On a submarine, every inch of space is precious, one third of the boat is taken up by engines and steering, leaving very little space for living and working. There are often more sailors than beds. Rows of bunks or 'racks' are stacked three high. Only the captain has a private room. Submariners sleep in shifts of 6 hours, when one leaves the bunk to go on duty, another person immediately rolls in. This is called hot-bunking. As well as sleeping sailors, the bunks are also used to store extra supplies!