Entry & progression
A day of sea service is not a day on board. And the letter that proves it must come from a different person depending on which system you are in — get that backwards and you lose months.
The eight-hour rule
In the domestic system, MO505 Schedule 4 clause 4.2 sets a day of sea service at 8 hours. Shifts may be added together to reach 8 hours. You may never credit more than one day per 24 hours.
In the international yacht system, MO74 Schedule 3 says the same thing and then adds something interesting. If you perform master or deck officer duties for at least 8 hours in a 24-hour period, the service is taken to be served as a whole day. If you perform them for less than 8 hours, the service is taken to be the actual hours served plus 25% of that time.
The 25% uplift
Read that carefully, because it is generous and it is easy to miss. A 6-hour day is not credited as 6 hours. It is credited as 6 hours plus 25% of 6 hours — 7.5 hours. The uplift exists because short days on small vessels involve the same responsibilities compressed.
But the ceiling is absolute. MO74 Schedule 3 clause 3.3: at least 8 hours in any 24 is a whole day and no more than a whole day. There is no such thing as a day and a half.
Whose hours count
MO74 Schedule 3 clause 3.1: service must have been performed as a master, deck officer, trainee deck officer, deck cadet or rating. That restricts the role — not the vessel type and not the trade. A season as a deckhand counts as a rating. A season as a steward does not count at all.
The sea service letter trap
WARNING — International: the letter must come from the COMPANY AMSA, verbatim, for international certificates: "Qualifying sea service must be confirmed by a letter issued by the company. Letters issued on board a vessel by a master will not be accepted."
For domestic near-coastal certificates, a letter from the master, owner, operator or chief engineer IS accepted.
It is exactly backwards from what most crew assume. A captain's signed letter — the thing you are most likely to be able to get on your last day aboard — is worthless for an international application. Get the company letter before you leave. Chasing a management company for a letter about a season you finished two years ago is how people lose sea time they actually did.
And AMSA can still discount it
Even correctly documented service is not automatically credited. MO70 section 18 lets AMSA discount all or part of a period of service it considers of little or no relevance to the seafarer certificate being applied for. Section 19 lets AMSA recognise service on other kinds of vessels according to its equivalence. Section 20 says service is qualifying to the extent that AMSA considers appropriate.
That is discretion, written down. It cuts both ways, and it is the reason no one can promise you in advance exactly what a given season will be worth.
Record hours, not vibes. Get the right letter from the right person, at the time, in the system you are actually in.