These are the bunks for the non-commissioned officers. On U.S. Navy submarines, to save space, a common practice was "hot-bunking", still in use today; you share your bed with 2 other people. Since you only sleep 8 hours a day, and there are 24 hours in a day, 2 other people will use the bed in the 16 hours you are working. In this way, usage is maximized while space is minimized.