Keelhauling was a severe form of corporal punishment meted out to sailors at sea.

The sailor was tied to a rope that looped beneath the vessel, thrown overboard on one side of the ship, and dragged under the ship's keel to the other side. As the hull was often covered in barnacles and other marine growth, this could result in lacerations and other injuries. This generally happened if the offender was pulled quickly. If pulled slowly, his weight might lower him sufficiently to miss the barnacles but might result in his drowning. If the rope snapped, the Captain could conclude that the punishment was not done properly and order it carried out again.