Inland vs. International Rules
Where International COLREGS end and U.S. Inland Rules begin — and the key differences that matter in practice
The COLREGS Demarcation Line
The United States applies two separate sets of navigation rules depending on where a vessel is operating. The boundary between them is called the COLREGS Demarcation Line, defined in 33 CFR Part 80.
Seaward of the Demarcation Line: International COLREGS (1972 Convention on the International Regulations for Preventing Collisions at Sea) apply. These are the rules that govern all vessels on the high seas and in waters connected to the high seas that are navigable by seagoing vessels.
Shoreward of the Demarcation Line (inland waters): U.S. Inland Navigation Rules apply. This includes most harbors, rivers, lakes, and inland waters of the United States.
Where is the line? The Demarcation Line is published in 33 CFR Part 80 and is marked on NOAA nautical charts with a dashed magenta line. Locations vary: at some ports the line runs along the mouth of a harbor entrance; at others it follows a breakwater, jetty, or specific geographic line. You need to know where the line is for every harbor you enter or exit.
Practical example: Chesapeake Bay is an inland waterway — Inland Rules apply. Once you cross the line at the entrance to the Bay at Cape Henry/Cape Charles and proceed seaward into the Atlantic, International COLREGS apply.
A critical legal point: ignorance of which rule set applies is not a defense. If you are operating in inland waters, you are expected to know and follow the Inland Rules. If you operate internationally, International COLREGS govern. When in doubt, apply the more conservative interpretation.
Before any passage that crosses from inland to coastal waters, identify where the COLREGS Demarcation Line is on your chart. The dashed magenta line on NOAA charts is easy to overlook — but crossing it changes which rule set applies to you, your lights, your signals, and your passing obligations.
Where is the COLREGS Demarcation Line defined and how is it marked on charts?
Which rule set applies on the Chesapeake Bay?
Key Differences in Lights, Shapes, and Vessel Types
The International COLREGS and Inland Rules are largely identical — both stem from the same 1972 framework and the Inland Rules were specifically rewritten in 1980 to align with COLREGS. However, there are meaningful differences that apply in practice.
Vessel Constrained by Draft (CBD): This category exists only in International COLREGS. Under International Rules, a CBD vessel displays three red lights vertically (or a cylinder by day) and has elevated right-of-way in narrow channels. The Inland Rules do not recognize the CBD category — the concept does not exist in inland rules.
Pushing and towing configurations — light displays: the specific light configurations for inland pushing/towing differ from international configurations. This matters most for commercial traffic on rivers and in ports. The most visible difference is that inland towing configurations use a distinct series of amber towing lights that differ from international specifications.
Anchor lights on small vessels: under International Rules, a vessel under 7 meters may, if not in or near a narrow channel, fairway, or anchorage, omit the anchor light. Under Inland Rules, a vessel under 20 meters may show an anchor light anywhere, and a vessel under 7 meters need not show it unless in a narrow channel or fairway.
Flashing blue lights for law enforcement: Inland Rules include a provision for U.S. law enforcement vessels to display a flashing blue light when engaged in direct law enforcement activities. International Rules have no equivalent.
Vessels engaged in minesweeping: the specific lighting provisions for minesweeping vessels differ slightly between the two rule sets, relevant primarily to military operations.
For recreational sailors, the CBD category is the most important structural difference to know. If you are operating internationally (offshore, Caribbean passages, crossing to the Bahamas) and you see a vessel displaying three red lights vertically, it is constrained by its draft and has elevated right of way — you must keep clear. This signal has no meaning in U.S. inland waters.
Which vessel category exists under International COLREGS but NOT under U.S. Inland Rules?
What unique provision in the Inland Rules has no equivalent in International COLREGS?
Sound Signal Differences and the Agreement Signal
The most operationally significant difference between International and Inland Rules for most sailors is the maneuvering sound signal system.
International Rules — intent signals: under International Rules, maneuvering signals (one blast = turning starboard, two blasts = turning port, three blasts = engines astern) are intention signals — they announce an action that is being taken. The other vessel is not required to respond. There is no agreement procedure.
Inland Rules — agreement signals: under Inland Rules, maneuvering signals are proposal and agreement signals. When a power vessel proposes a passing arrangement with one or two blasts, the other vessel must either agree (same signal back) or disagree (five or more short rapid blasts). This creates an explicit mutual-agreement protocol that does not exist under International Rules.
Practical impact: if you are sailing inland waters and you hear one blast from an approaching vessel, you are expected to respond. If you agree with the proposed arrangement, reply with one blast. If you don't understand or disagree, give five short blasts. Not responding is itself a problem — the other vessel is waiting for your agreement before maneuvering.
The 'same direction' passing on inland waters: the Inland Rules also govern meeting situations on rivers where vessels may be overtaking in the same lane. The upstream vessel typically has priority, and the agreement signal system is used to negotiate who passes where.
Crossing and head-on situations: the rules for crossing (Rule 15) and head-on (Rule 14) encounters are identical under both rule sets. A vessel on your starboard side in a crossing situation has right of way under both International and Inland Rules.
The give-way/stand-on structure: identical under both rule sets. The vessel hierarchy (NUC, RAM, CBD [international only], fishing, sailing, power) is essentially the same, minus the CBD category in Inland Rules.
Summary of the key practical differences for sailors:
1. Agreement signal required for maneuvering proposals (Inland only)
2. CBD category does not exist (Inland only)
3. Law enforcement blue light provision (Inland only)
4. Anchor light exemptions vary slightly by vessel length
5. Some towing/pushing light configurations differ for commercial traffic
When transiting between inland and offshore waters on a passage, make a mental note as you cross the Demarcation Line. Your maneuvering signal obligations change: offshore you are announcing actions; inland you are proposing and must wait for agreement. A sailor unfamiliar with the inland system who hears two blasts from a commercial vessel and does not respond may find that vessel unable to safely proceed.
Under Inland Rules, when a vessel proposes a passing arrangement with two short blasts and you agree, you must:
Under International Rules, when you hear one short blast from an approaching vessel, you should:
Summary
The COLREGS Demarcation Line (33 CFR Part 80, marked as a dashed magenta line on NOAA charts) separates U.S. inland waters (Inland Navigation Rules) from seaward waters (International COLREGS). The most important practical differences for sailors are: the Inland Rules require a maneuvering signal agreement response while International Rules treat signals as announcements; the Vessel Constrained by Draft category exists only under International COLREGS; and Inland Rules include a law enforcement flashing blue light provision. The crossing and give-way/stand-on structure is essentially identical under both rule sets.
Key Terms
- COLREGS Demarcation Line
- The boundary defined in 33 CFR Part 80 separating U.S. inland waters (Inland Rules) from seaward waters (International COLREGS). Marked as a dashed magenta line on NOAA charts.
- U.S. Inland Navigation Rules
- The set of navigation rules (based on COLREGS but with modifications) that govern vessel operation in U.S. inland waters shoreward of the Demarcation Line.
- Agreement Signal
- The Inland Rules requirement for the receiving vessel to respond to a proposed maneuvering signal with the same signal to confirm agreement before the maneuver is executed.
- Vessel Constrained by Draft (CBD)
- A power-driven vessel that cannot deviate from its course due to its draft relative to available water depth. Recognized only under International COLREGS, not Inland Rules.
- 33 CFR Part 80
- The Code of Federal Regulations section that defines the exact positions of the COLREGS Demarcation Lines for U.S. waters.