Keelhauling is a form of punishment meted out to sailors at sea. The sailor was tied to a line that looped beneath the vessel, thrown overboard on one side of the ship, and dragged under the ship's keel, either from one side of the ship to the other, or the length of the ship (from bow to stern). As the hull was usually covered in barnacles and other marine growth, if the offender was pulled quickly, keelhauling would typically result in serious cuts, loss of limbs and even decapitation. If the victim was dragged slowly, his weight might lower him sufficiently to miss the barnacles, but this method would frequently result in his drowning.
Keelhauling was legally permitted as a punishment in the Dutch Navy. The earliest official mention of keelhauling is a Dutch ordinance of 1560, and the practice was not formally abolished until 1853. Keelhauling has become strongly associated with pirate lore.
Today, keelhauling can refer to the spinnaker sheets getting stuck under the hull after dousing the sail. This occurs especially in dinghy sailboats such as Laser 2 because nothing prevents the sheet from being pulled under the bow.