Comparatifs
COLREG vs US Inland Rules — what changes inside the demarcation line
On a chart of US coastal waters, magenta dashed lines mark the COLREG Demarcation Lines. Seaward of those lines, the international COLREGs apply. Landward — bays, sounds, harbours, the Great Lakes, the Western Rivers, the ICW — the US Inland Navigation Rules apply.
The two rule sets are almost identical, which is why the USCG exam tests the differences relentlessly. This page lists every meaningful one.
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COLREG (International) vs US Inland — major differences
| Topic | COLREG (International) | US Inland Rules |
|---|---|---|
| Where each applies | Seaward of the COLREG Demarcation Lines and on the high seas worldwide | Landward of the Demarcation Lines (33 CFR 80) — bays, sounds, the Great Lakes, Western Rivers, ICW |
| Towing — flashing yellow light | Not used | ALL towing vessels pushing ahead OR towing alongside show a SPECIAL FLASHING YELLOW light forward (Rule 24(f) Inland) |
| Whistle signals — meaning | INTENT signals: "I am altering course to starboard" (1 short), etc. | AGREEMENT signals: "I propose to leave you on my port side" (1 short) — the other vessel agrees with the same signal |
| 5+ short blasts (danger signal) | Required when unsure of intent (Rule 34(d)) | Same meaning, same minimum (but Inland also requires it before getting underway from a berth in certain situations) |
| Power-driven vessel < 12 m — masthead light | MAY substitute an all-round white for masthead+sternlight (Rule 23(c)(i)) | Same provision; additionally, vessels < 12 m on Inland waters may show sidelights as a combined lantern even when ≥ 12 m would not be allowed |
| Vessel at anchor in a "special anchorage area" | No equivalent — anchor lights required | Vessels < 20 m anchored in a special anchorage area (CFR-designated) are NOT required to exhibit anchor lights |
| Sailing vessel < 7 m | Show sidelights+sternlight if practicable; otherwise an electric torch on approach (Rule 25(d)(ii)) | Same in substance; Inland slightly different wording — "have ready at hand" an electric torch |
| Western Rivers special rules | Not applicable | Special rules for descending vs ascending vessels (descending has the right of way in narrow channels); explicit pilot rules; bridge-to-bridge radio (VHF Ch 13) |
| Bridge-to-bridge radiotelephone | Not mandated by COLREG | Required (Bridge-to-Bridge Radiotelephone Act, 33 USC 1201): commercial vessels ≥ 20 m, towing vessels, vessels carrying dangerous cargo must maintain VHF Ch 13 watch |
| Vessel approaching a bend — sound signal | Rule 9(f): one prolonged blast | Same wording, same blast, but Inland Rule 34 phrasing differs slightly — exam-trap territory |
Which rule set do I use?
Check the chart for the magenta Demarcation Line and look up the relevant CFR section.
- 33 CFR 80 — "COLREGS Demarcation Lines" — is the authoritative source. It is published on every NOAA chart of US coastal waters.
- Seaward of the line: COLREG (International) applies.
- Landward of the line, plus the Great Lakes, plus the Western Rivers (Mississippi system, parts of the Atchafalaya, Red, etc.): US Inland Rules apply (33 CFR 83).
- On the boundary itself: COLREGs apply.
- If your USCG exam scenario mentions a "vessel pushing ahead with a special flashing yellow light", it's Inland. The yellow flashing light does not exist in COLREG.
Drill the rules that actually differ
Frequently asked questions
Where do the US Inland Rules apply?
Landward of the COLREG Demarcation Lines specified in 33 CFR 80, plus the Great Lakes, plus the Western Rivers (Mississippi River system and selected tributaries). The Demarcation Lines are drawn on every NOAA chart of US coastal waters and define exactly where COLREG ends and Inland begins.
Why does Inland have a yellow flashing light?
It is a US-specific safety feature for vessels pushing barges ahead or towing alongside — situations common on US inland waterways. The special flashing yellow light forward (Rule 24(f) Inland) makes the geometry of an integrated tug-and-tow group unambiguous from a distance. COLREG handles the same case with masthead lights and configuration alone.
Are the whistle signals the same in COLREG and US Inland Rules?
Same blasts, different meaning. Under COLREG Rule 34, whistle signals announce INTENT ("I am altering course to starboard"). Under Inland Rule 34, the same blasts propose an AGREEMENT ("I propose to leave you on my port side") which the other vessel confirms with the identical signal. The Inland exam tests this swap directly.
Do USCG candidates have to learn both rule sets?
Yes. The USCG OUPV and higher licence examinations test both COLREG (International) and the US Inland Rules, and candidates are expected to know which differences matter on which side of the Demarcation Line. The Inland-specific provisions tested most often are the flashing yellow towing light, the agreement-style whistle conventions, the Western Rivers descending-vessel right of way, and bridge-to-bridge VHF Ch 13.
Do COLREG Rules apply on the Great Lakes?
No. The Great Lakes are governed by the US Inland Navigation Rules (and parallel Canadian Collision Regulations on the Canadian side). The Demarcation Lines at the seaward end of the St Lawrence are where COLREG begins for vessels exiting toward the Atlantic.
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