The operation of sailing ships necessarily tended to take a capitalist form. Not only were the ships expensive, necessitating partnership forms that presaged the joint stock company [Banaji, 2016], shipping was, in the precapitalist economies, the main instance of production by means of powered machines. The sailing ship used wind power to replace what would otherwise have required a large number of galley slaves. In this it shared one of the archetypal traits of capitalist industry—the replacement of human labor with powered devices. The anomaly of merchant capital existing in antiquity and the Middle Ages should be understood as arising from shipping being the first field to which such machines were applied. The profit of merchant capital should then be understood as a special early and precocious case of the production of relative surplus value.

By using sail-power shippers in, say, first-century Italy could convert grapes for wine into Egyptian corn for sale in Italy such that the labor that would go into growing the grapes, plus the labor of shipping, was less than the labor that would be required to grow the same amount of corn in Italy. To the extent that the corn imported from Alexandria entered into the subsistence of slaves exploited in Italy, the cheapening of corn would have decreased the fraction of time that slaves had to work to produce their subsistence, increasing the number of hours a week that yielded an income for the slave owners. A portion of this increased surplus was then appropriated by the sea captains and shippers as monetary profit.

-Paul Cockshott, How The World Works