At sea, the typical submarine day is 18 hours long, not 24 hours.
原文地址 (潜艇生活FAQ)：
http://www.navy.mil/navydata/cno/n87/faq.html
22. What does the crew do during its free time?
At sea, the typical submarine day is 18 hours long, not 24 hours. Submarine crews are divided into three watch sections. Each section is on duty (on watch) for 6 hours, and then spends 12 hours off watch. When on watch, the crew members are actively operating their assigned equipment. Examples of watch stations that are manned continuously at sea are: Radioman of the Watch (operates radio equipment), Quartermaster of the Watch (determines the submarine's position in the ocean), Engine Room Lower Level Watch (operates all equipment located in the lower level of the engine room), Throttleman (operates the throttles which control the main engines, which, in turn, control the speed of the ship's propeller) and Planesman (operates the submarine's diving or steering controls). Under normal conditions, there are approximately 25 crew members "on watch" at the same time. Under special conditions, such as battle stations and when entering or leaving port, everyone has a watch station.
During the 12 hours out of each 18-hour day that submarine crewmen are not actually on watch, they engage in a wide variety of activities. Crew members who are off watch eat, attend training sessions and study, both for advancement examinations, and in order to become qualified to stand other watchstations. Others may perform routine preventive maintenance on the equipment that they are responsible for (e.g., a radioman periodically changes emergency batteries on some of his radio gear, an electrician periodically inspects the ship's wiring for problems, etc.). A wide variety of activities are available during free time. The ships carry about 400 movies, which are exchanged for newer ones in each port. Card games and various board games, such as a Backgammon or Cribbage, are also popular. There are also some athletic equipment on board, such as an exercise bike, versa climber, rowing machines, and free weights. U.S. fleet ballistic missile submarines have more athletic equipment than SSNs because they have more space. SSBNs are so large that some people even run marathons by running around the perimeter of the missile compartment many thousands of times!
