Why do the ship's crew members flog an enslaved person at the end of the excerpt?

The ship's crew members flog an enslaved person in order to move the ship fast.

Explanation:

In ancient times, enslaved people were used in many fields such as planting of trees and vegetables and for moving ships. At that time, human force especially enslaved persons were used for the movement of ships. The crew members of the ship beat the enslaved person with the whip in order to enhance the performance of the enslaved people which increases the speed of the ship.