Two international instruments set rest minimums at sea, and they deliberately share the same headline numbers: at least 10 hours of rest in any 24-hour period, at least 77 in any 7 days. The differences live around the edges, and the edges are what an auditor asks about. As always, your flag state's notices are the authority for your vessel; this is the orientation.
Who each one covers
- MLC 2006, Standard A2.3 is labour law. It covers seafarers, which is read broadly: on a commercial yacht that is effectively everyone working aboard, from the captain to the junior stewardess.
- STCW, section A-VIII/1 is about safe watchkeeping. It covers those assigned duty as officer in charge of a watch or as a rating forming part of a watch, and those with designated safety, security and pollution-prevention duties.
On a working yacht the two groups overlap almost completely, which is why the practical answer to "which applies to us" is usually both, and the operating rule is: comply with the stricter reading wherever they differ.
Where they differ
- Exceptions. STCW carries its own exception clauses: parties may allow the weekly total to drop as low as 70 hours, for no more than two consecutive weeks, with conditions on how often exceptions can follow each other, and the 10 hours may under exception be divided into up to three periods under strict conditions. MLC allows exceptions only through collective bargaining agreements given effect by the flag; a vessel cannot grant itself one. If your SMS does not name an applicable exception, assume there is none.
- Drills. STCW says it directly: musters, fire-fighting and lifeboat drills shall be conducted in a manner that minimises the disturbance of rest periods and does not induce fatigue. A 0300 drill after a harbour day is a choice the regulation asks you to justify. MLC requires compensatory rest where rest is disturbed by call-outs to work.
- Purpose, and therefore enforcement. MLC compliance is checked as part of labour-condition inspections and certification; STCW rest sits behind watchkeeping certificates and manning. Port State Control can pull on either thread, and the same rest record answers both.
What your flag adds on top
Flag administrations bind the two regimes together in their own notices, and add interpretation with teeth. For the UK flag, MSN 1877 (as amended) is the operative document: it carries the 1-hour floor for a break to count as rest, and confirms that additional rest periods of an hour or more count toward the weekly total. Other registries publish equivalents. When your vessel's SMS quotes a rule, it is usually quoting the flag notice rather than the convention text, and the flag notice is the one an inspector will hold you to.
The rules themselves, and the subtleties that decide borderline days, are unpacked in MLC hours of rest explained.
How The Muster App handles the two regimes: the app checks every logged day against the shared minimums, the 10 in 24 and the 77 in 7 over rolling windows, plus the two-period structure, which satisfies the arithmetic both regimes share. It does not assume any exception: a day below the limits is flagged as a non-conformity and the crew member must comment before the day saves. If your vessel operates under a documented exception, the comment thread is where that context lives, attached to the exact day it applies to.