Slave ships were large cargo ships specially converted for the purpose of taking slaves, especially newly captured African slaves to the Americas. Living conditions for slaves on these ships was inhuman. Men, women and children were crowded into every possible space leaving no room to move or even breathe. There was little food and the smell could not be described. Between 1526 and 1867 about 12.5 million slaves were sent by slave ships from Africa to the Americas. But only about 10.7 million slaves actually arrived. Of all human migrations it was the most costly in terms of human lives lost. The average time to sail across the Atlantic took from 60 to 90 days. Sometimes the trips took up to four months.

Saleable slaves 
Ship captains were under orders by slavers to deliver only saleable slaves. Starving, sick or troublesome slaves were often thrown overboard into the ocean to drown. Sometimes Africans were kept on deck chained together with long chains. There were times when they would jump overboard, taking every other chained slave with them, to avoid their unknown fate. Many ships kept extra crew members on deck to prevent this from happening. At other times crew members would throw slaves overboard as a warning to other slaves. If the ship was running out of food or water, all the slaves would be killed to save the crew from starving.

Zong massacre 
Slave ships could at times get insurance on their cargos of slaves. But insurance would not pay if the slavers starved the slaves or they got sick. Insurance paid out if the slaves drowned, but not if they were severely starved to death. In 1781 the slave ship Zong was hauling 470 slaves—more than the ship could handle. Many began to get sick. By the time the ship reached the Doldrums, a mid-Atlantic region that at times had no wind, several of the crew had died from the spreading diseases. To save themselves, the remaining crew threw 132 dying and sick slaves into the ocean. Another 10 slaves jumped with them. When they tried to collect the insurance the Insurance underwriter denied the claim. In 1782 in a court in Jamaica, ruled for the owners. The insurance company appealed the decision. This brought a good deal of public attention. It began to be called the Zong massacre. No member of the crew or the owners were ever charged with murder.

Slave ship takeovers 

About 15% to 20% of the ships leaving Africa never arrived in the Americas. In thousands of cases the crews were overtaken by captured slaves, some of them warriors. Often the crew was killed and the Africans managed to sail back to Africa. In other cases the ships were lost at sea.

Some of the documented slave ship revolts are:

 The Little George ship revolt happened after the ship Little George left Guinea in June of 1730. It was carrying 96 slaves to Rhode Island. Several of the African men slipped out of their shackles and overpowered the crew. They killed three of the crew who were on deck at the time. The rest were taken prisoner. The Africans sailed the ship back to the Sierra Leone River where they abandoned the ship and still imprisoned crew.

 In 1732 a ship out of Portsmouth, New Hampshire captained by John Major had just loaded hundreds of slaves and was leaving when about 50 Africans took over the ship. The crew was killed with guns, axes, swords and other weapons taken from the crew members.

 In 1747 an American ship with a captain named Beers was taken over by hundreds of captive Africans in West Africa. All but two of the crew were killed with their own weapons. There was no record of how many Africans died during the short battle.

 A 1764 ship incident. An unnamed ship from New London, Connecticut had just taken on a load of slaves in Senegal. That night, while the ship was still in port, the Africans freed themselves and, using clubs, killed the entire crew and captain. The Africans then disappeared back into Senegal.

 In 1839 the Spanish ship Amistad was sailing off the coast of Cuba in the process of smuggling slaves. By this time slave trading was illegal except in Cuba. Joseph Cinqué, who had been captured as a slave in Sierra Leone, led a revolt that had far reaching consequences. The court case in Connecticut that followed was appealed to the Supreme Court of the United States where it was upheld. Cinqué and the surviving Africans returned to Sierra Leone in 1842.

 The Creole Ship Uprising. The ship Creole sailed from Richmond, Virginia on Oct. 27, 1841. It was bound for New Orleans with a cargo of 135 slaves. The slaves were able to kill some of the crew, the rest were taken captive. The ship arrived at Nassau, Bahamas where the slaves were set free by the British.

References 

Historic slave trade
Ship types