Cat O' Nine Tails

Until the middle of the nineteenth century, flogging was the most common form of punishment used to maintain discipline aboard ship whether the vessel was military or merchant. Flogging was a whipping using a cat-o'nine-tails - a diabolical device designed especially for its task. The cat-o'nine-tails consisted of nine lengths of cord with each length containing up to three knots. The cords were attached to a handle often made of a short piece of thick rope. The knotted cords would rip into a victim's skin with each lash causing excruciating pain. Repeated blows often left the victim unconscious. The number of lashes meted out to a victim depended on the offense committed and the Captain's discretion. Typically, they would range between 5 and 100.

Anything over 50 lashes would be likely to cripple and 100 would kill. Especially cruel captains would splash salt water on the offenders back during the lashing to cause increased pain.

The Cat was often nicknamed the 'Captain's Daughter'.

On board ship the whip was kept in a bag hung from the mast and when it was to be used the Captain would call for the 'cat' to be let out of the bag. Plenty of room was needed to swing the whip without the tails getting caught, hence the saying 'no room to swing a cat'.
 
Sailors and soldiers in the British navy and army were subject to flogging with the dreaded 'cat' until well into the 19th century.

Most captains wouldn't deliver the punishment themselves but would rather assign one of the other members of the crew to carry out the flogging. The sailors aboard ship would agree beforehand to strike hard enough to scratch the offenders back but not so hard that it would cause deep gashes. In other words they agreed to 'scratch your back if you scratch mine.