The term watch is used in several ways in the Royal Naval Service. First, it is used to signify a portion of the crew which is on duty at a given time. There are seven watches during the day, five of which are four hours long, and two of which are two hours long. The two hour long "dog watches" serve to create an uneven number of watches, so that the same officers do not serve on the same watch every day. Since "day" and "night" have only arbitrary meaning on a starship, the ship's time is set to be equivalent to Enaj Mean Time.
|0000 - 0400|
|0400 - 0800|
|0800 - 1200|
|1200 - 1600|
|1600 - 1800|
|1800 - 2000|
|2000 - 2400|
In a custom that goes back to time immemorial, time is struck aboard ship by bells. A bell is struck every half hour, one for each half-hour in the watch, with eight bells signifying a change of watch. The bells are struck in pairs, i.e. ding-ding [pause] ding-ding [pause]. Time can be kept in terms of bells in a watch - for example, "three bells in the afternoon watch" would be 1:30 p.m..
In the two-hour dog watches, time is kept as normal from one bell to three. Eight bells is then struck for change of watch. The count then starts over at one for the next dog watch.
On ships with particularly large crews, a watch also serves as a division of one half of the crew, generally called port and starboard, each watch having its own "set" of officers, from second lieutenant on. There is only one First Lieutenant on RNS vessels.