364 slave trade, Africa and the and direction of wind to sail Seldom less than a month, the wait on the coast sometimes stretched to half a year All the while slaves, packed 100–1,000 per ship, depending on its size, occupied little more than six square feet of space with two or three feet of headroom Slavers separated men from women, shackling the men in pairs to reduce the danger of rebellion Long chains tethered groups of slaves, kept below deck most of the time, for movement to the deck for fresh air and meals The duration of the wait on the coast and the voyage to the Americas tempted the all-male crews to rape female slaves Once a ship set sail, slaves were vulnerable to the vicissitudes of weather and wind Rain prevented them from getting fresh air on deck and increased the incidence and spread of diseases Storms imperiled even the most promising crossing In 1738, a storm assailed the Dutch ship Leusdan only days from its destination When it began to leak, the crew, fearing a fight over the lifeboats, locked some 660 slaves below deck, leaving them to drown Only the crew and 14 slaves on deck survived The absence of wind brought ships to a standstill and strained the food supply Ship captains rarely had more than three months of food at the start of a voyage and reduced slave rations on long trips The crossing from the Guinea Coast was especially perilous because ships had to traverse the doldrums twice and thereby risk a lengthy calm One study estimated the mortality rate for ocean crossings of fewer than 20 days at percent, though the death rate increased to nearly one-quarter for voyages longer than two months Malaria, yellow fever, and intestinal ailments accounted for two-thirds of deaths, and smallpox, scurvy, and suicide the remaining third Once a ship reached its destination, an inspector boarded to check slaves for disease, quarantining all slaves if he found one with a communicable disease and prolonging their stay aboard ship until contagion had passed On land, slaves at last had fresh food and water Traders amassed slaves for sale once ashore, selling the young and old first and holding men and women of childbearing age for sale until last in the expectation that prices would rise with the eagerness of buyers to close the deal The fact that ovulating women fetched a higher price than pre- and postmenopausal women contradicts the assertion of slave traders that they did not sell slaves for the purpose of breeding Traders sold most slaves by auction, though an alternative was to fix the price for a group of slaves of similar age and physical condition and allow buyers to choose from among this group An illustration showing the dimensions allowed to slaves as cargo in a slave ship By the 1760s, slave imports averaged between 10,000 and 15,000 per year By 1787, the number exceeded 40,000 per year