<P> An estimated 15% of the Africans died at sea, with mortality rates considerably higher in Africa itself in the process of capturing and transporting indigenous people to the ships . The total number of African deaths directly attributable to the Middle Passage voyage is estimated at up to two million; a broader look at African deaths directly attributable to the institution of slavery from 1500 to 1900 suggests up to four million African deaths . </P> <P> For two hundred years, 1440--1640, Portuguese slavers had a near monopoly on the export of slaves from Africa . During the 18th century, when the slave trade transported about 6 million Africans, British slavers carried almost 2.5 million . </P> <P> The duration of the transatlantic voyage varied widely, from one to six months depending on weather conditions . The journey became more efficient over the centuries; while an average transatlantic journey of the early 16th century lasted several months, by the 19th century the crossing often required fewer than six weeks . </P> <P> It is believed that African kings, warlords and private kidnappers sold captives to Europeans who held several coastal forts . The captives were usually force - marched to these ports along the western coast of Africa, where they were held for sale to the European or American slave traders in the barracoons . Typical slave ships contained several hundred slaves with about 30 crew members . </P>

How long was the atlantic slave trade journey