Depending on the number of crew available and the number of trained watch keeping officers, the crew could be divided into two or three watches. Usually it was two, the starboard and larboard watches. The three watch system was more popular with crews as they got a full eight hours off but not as widely used.
The 24 hours were divided into 5 four hour watches and 2 two hour watches. The Latter had the effect of swapping the crew over so that the same people were not always on duty at the same time. The watches were named :
During these watches the bell was rung each half hour with the number of rings being increased by one each time. So half hour into the watch was one bell, one hour in was two bells 1.5 hours 3 bells etc. with the watch ending at eight bells (except the First Dog watch which ended at four bells).
You can therefore use the above to calculate the time used in the books. If a cutting out expedition is to set out at four bells in the morning watch it will be leaving at 6 o’clock in the morning.