Ship's Bells

At sea you can tell the time from the ship's chiming bell clock. Bell times are organized according to the six watches during each 24 hour day. Eight bells announce the beginning and end of every watch. At thirty minutes into a watch it's time for one bell, then two bells on the next hour. Another bell is added to the series every half hour. 

For example, at thirty minutes into a watch one hears ding. At the beginning of the next hour it's ding ding. After another thirty minutes three bells are heard ding dingding.

Bells are struck in pairs. Eight bells are sounded: ding dingding ding ding ding ding ding.

Bells once were rung manually based on the time determined at the ship's local noon, sounded by the mate on watch at half hour intervals, timed by 30 minute hourglass. Mechanical chiming clocks and electronic ones have replaced this system but the number of bells struck remain the same.

Survey ship USNS Michelson observed the US merchant marine standard of six four hour watches per day. Aboard regular US Navy ships the 16:00 to 20:00 evening watch may be split into two "dog watches" to facilitate personnel rotation and better accommodate the evening meal.

On the web you can find free ship's bell programs for your PC and apps for smartphones. 

Here is a ship's bell clock reference chart: 

The Ship's Bell Clock Tells the Time During Each Four Hour Watch