1.1 COLREGS vs Inland Rules

Why Rules Foundation Comes First on Master 100

On the ONC06 Master Less Than 100 GRT path, Q100 Rules of the Road is a stand-alone 50-question module scored at 90%. That is the same bar as Chart Plot (Q163) and a full 20 points above Deck General, Deck Safety, and Navigation General Near Coastal (all 70%). Miss six Rules items and the module fails even if every other module is perfect. The Coast Guard draws those 50 items from both the International Regulations for Preventing Collisions at Sea, 1972 (72 COLREGS) and the U.S. Inland Navigational Rules (Inland Navigational Rules Act of 1980; codified primarily in 33 CFR Part 83). A Master who only ever ran rivers and memorized "Inland" wording will bleed points on International items; a coastal operator who never learned Inland whistle intent signals will do the same on harbor scenarios.

Master 100 framing: Near Coastal (NC) and Inland route endorsements both sit under the Master LT 100 credential. Q100 does not grade only the waters on your MMC. You are tested on both systems. Treat inland-only prep as an automatic 90% risk.

Two Books, One Credential

The Coast Guard publishes both side by side in Navigation Rules, International–Inland. Rule numbers match (Rule 5 is lookout in both books; Rule 15 is crossing in both). Roughly nine-tenths of the text is identical. Q100 exploits the remaining tenth: candidates who "know the idea" of a rule but not which book is active, or who confuse action with intent, lose the point.

COLREGS Demarcation Lines (33 CFR Part 80)

The switch is not "when you leave the jetties," "beyond three nautical miles," or "when you clear the sea buoy" unless a charted line happens to coincide with those landmarks. The legal boundary is a set of COLREGS Demarcation Lines in 33 CFR Part 80, drawn on NOAA charts (often as a magenta line across a harbor mouth, bay entrance, or river approach). Where practical they run between fixed, eye-visible objects — headlands, jetty tips, lights — so you can fix the boundary without electronics.

• Outside / seaward of the line → International Rules.
• Inside / landward of the line → Inland Rules.

The moment your vessel crosses the line outbound, the governing book can change mid-passage. For a Master on a passenger run (six-pack, uninspected passenger vessel, or inspected small passenger vessel under 100 GRT), that can flip what a whistle blast means and whether Constrained by Draft even exists as a category — without any change in sea state.

Worked boundary scenario (Master passenger charter)

You hold Master NC LT 100 and run a 49-passenger T-boat from a coastal inlet. Up the river and inside the jetties you are landward of the charted demarcation line: Inland Rules apply. A meeting power-driven vessel's one short blast is an intention signal ("I intend to leave you on my port side"), and there is no Constrained-by-Draft category. Once you clear the jetty tips and cross the magenta demarcation line into the approaches, you are seaward: International Rules take over. The same one short blast now reports an action already being taken ("I am altering my course to starboard"), and a deep-draft freighter in the fairway may lawfully show the three all-round red lights of a vessel constrained by her draught. Nothing about the water "felt" different — only the statute did.

Shared Structure: Parts A–E

Both books share the same skeleton. Learning the skeleton once multiplies every later chapter.

Annexes (I–IV and related technical annexes) cover light placement, additional fishing signals, sound-signal equipment, and distress signals. Critically for later study: Part B Section II (Rules 11–18) applies only when vessels are in sight of one another; Section III (Rule 19) governs restricted visibility. If you cannot observe the other vessel visually, you are not in a Section II "crossing/head-on/overtaking" geometry problem — you are in Rule 19 territory.

Where International and Inland Diverge (Q100 high-yield)

1. Whistle-signal logic. International: a maneuvering blast reports an action taken or being taken. Inland: the corresponding blasts often announce intent and may require agreement in meeting/overtaking situations. Same blast pattern; different legal meaning.
2. Constrained by Draft (CBD). Exists under International Rules only (power-driven vessel severely restricted by draft relative to available depth and width). No CBD category under Inland Rules.
3. Downbound with following current (Inland). On the Great Lakes, Western Rivers, and certain specified waters, a power-driven vessel proceeding downbound with a following current has right-of-way over an upbound vessel and proposes the manner of passage (Inland Rule 14(d) and related provisions). COLREGS has no equivalent.
4. Lights and special signals. Inland allows arrangements (for example certain towing/pushing lights and special flashing yellow lights on some vessels) that differ from International text. Do not assume a light picture valid in one book is valid in the other.

Rule 1 also allows special rules for harbors, rivers, and inland waters — local pilot rules, VTS participation, and Coast Pilot notes can add duties without erasing COLREGS/Inland. On unfamiliar water, chart the demarcation line and read the local notes before you count on "standard" signals.

Exam Trap: Inland-Only Mastery Is Not Enough for Near Coastal

Candidates upgrading from inland OUPV/Master Inland routes sometimes bank everything on river practice and skip International nuances. Q100 still asks International items: CBD existence, action-vs-intent blasts, open-sea restricted-visibility conduct, and demarcation-line application. Conversely, pure offshore fishermen who never ran Western Rivers miss the downbound following-current privilege. Master NC prep must drill both books to the verbatim level — "does not appreciably change," "early and substantial," "more than 22.5° abaft the beam" — because the 90% cut leaves no room for approximate wording.

Precision Standard for the Rest of This Chapter

This foundation chapter (Rules 1–3, 5–7) builds the vocabulary and risk framework every later Rules chapter uses. When a quiz answer hinges on whether you are "underway," "engaged in fishing," or "in sight," the wrong category silently flips the entire give-way analysis. Read the statutory phrases in bold as if they were the answer choices — on Q100, they often are.