (Motor Vessel, (powered by Diesel). "SS" (Steam Ship, now seldom seen, powered by steam). "TS", sometimes found in first position before a merchant ship's prefix, denotes that it has Twin Screws. (For further discussion, see Ship prefixes.)
Famous cargo ships would include the Liberty ships of World War II, partly based on a British design, the sections for which were prefabricated all over the USA and then assembled by shipbuilders in an average of 6 weeks with the record being just over 4 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.
Sizes of cargo ships
Cargo ships are categorized partly by their capacity, partly by their weight, and partly by their dimensions (often with reference to the various canals and canal locks through which they can travel). Some common categories include:
- Small Handy size, carriers of 20,000-28,000 deadweight tonnage
- Handy size, carriers of 28,000-40,000 deadweight tonnage
- Handymax, carriers of 40,000-50,000 dwt
- Seawaymax, the largest size which can traverse the St Lawrence Seaway
- Aframax, oil tankers between 75,000 and 115,000 dwt. This is the largest size defined by the average freight rate assessment (AFRA) scheme.
- Suezmax, the largest size which can traverse the Suez Canal
- Panamax, the largest size which can traverse the Panama Canal (generally: vessels with a width smaller than 32.2 m)
- Malaccamax, the largest size which can traverse the Straits of Malacca
- Capesize, vessels larger than Panamax and Suezmax, which must traverse the Cape of Good Hope and Cape Horn in order to travel between oceans
- VLCC (Very Large Crude Carrier), supertankers between 150,000 and 320,000 dwt
- ULCC (Ultra Large Crude Carrier), enormous supertankers between 320,000 and 550,000 dwt