What counts as a sea day for a captain's license
On a vessel of less than 100 gross register tons, one day of sea service is a calendar day on which you were underway for at least 4 hours. Under 4 hours earns nothing at all, not a part day. On larger vessels a day is 8 hours.
A long day earns no extra credit on a small boat: 8, 10 or 12 hours is still one day, and one date counts only once however many boats you were on.
Where the rule comes from
The definitions section of the credentialing rules defines a day as 8 hours of watchstanding or day work, not counting overtime. For vessels under 100 GRT it adds that a day is considered 8 hours unless the vessel's operating schedule makes that inappropriate, and never less than 4 hours (46 CFR 10.107, and the same wording in 10.232(h)(3)). The crediting guidance from the National Maritime Center says it more plainly: only on vessels under 100 GRT, a full day is credited for 4 hours or more, and no credit is ever given for a day of less than 4 hours.
The small vessel sea service form asks for your average hours underway per day, and the same guidance says that average can be no less than 4 hours for credit to be given.
Why 12 hours is still one day
The rules do allow a 12 hour day to count as a day and a half, but only on vessels authorized to run a two watch system under the manning law (46 CFR 10.232(h)(2)). The NMC's time and a half guidance, dated July 2025, says that a letter or small vessel form claiming 12 hour days on a recreational vessel under 100 GRT is not credited at time and a half. Most vessels under 100 GRT have no certificate of inspection authorizing two watches. So on your own boat, a long passage day is one day.
Claiming extra credit you are not entitled to is worse than useless: the person who signs, and you, are certifying the numbers.
One date, one day
- Two trips on the same boat on the same day can be added: a morning and an evening sail of 2 hours each make a 4 hour day.
- Hours on two different boats on one day are separate. Each boat's form stands alone, and the guidance warns against claiming the same days on forms for different vessels.
- An overnight passage is split at midnight into the hours on each date.
Worked example: one week on a 6 GRT skiff
| Day | Logged | Result |
|---|---|---|
| Monday | 3.5 hours on the skiff | Does not count: under 4 hours |
| Tuesday | 4 hours on the skiff | Counts: exactly 4 hours |
| Wednesday | 2 hours in the morning and 2.5 in the evening on the skiff | Counts: 4.5 hours on one boat in one day |
| Thursday | 12 hours on the skiff | Counts as one day, not one and a half |
| Friday | 5 hours on the skiff and 7 on a friend's trawler | Counts once, on the trawler (more hours) |
7 trips and 36 hours underway make 4 sea days. The 12 hour Thursday did not make up for the short Monday, and Friday counts once. You can check any week of your own the same way in the sea time log.
Common mistakes
- Rounding 3.5 hours up to a day. Under 4 hours is zero.
- Claiming a day and a half for a 12 hour day on a recreational boat.
- Adding hours from two different boats on the same date.
- Counting time at the dock or at anchor with the engine off as underway.
- Treating the 4 hour rule as applying to every vessel. At 100 GRT or more a day is 8 hours.
Count your own days
The log applies the day rule to every trip as you enter it, one day per calendar day, and the progress page shows your totals by waters against each credential's published numbers. Everything stays in this browser.
Frequently asked questions
Is 4 hours exactly enough?
Yes. The rule is at least 4 hours on a vessel under 100 GRT, so 4.0 hours counts and 3.9 does not.
Can I claim 1.5 days for a 12 hour day on my boat?
Almost certainly not. The day and a half credit applies only on vessels authorized to run a two watch system, and the guidance says 12 hour days on recreational vessels under 100 GRT are not credited at time and a half.
What about a vessel with no gross tons on its paperwork?
Many small boats have no admeasured tonnage. The form asks for length, width and depth so tonnage can be worked out. This site treats a boat with no tons entered as under 100 GRT for the day rule.
Sources
Last reviewed 27 September 2026 against:
- 46 CFR 10.107, Definitions in subchapter B (Day, Month, Year, Near-coastal, Inland waters, Great Lakes), Electronic Code of Federal Regulations, current text, up to date as of 24 September 2026, read 27 September 2026. Used for: The definition of a day, month and year.
- 46 CFR 10.232, Sea service, Electronic Code of Federal Regulations, current text, up to date as of 24 September 2026, read 27 September 2026. Used for: The day on vessels under 100 GRT and the two watch day and a half, paragraph (h).
- Sea Service (crediting sea service, what counts as a day, completing the small vessel sea service form), National Maritime Center, dated 12 August 2024, read 27 September 2026. Used for: A full day for 4 hours or more only under 100 GRT; average hours on the form.
- 12 Hour Day / Time-and-a-Half Credit, National Maritime Center, dated 16 July 2025, read 27 September 2026. Used for: No time and a half for 12 hour days on recreational vessels under 100 GRT.
This guide is our own summary of the published documents, not legal advice and not a finding about your eligibility; the credentialing authority decides. If it disagrees with the current documents, the documents win. Sea Time Ledger is not affiliated with, endorsed by, or connected to the United States Coast Guard, the National Maritime Center or any other government body.