Bells are struck in pairs. Eight bells are sounded: ding ding ● ding 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 |