A Ship's bells are used to indicate time onboard ship. One bell sounds for each half hour. Bells are rung in pairs to make it easier to count. Thus, for a watch starting at 12:00, one bell means 12:30, two bells mean 1:00, two bells pause 1 bell means 1:30, and so on until 4:00 ("eight bells"). This cycle starts again for the next watch.
The use of 30 minutes as the interval between bells originates from the use of a half hour glass to keep time. Each half hour, the glass would be flipped and the appropriate count of the bell would be rung.
|Number of bells||Hour|
- U.S. Department of Transportation Maritime Administration