Only a few decades after the discovery of America by Europeans, demand for cheap labor to work plantations made slave-trading a profitable business. The peak time of slave ships to the Atlantic passage was between the 18th and 19th centuries, when large plantations developed in the British colonies of North America.
In order to achieve profit, the owners of the ships divided their hulls into holds with little headroom, so they could transport as many slaves as possible. Unhygienic conditions, dehydration, dysentery and scurvy led to a high mortality rate, on average 15% and up to a third of captives. Often the ships, also known as Guineamen, transported hundreds of slaves, who were chained tightly to plank beds. For example, the slave ship Henrietta Marie carried about 200 slaves on the long Middle Passage. They were confined to cargo holds with each slave chained with little room to move.
The most significant routes of the slave ships led from the north-western and western coasts of Africa to South America and the south-east coast of what is today the United States, and the Caribbean. As many as 20 million Africans were transported by ship. The transportation of slaves from Africa to America was known as the Middle Passage.
After abolition, slave ships adopted quicker, more maneuverable forms to evade capture by naval warships, one favorite form being the Baltimore Clipper. Some had hulls fitted with Copper sheathing. This was very expensive work that at this time was only commonly done to Royal Navy vessels. However it increased speed by preventing the growth of marine weed on the hull, which would otherwise cause drag. The speed of slave ships made them attractive ships to repurpose for piracy, and also made them attractive for naval use after capture; the USS Nightingale (1851) and HMS Black Joke (1827) were examples of such vessels. The size of slave ships also allowed for many cannons and other guns to be placed on them.
Whydah Gally, slave ship turned into pirate ship, sank 1717.
Zong, a British slave ship infamous for the 1781 massacre of 132 sick and dying slaves who were thrown overboard in an attempt to guarantee that the ship's owners could collect on their cargo insurance.
Note: While La Amistad is often called a slave ship, it was in fact a general-purpose cargo ship that occasionally carried slaves. See the article about the ship, and the resulting court case, for more information.