Slave ships used to be cargo ships converted for the only purpose and that was transporting slaves, especially those newly captured African slaves.
The slave ships routes led from the northern or middle african coasts to South America and to the south coast of what is today known as the Caribbean and also the United States of America. The captains and sailors of these boats were allowed to do anything they wished with the slaves. This included things like rape, torture or even murder because those slaves were considered as their property.
More than 20 millions of Africans were transported by these ships. This transportation from Africa to America was also called as the Middle Passage. The trade with African slaves was outlawed in the year 1807 by the United States of America and also the United Kingdom. The US law fully took effect on 1th. January 1808. After this date all US and English slave ships that were leaving Africa were considered as pirate vessels and were subject to capture by the American and British navies.
At the Council of Vienna in 1815 when Spain, France, Portugal and The Netherlands agreed to finish their slave trade. During that time, the remaining slave ships became much smaller and more cramped but were improved in performance as thez had a new role as smuggling craft or blockade runners.