A boater in U.S. waters can be under two slightly different rulebooks in the same afternoon. The International Regulations apply offshore; the Inland Rules apply inside the demarcation lines and on the Great Lakes. Most rules read the same in both. A few do not, and the ones that differ are exactly the ones that matter when two boats are close together.
Rule 1 answers this directly. The International wording applies on the high seas and in the waters connected with them that are navigable by seagoing vessels. The Inland wording applies on the inland waters of the United States and to U.S. vessels on the Canadian waters of the Great Lakes, to the extent there is no conflict with Canadian law.
The boundary between the two is the COLREGS demarcation line. Rule 3 defines “inland waters” as the navigable waters shoreward of the navigational demarcation lines dividing the high seas from harbors, rivers and other inland waters, plus the U.S. side of the Great Lakes. Nautical charts mark the line, so the practical way to know which wording applies before you are in a crossing situation is to look at your chart before you cross it.
One more inland-specific duty is easy to overlook: on inland waters, every self-propelled vessel 12 meters (about 39 feet) or longer must carry a copy of the Rules on board (Rule 1(g)).
The core of collision avoidance does not change with the water you are on. The lookout duty (Rule 5), safe speed (Rule 6), judging risk of collision (Rule 7), acting early and obviously (Rule 8), the overtaking rule (Rule 13), the duties of give-way and stand-on vessels (Rule 16, Rule 17) and the restricted-visibility rule (Rule 19) are written once and apply everywhere. If you have learned those, most of what you need is the same in both versions.
| Topic | International | Inland |
|---|---|---|
| Downbound vessel with a following current | No special right-of-way. | On the Great Lakes, Western Rivers and waters specified by the Secretary, a power-driven vessel going downbound with the current has the right-of-way over an upbound vessel, proposes the manner and place of passage, and starts the signals. The upbound vessel holds as needed (Rule 9, Rule 14). |
| Crossing a river | Ordinary crossing rule: the vessel with the other on her starboard side gives way (Rule 15). | On the Great Lakes, Western Rivers and specified waters, a power-driven vessel crossing a river keeps out of the way of one going up or down the river (Rule 15(b)). |
| One short whistle blast | “I am altering my course to starboard.” | “I intend to leave you on my port side.” A proposal, not a completed turn (Rule 34(a)). |
| Two short whistle blasts | “I am altering my course to port.” | “I intend to leave you on my starboard side.” |
| Answering a passing signal | The signal announces what you are doing; no reply is required. | The other vessel, if she agrees, sounds the same signal back. If she doubts the maneuver is safe she sounds the five-blast danger signal and both vessels take precautions until a safe passing is agreed. |
| Signals for overtaking | In a narrow channel: two prolonged blasts then one short (overtake on your starboard side) or two short (port side). The overtaken vessel agrees with prolonged-short-prolonged-short (Rule 34(c)). | One short blast (starboard side) or two short (port side); the vessel being overtaken, if she agrees, sounds a similar signal. |
| Radio agreement | Not addressed in the rule text. | Vessels that reach agreement by radiotelephone in a head-on, crossing or overtaking situation need not sound whistle signals. If they do not reach agreement, whistle signals are exchanged and prevail (Rule 34(h)). |
| Deep-draft ships | Other vessels must, where circumstances allow, avoid impeding a vessel constrained by her draft that shows the signal (Rule 18(d)). She may show three red lights or a cylinder (Rule 28). | No equivalent. Rule 18(d) and Rule 28 are reserved. |
| Lights on a tow | Two or three masthead lights in a vertical line, a sternlight and a yellow towing light (Rule 24). | Same base pattern, plus a yellow “special flashing light” on a pushed or alongside tow, and on the Western Rivers a simpler two-towing-light pattern (Rule 24(f), (j)). |
| Power-driven vessel lights | Small-vessel simplifications by length and speed (Rule 23(d)). | On the Great Lakes, one all-round white light may replace the second masthead light and the sternlight (Rule 23(e)). Lights may be lowered to clear a bridge (Rule 20(f)). |
| Anchored barges and small craft | Small vessels have relief by size. | Detailed barge lighting rules, and vessels under 20 meters in a Coast Guard special anchorage area need no anchor lights (Rule 30(g)-(l)). |
| Older vessels | Limited grandfather exemptions for vessels whose keel was laid before the effective date, in 1977 (Rule 38). | Same idea with a December 24, 1980 cutoff. Any boat built since is not covered. |
Whistle signals are where the two rulebooks look the most alike and mean the most different things. Under the International Rules, one short blast tells another vessel “I am altering my course to starboard.” Under the Inland Rules, within half a mile of another power-driven vessel that you are meeting or crossing, the same single blast means “I intend to leave you on my port side.”
That is a proposal. The Inland maneuver is not supposed to start until the other vessel answers with the same signal. If you treat an inland one-blast signal as an announcement, you can end up committed to a maneuver nobody else has agreed to. If you are on the receiving end and you are not comfortable with what was proposed, Rule 34(d) gives you the answer that is the same in both versions: at least five short and rapid blasts, immediately, to say you are in doubt.
Several Inland Rules apply only on the Great Lakes, the Western Rivers or waters the Secretary specifies. Rule 3 defines the Western Rivers as the Mississippi River and its tributaries, South Pass and Southwest Pass, and a few connected waters listed in the rule, and the Great Lakes as the lakes and their connecting tributary waters listed there. The downbound-current right-of-way, the river-crossing rule and the Western Rivers towing lights are all limited to these waters. Do not carry them into a harbor or a coastal passage where they do not apply.
The fog-signal rule (Rule 35) is largely shared: one prolonged blast every two minutes when making way, two when stopped, and one prolonged plus two short for vessels that are not under command, restricted in their ability to maneuver, sailing, fishing or towing. The lists differ slightly at the edges: the Inland version groups a vessel restricted in her ability to maneuver in that last category whether she is underway or at anchor, and the International version includes a vessel constrained by her draft. The Inland version also excuses vessels under 20 meters and barges from the anchor signals when anchored in a Coast Guard special anchorage area.
Before a trip, note on your chart or plan where the demarcation line lies on your route and which special-water rules (Great Lakes or Western Rivers) you will be in. The steering rules in the manual (Rule 13 to Rule 17) are the same in both versions; what changes is how you talk to the other vessel and a few right-of-way exceptions. For those, knowing which version you are under is half the answer. When you meet another vessel and are unsure what they mean, slow down, keep watching, and use the five-blast doubt signal rather than guessing.