Tramp steamers

A boat or ship engaged in the tramp trade is one which does not have a fixed schedule or published ports of call. As opposed to freight liners, tramp ships trade on the spot market with no fixed schedule or itinerary/ports-of-call(s). A steamship engaged in the tramp trade is sometimes called a tramp steamer; the similar terms tramp freighter and tramper are also used. Chartering is done chiefly on London, New York, Singapore shipbroking exchanges. The Baltic Exchange serves as a type of for the trade.

The term tramper is derived from the British meaning of "tramp" as itinerant beggar or vagrant; in this context it is first documented in the 1880s, along with "ocean tramp" (at the time many sailing vessels engaged in irregular trade as well).

The tramp trade first took off in Britain around the mid 19th century. The dependability and timeliness of steam ships was found to be more cost-effective than sail. Coal was needed for ships' boilers, and the demand created a business opportunity for moving large amounts of best Welsh coal to various seaports in Britain. Within a few years tramp ships became the workhorses of trade, transporting coal and finished products from British cities to the rest of the world.

The size of tramp ships remained relatively constant from 1900 to 1940, at about 7,000 to 10,000 deadweight tons (dwt.). During the Second World War, the United States created the Liberty Ship; a single design that could be used to carry just about anything, which weighed in at 10,500 dwt. The U.S. produced 2,708 Liberty Ships and they were used on every international trade route. After the Second World War, economies of scale took over and the size of tramp ships exploded to keep up with a booming supply and demand cycle. During this time the bulk carrier became the tramp of choice for many owners and operators. The bulk carrier was designed to carry coal, grain and ore, which gave it more flexibility and could service more ports than some of its ancestors, which only carried a single commodity.

...
Wikipedia