A ship is a large watercraft that travels the world's oceans and other sufficiently deep waterways, carrying cargo or passengers, or in support of specialized missions, such as defense, research and fishing. Ships are generally distinguished from boats, based on size, shape, load capacity and purpose. Ships have supported exploration, trade, warfare, migration, colonization, and science. After the 15th century, new crops that had come from and to the Americas via the European seafarers significantly contributed to world population growth. Ship transport is responsible for the largest portion of world commerce. The word ship has meant, depending on the era and the context, either just a large vessel or specifically a ship-rigged sailing ship with three or more masts, each of which is square-rigged. As of 2016, there were more than 49,000 merchant ships, totaling almost 1.8 billion dead weight tons. Of these 28% were oil tankers, 43% were bulk carriers, and 13% were container ships. The earliest historical evidence of boats is found in Egypt during the 4th millennium BCE Nomenclature Ships are typically larger than boats, but there is no universally accepted distinction between the two. Ships generally can remain at sea for longer periods of time than boats. A legal definition of ship from Indian case law is a vessel that carries goods by sea. A common notion is that a ship can carry a boat, but not vice versa. A ship is likely to have a full-time crew assigned. A US Navy rule of thumb is that ships heel towards the outside of a sharp turn, whereas boats heel towards the inside because of the relative location of the center of mass versus the center of buoyancy. American and British 19th century maritime law distinguished "vessels" from other watercraft; ships and boats fall in one legal category, whereas open boats and rafts are not considered vessels. Particularly in the Age of Sail, the word ship might apply generally to a seagoing vessel or particularly to a full-rigged sailing ship with three or more masts, all square-rigged. Other rigs on seagoing vessels included brig, barque, and barquentine. Some large vessels are traditionally called boats, notably submarines. Others include Great Lakes freighters, riverboats, and ferryboats, which may be designed for operation on inland or protected coastal waters. In most maritime traditions ships have individual names, and modern ships may belong to a ship class often named after its