1.1 The Navigation Rules: International (COLREGS) vs Inland

Two Rule Sets, One Waterway System

Every mariner operating in United States waters must obey one of two closely related sets of navigation rules. Offshore you follow the International Regulations for Preventing Collisions at Sea, 1972 — universally abbreviated 72 COLREGS or simply the International Rules. Close to shore and on rivers, bays, and the Great Lakes you follow the U.S. Inland Navigation Rules, enacted by the Inland Navigational Rules Act of 1980 and now codified in 33 CFR Part 83. The OUPV exam draws its 50 Rules-of-the-Road questions from both systems, so you must know each rule and the specific points where the two disagree.

Quick Answer: COLREGS apply seaward of the COLREGS Demarcation Lines; the Inland Rules apply landward of (inside) those lines. The rule numbers are identical in both books, and roughly 90% of the text is word-for-word the same.

The COLREGS Demarcation Lines

The boundary between the two systems is not a vague "three miles out" — it is a set of COLREGS Demarcation Lines published in 33 CFR Part 80 and printed on nautical charts. These lines are drawn, wherever practical, between fixed, eye-visible objects such as prominent headlands, jetties, and lighthouses, so a mariner can tell which rules apply without instruments.

• Seaward (outside) the line → International Rules (72 COLREGS).
• Landward (inside) the line → Inland Rules.

A demarcation line typically runs across the mouth of a harbor, river, or bay. The instant your bow crosses it, the governing rule set can change — most often affecting the meaning of your whistle signals and the required lights on a tow. On the OUPV chart you should be able to point to a demarcation line and state which rules govern each side.

Shared Structure: Parts A Through E

Both rule books are organized identically, which makes them far easier to learn than their length suggests:

Both books also carry Annexes covering the technical details of light positioning and spacing (Annex I), fishing-vessel signals (Annex II), sound-appliance specifications (Annex III), and distress signals (Annex IV). Because Part B, Section II (Rules 11-18) applies only to vessels in sight of one another, while Section III (Rule 19) applies in restricted visibility, knowing which section you are in tells you which rules are even active.

Where International and Inland Diverge

Most of the text is identical, but a handful of differences appear repeatedly on the exam. Commit these to memory:

1. Whistle signals mean different things. Under the International Rules a whistle blast reports an action already taken ("I am altering my course to starboard"). Under the Inland Rules a blast announces an intention ("I intend to leave you on my port side"). Same one short blast, different logic.
2. Constrained by Draft (CBD) exists only offshore. The vessel category constrained by her draught, with its three all-round red lights, is an International-Rules-only concept. There is no CBD in the Inland Rules.
3. Western Rivers and Great Lakes get special right-of-way. In Inland waters a power-driven vessel proceeding downbound with a following current on the Great Lakes, Western Rivers, or specified waters has the right-of-way over an upbound vessel and proposes the manner of passage (Inland Rule 14(d)). COLREGS has no such provision.
4. Some lights differ. Inland allows certain vessels a special flashing yellow light, and towing-light arrangements differ for pushed and towed barges.

A Practical Boundary Example

Picture a charter running out of a coastal inlet. Inside the jetties and up the river the vessel is landward of the demarcation line, so the Inland Rules apply: a meeting vessel's single short blast means "I intend to leave you on my port side," and there is no constrained-by-draft category to consider. As the charter clears the jetty tips and crosses the charted magenta demarcation line into the open approaches, it passes seaward of the line and the International Rules take over — now a single short blast means "I am altering my course to starboard," and a deep-draft ship in the fairway may legitimately display the three all-round red lights of a vessel constrained by her draught. Nothing about the water changed; only the rule book did, at the instant the bow crossed the line.

Both systems are published together by the Coast Guard in a single volume, Navigation Rules, International-Inland, with the two texts printed side by side so differences are easy to spot. Local pilot rules and harbor regulations, authorized by Rule 1, can add further requirements on top of the Inland Rules in specific waters, so on unfamiliar water you should also consult the Coast Pilot for that area.

Why Precision Matters for the OUPV

The Rules-of-the-Road module is scored at 90% — the highest threshold on the exam alongside Chart Plotting. On a 50-question module that means you can miss no more than five. Questions are frequently written to punish the candidate who remembers the idea of a rule but not its exact wording ("early and substantial action," "more than 22.5° abaft the beam," "does not appreciably change"). Throughout this chapter the exact statutory phrasing is highlighted in bold, because on this module the difference between an approximate answer and the verbatim rule is the difference between passing and failing.