A French slave ship in 1741
A slave ship was a large cargo ship that was converted with the purpose of transporting slaves. Slave ships were often former military or merchant vessels that were made to carry large numbers of people from their homes in Africa to slavery in the Americas, and the number of slaves on board depended on if they were tied shoulder-to-shoulder or "packed-tight" (meaning that they were made to lay down sideways, making more room for more slaves to be put on board). Disease often killed between 15% and 33% of the slaves on board, and the naked and shackled slaves were treated like animals and forced to endure horrific conditions. After 1808, the United States and the United Kingdom passed laws that outlawed the trans-Atlantic slave trade, meaning that any US or British slave ships leaving West Africa were to be treated as "pirate ships". After the 1815 Congress of Vienna, France, Spain, Portugal, and the Netherlands also agreed to end the slave trade, and slave ships were no longer used legally.