Watch System
F5. The Reference Layer: Glossary, Units, Signals and Information SourcesDefinition
Traditional four-on, eight-off three-watch or two-watch system, with the dog watch from 1600 to 2000 divided into two two-hour watches.
A watch system is the rotation that divides the 24-hour day into periods of duty so the bridge and engine room are continuously manned. The traditional pattern is three watches in four-hour periods (the 4-on, 8-off cycle), with the 1600 to 2000 dog watch split into two two-hour halves so the rota shifts daily and no one keeps the same hours indefinitely. Two-watch (six-on, six-off) systems run on short-handed ships. STCW 1978, Chapter VIII and the STCW Code require minimum rest hours that constrain how watches can be arranged.
Source: STCW 1978 (as amended) Chapter VIII and STCW Code Part A-VIII/1 (watchkeeping and rest hours).