Training: Bells and watches

The passage of time on board ship is indicated by the striking of the ship's bell at half-hour intervals.
There are seven Watches each day, five of four hours each and two of two hours (the two Dog Watches).

You may notice the absence of five bells in the last Dog Watch. In 1797, five bells of the Last Dog Watch (1830 hours) was to be the signal for the mutiny at the Nore.
Since that time, the Admiralty decreed that the five bells (half an hour after the start of the Last Dog Watch) should never again be struck on British vessels. Hence the one bell at 1830 hours, and the watch concluding at eight bells as it always had.