Sub M's 12-Hour Rule vs STCW 77/10: The Rest-Hour Confusion That Puts Tugboats at Risk

A captain working 14 hours on a Subchapter M towing vessel is not compliant — even if the compliance software says "green." That's the kind of error that surfaces in a Jones Act injury case with consequences measured in seven figures. Here's the regulatory distinction every operator and compliance-software buyer needs to understand.

The 3 rest-hour regimes in US commercial maritime

If you run a US-flag vessel, you operate under one of three rest-hour regimes. The regulation that applies to you depends on your vessel's subchapter, gross tonnage, and route. Getting it wrong doesn't just create a paper-compliance gap — it creates an operational risk and a product-liability exposure for whoever built your compliance system.

STCW 2010 Manila (46 CFR 15.1111)

Applies to: vessels 500+ GT on international voyages, or vessels carrying STCW-certified officers

Rule:

• Minimum 10 hours of rest in any 24-hour period
• Minimum 77 hours of rest in any 7-day period
• Rest may be divided into no more than two periods, one of which must be at least 6 hours long

This is the famous "77/10 rule" you read about in industry journals. It came out of the 2010 Manila amendments to the STCW Convention and applies to most large ocean-going vessels.

46 CFR 15.705 — the 12-hour rule

Applies to: Subchapter M (towing), Subchapter T + K (inspected small passenger), Subchapter L (offshore supply), Subchapter H passenger vessels under 500 GT, and most inspected vessels on domestic voyages

Rule:

• No person may be required to work more than 12 hours in any 24-hour period
• Exception for emergencies
• No 7-day aggregation requirement (no "77 per week" rule)

This is the rule that governs the vast majority of US commercial vessels. If you run tugboats, dive charters, whale watch vessels, or harbor ferries, this is your rule.

46 CFR 15.710 — watch/off-watch system

Applies to: vessels operating a three-watch system, typically 100+ GT oceangoing

Rule:

• At least two watches if the vessel operates >12 hours in a 24-hour period
• Not a rest-hour rule per se — it's a watchstanding requirement that has rest-hour implications

Most commercial operators don't hit this rule directly — it operates in the background of how larger vessels structure their crew rotations.

Why Subchapter M matters specifically

The January 2018 Subchapter M final rule was a watershed for the US towing industry. Before Sub M, tugboats were largely uninspected and operated with minimal federal oversight. After Sub M, every US towing vessel needs:

• A Certificate of Inspection (COI)
• Either a Towing Safety Management System (TSMS) under 46 CFR 138 or annual Coast Guard inspections
• Mandatory crew credentialing per manning table
• Rest-hour compliance under 46 CFR 15.705 — the 12-hour rule

That last piece matters. A lot of operators came into Sub M from an uninspected past, and a lot of compliance software was built assuming STCW was the universal US rule. When your software says "green" to a captain who just worked 14 hours, two things are true:

1. The software is wrong — you're over the Sub M 12-hour limit
2. That wrongness is now evidence in any subsequent Jones Act case

The negligent tool problem

Here's where the stakes go up.

Imagine a tugboat running harbor work in Lake Charles, LA. The captain's work/rest log in Binnacle AI (or Helm, or any other compliance tool) says "compliant" every day for a month. On day 31, the captain — who has been working 13-14 hours most days because that's how harbor work is done — misses a bollard during a docking maneuver. The barge allides with a pier. One deckhand breaks his collarbone.

Six months later, the deckhand's Jones Act lawyer subpoenas the work/rest records. The records show every day "compliant." The lawyer's expert witness points out that compliance was evaluated against STCW 77/10 — but the vessel was Sub M, governed by 46 CFR 15.705's 12-hour rule. Every one of those 13-14-hour days was a violation. The "compliance" was an artifact of the software applying the wrong regulation.

The deckhand's case just got a lot stronger. And Binnacle AI — the maker of the compliance tool — just got named as a third-party defendant under a failure-to-warn theory.

This is a novel legal theory. There's no directly-on-point case law yet, because maritime compliance software is a relatively new category. But the theory is being developed in real depositions right now. The fact pattern — vendor's software said compliant, reality disagreed, someone got hurt — is exactly the fact pattern plaintiffs look for.

The right answer is not to remove the "compliant" label. The right answer is to make sure the software applies the correct regime.

What software vendors should do

If you're evaluating compliance software — or if you're an operator whose vendor hasn't thought about this — here are the five things to check:

1. Does your vendor branch rest-hour calculations by vessel subchapter?

Ask specifically: "How does your system know whether to apply STCW 77/10 or 46 CFR 15.705?" The answer should be: "We check the vessel's subchapter, gross tonnage, and route, and select the regime accordingly." If the answer is "we apply STCW because it's the safest default," walk away.

2. Does every rest-hour display show the applicable CFR citation?

A work/rest log that says "compliant" without telling you which rule it's checking is a black box. A log that says "Compliant under 46 CFR 15.705 — no day exceeds 12 hours" is transparent.

3. Can the system generate different reports for USCG vs Class?

USCG inspectors want to see 46 CFR 15.705 compliance for Sub M vessels. ABS/DNV/LR Class surveyors want to see STCW 77/10 for the STCW-applicable vessels they class. If your software can't produce both views, it's not doing its job.

4. Does it warn before the violation, not after?

A compliance log that flags violations in last week's records is an audit trail. A system that alerts when a crew member is 2 hours away from exceeding their 12-hour limit today is a compliance tool. Buy the latter.

5. Is "subchapter" a required field?

If your vessel record lets you leave subchapter blank, the software has no way to apply the right regime. Check every vessel in your fleet right now — if any have "other" or blank subchapter, that's a bug to fix before your next USCG boarding.

What operators should check

Beyond vendor due diligence:

Know your own vessel's subchapter. This sounds obvious but I've had conversations with owners who didn't know whether their 95-ft aluminum passenger catamaran was Sub T (less than 100 GT) or Sub K (inspected passenger under 100 GT on certain routes). Look at your COI — the subchapter is printed on it.

Make sure your captain + chief engineer know the rule. If you're a Sub M tug, the captain and chief should be able to recite "12 hours, 24 hours" the way a Sub H captain recites "10 hours, 24 hours — 77 hours, 7 days." Test them.

Train your crew to flag their own overages. The best compliance culture is one where a deckhand will tell the captain "hey, we're coming up on 12 hours" before anyone has to enforce it. That's an owner-operator conversation, not a software feature.

Audit your prior logs. If you've been using STCW rules on Sub M vessels for a year, you have evidence of non-compliance in your records. That's a liability waiting to be discovered. Talk to your maritime attorney about what to do — usually the answer involves a one-time corrective memo + a prospective fix.

Binnacle AI's approach

We had this exact bug in our own system until mid-April 2026. WorkRestEntry compliance was evaluated against STCW 77/10 for every vessel regardless of subchapter. We caught it during a legal audit and fixed it.

Our current implementation:

• selectRestRegime() in src/lib/rest-hour-rules.ts branches by vessel.subchapter, grossTonnage, and internationalVoyage
• Sub M, Sub T, Sub K, Sub L vessels → 46 CFR 15.705 12-hour rule
• 500+ GT on international voyages → STCW 77/10
• Other vessels → conservative default (STCW or 12-hr, whichever is stricter for the specific scenario)
• Every rest-hour display shows the applicable CFR citation inline
• Reports can be generated separately for USCG vs Class

Subchapter is a required field on vessel creation. You literally cannot create a vessel in Binnacle without picking a subchapter.

We share this not to brag — we had the bug too — but to tell you what fixing it looks like so you can evaluate your own vendor (or yourself if you're building in-house).

Start with your own vessels

Go check right now. Open your compliance software. Pick a tugboat — any Subchapter M vessel. Look at a recent work/rest report. Does the report tell you which regulation it's checking? If it doesn't, your first conversation tomorrow is with your vendor.

If you don't have compliance software yet, try the free Binnacle AI calculator:

[Try the free compliance calculator →](/compliance-calculator)

Pick Sub M, enter your crew size and GT, and we'll generate a starting checklist keyed to 46 CFR sections that actually apply to your vessel — including the 12-hour rule.

Capt J is the founder of Binnacle AI. He runs a small maritime tech company on Oʻahu that builds compliance tools for commercial fleets. None of this article is legal advice — consult a qualified maritime attorney for specific regulatory questions.