Scale and context
Between the sixteenth and nineteenth centuries, ships carried millions of enslaved Africans across the Atlantic to the Americas. The best-documented estimates, drawn from surviving voyage records, put the number embarked at roughly 12 million, with something over 10 million surviving the crossing, although these are scholarly estimates rather than exact counts. Portuguese, Spanish, British, French, Dutch, Danish and later American and Brazilian ships all took part, and the trade was organised by a mix of chartered companies, private merchants and, in some periods, state-licensed contractors.
The central purpose here is to describe the ships. That history cannot be separated from the human suffering it involved, and it is treated here as a factual record of a system of forced transport and violence, not as an engineering story.
Were there special slave ships?
There was no single “slave ship” design. Most vessels were ordinary merchant ships adapted for the purpose: ships, brigs, snows, schooners and sloops of widely varying size. Large numbers of voyages used vessels of roughly 100 to 300 tons, though sizes ranged more widely and changed over time. A ship might carry enslaved people on one voyage and general cargo on another.
Adaptation typically meant adding platforms or temporary decks in the hold to create more sleeping space, installing barricades to separate groups on deck, fitting additional grating and ventilation, and carrying extra water, food and chains. Crews were usually larger than for a normal cargo run, since a ship holding many captives required guards as well as sailors. Slave voyages are therefore best understood as an extreme use of ordinary merchant shipping rather than as a separate class of vessel, though some owners did prefer ships built or refitted with the trade in mind.
Shipboard conditions
Evidence about conditions comes from ships’ logs, surgeons’ journals, testimony given in Britain during the abolition campaign and a small number of accounts by formerly enslaved people. The picture is consistent: severe crowding, little headroom, poor ventilation in warm latitudes, and shortages of water and fresh food. Disease, especially dysentery and fever, spread quickly, and the crossing itself, known as the Middle Passage, typically lasted several weeks to a few months depending on the route and winds. Captives also resisted, and revolts on board are well documented.
Mortality varied by period and route. Estimates for the average share of captives who died during the crossing are often placed at somewhere around one in eight or lower, with earlier centuries generally worse than later ones. Crew deaths were also high, since sailors were exposed to the same diseases and to the dangers of the African coast. The relevant medical knowledge of the time, such as it was, is covered by ship surgeons and medicine afloat.
The Brookes diagram and regulation
One image from this history is widely known: the 1788 print of the British slave ship Brookes, which showed a plan of the lower decks with rows of human figures packed into the available space. It was based on measurements of a real Liverpool vessel and was circulated by abolitionists. Historians point out that it should be read as campaign material, produced to make an argument, and that it simplifies actual practice, but it did bring the reality of the trade to a wide audience.
In 1788 Parliament passed what is known as Dolben’s Act, which for the first time limited the number of captives that British ships could carry in relation to their tonnage and required a surgeon on board. The act did not end the trade and was criticised as inadequate, but it reflected a growing public debate.
Routes and commerce
Voyages followed several patterns. In the often-cited “triangular” pattern, ships carried manufactured goods from Europe to West Africa, then enslaved people to the Americas, then plantation products such as sugar, tobacco and cotton back to Europe. Many voyages did not fit the pattern neatly: Portuguese and Brazilian ships, for example, sailed more directly between Africa and Brazil. These routes formed part of the wider Atlantic crossings and the Caribbean sea lanes that connected the continents, and they also drew on the African coastal seafaring that carried regional trade along the continent’s shores.
Ports such as Liverpool, Bristol, Nantes, Lisbon, Rio de Janeiro and Charleston grew in part through the trade, and its profits and insurance arrangements were embedded in the economies of the time.
Abolition and the ships of suppression
Britain made the trade illegal for its ships in 1807, and the United States banned the import of enslaved people from 1808. Other nations followed at different times. Britain’s Royal Navy then established a squadron off West Africa to intercept illegal slave ships, and it was joined, with different levels of commitment, by other navies. Tens of thousands of people were reportedly freed from captured ships, though many illegal voyages continued for decades, especially to Cuba and Brazil.
Illegality changed the vessels. Traders who faced capture often favoured fast, sleek ships, including fore-and-aft rigged schooners built for speed, and concealed their purpose with false papers and flags. Overcrowding could become worse on some of these voyages, since the aim was to make each run as profitable as possible. The naval ships chasing them were often brigs and small steamers, and the use of steamers is part of the broader steamship era.
Remembering the ships
Wrecks of slave ships, and the documentary record behind them, are now studied by maritime archaeologists, and museums in several countries present the history to the public. The voyage databases built from archives continue to be refined, which means that estimates may change as new records are examined. What is clear is that ordinary-looking ships, financed and insured like any other trading vessels, were central to one of the largest forced migrations in history.