COLREGS Inland Rules
The complete exam guide to US Inland Navigation Rules versus International COLREGS (72) — covering demarcation lines, sound signals, lights, Rule 9, Rule 34 flashing lights, Western Rivers, and every difference the USCG captain exam tests.
Where Each Set of Rules Applies
The COLREGS demarcation lines determine which rule set governs. These lines run across the mouths of US harbors, rivers, and bays. The moment a vessel crosses the line, the applicable rules change.
| Item | Detail |
|---|---|
| Governing regulation | 33 CFR Part 80 |
| Where Inland Rules apply | Shoreward of demarcation lines — harbors, rivers, bays, coastal waters |
| Where COLREGS apply | Seaward of demarcation lines — high seas and ocean-connected waters |
| When rules switch | At the moment a vessel crosses the demarcation line |
| Great Lakes | Inland Rules apply with some supplemental provisions |
| Western Rivers | Inland Rules apply with additional Western Rivers passing conventions |
| Exam shortcut | Harbor / river / bay = Inland Rules. High seas / offshore = COLREGS. |
The question says "international waters," "high seas," "offshore," or "seaward of the demarcation line."
The question says "harbor," "river," "bay," "inland waters," or "shoreward of the demarcation line."
Sound Signals — The Most Tested Difference
The most frequently examined Inland vs. International distinction is the maneuvering sound signal system. Under COLREGS, signals describe what a vessel IS doing. Under Inland Rules, signals describe what a vessel INTENDS to do — and require agreement before the maneuver begins.
Signals are informational. One blast = I am altering to starboard (the action is happening). No answer required from the other vessel. Maneuver proceeds immediately.
Think: COLREGS = I am doing this.
Signals are proposals. One blast = I intend to leave you on my port side (proposal). The other vessel must respond with the same signal. Only then does the maneuver begin.
Think: Inland = I want to do this — do you agree?
| Signal | Duration | COLREGS Meaning | Inland Meaning |
|---|---|---|---|
| 1 Short Blast | approx. 1 second | I am altering my course to starboard (action signal — no answer required) | I intend to leave you on my port side (proposal — other vessel must respond with same blast to agree) |
| 2 Short Blasts | approx. 1 second each | I am altering my course to port (action signal — no answer required) | I intend to leave you on my starboard side (proposal — other vessel must respond with same two blasts to agree) |
| 3 Short Blasts | approx. 1 second each | My engines are going astern (does not necessarily mean vessel is moving astern — only engines reversed) | My engines are going astern (same meaning as COLREGS under both rule sets) |
| 5 or More Short Blasts | rapid succession | Doubt / danger signal — I doubt your intentions or question whether you are taking sufficient action | Doubt / danger signal — same meaning, also used to reject a proposed maneuver from another vessel |
| 1 Prolonged Blast | 4 to 6 seconds | Power vessel in restricted visibility (every 2 min); vessel leaving a berth; vessel approaching a bend | Same — vessel in restricted visibility; leaving berth; approaching a bend in a narrow channel |
| 1 Prolonged + 1 Short | — | Not a standard in-sight maneuvering signal under COLREGS Rule 34 | Overtaking signal: I intend to overtake you on your starboard side (in narrow channel) — Inland Rule 34(c)(i) |
| 1 Prolonged + 2 Short | — | Towing vessel in restricted visibility (Rule 35); also sailing/fishing/NUC/RAM fog signal | Overtaking signal: I intend to overtake you on your port side (in narrow channel) — Inland Rule 34(c)(ii) |
Sky rows highlight the most exam-critical distinctions.
Whistle Signal Meanings at a Glance
Under Inland Rules, one blast = leave you on my PORT side; two blasts = leave you on my STARBOARD side. This is counterintuitive to many exam takers — the blast count and direction reference do not match what you might expect.
| Blasts | COLREGS Meaning | Inland Meaning |
|---|---|---|
| 1 short blast | I am altering course to starboard (action taken) | I intend to leave you on my port side (proposal — awaiting agreement) |
| 2 short blasts | I am altering course to port (action taken) | I intend to leave you on my starboard side (proposal — awaiting agreement) |
| 3 short blasts | My engines are going astern | My engines are going astern (same) |
| 5+ short blasts | Doubt / danger signal | Doubt / danger signal; also rejection of proposed maneuver |
| 1 prolonged + 1 short | Not a standard in-sight signal | Intent to overtake on starboard side of vessel ahead (narrow channel) |
| 1 prolonged + 2 short | Fog signal: towing vessel / sailing / fishing / NUC / RAM | Intent to overtake on port side of vessel ahead (narrow channel) |
Rule 9 — Narrow Channels: Inland-Specific Provisions
Rule 9 (Narrow Channels) is largely the same under both rule sets, but Inland Rules add critical overtaking signal requirements and the Western Rivers right-of-way rule. These are tested directly.
Sound: 1 prolonged + 1 short blast
Vessel ahead must answer: 1 prolonged + 1 short blast
Sound: 1 prolonged + 2 short blasts
Vessel ahead must answer: 1 prolonged + 2 short blasts
Compare to COLREGS: 2 prolonged + 1 short (starboard) or 2 prolonged + 2 short (port) — no agreement system
Rule 10 — Traffic Separation Schemes
Rule 10 (Traffic Separation Schemes — TSS) governs vessel conduct in designated traffic lanes. TSS exist primarily in COLREGS waters, but Inland Rules recognize them as well where established.
Vessels crossing a TSS lane shall do so as nearly as practicable at right angles (90 degrees) to the direction of traffic flow. This is the most tested Rule 10 fact.
Vessels using a TSS shall keep to the starboard side within the lane. Do not travel on the port side of the centerline.
Vessels shall join or leave a traffic lane at the termination of the lane or when joining from the side, at as small an angle as practicable.
Vessels under 20 m, sailing vessels, and fishing vessels shall not impede the safe passage of power-driven vessels following a traffic lane.
Vessels shall not enter the separation zone unless: in an emergency, to avoid immediate danger, or to engage in fishing within the zone.
Where Traffic Separation Schemes exist on Inland Waters (such as near certain port approaches), the same Rule 10 crossing and lane requirements apply.
Navigation Lights — Inland vs. International Differences
Most light requirements are identical between the two rule sets. The key differences center on the vessel constrained by draft category, the push-tow flashing yellow light, and minor height specifications in the respective Annex I.
Category does not exist under Inland Rules. No lights or shapes recognized.
Three all-round red lights in vertical line at night; cylinder day shape (Rule 28). All vessels must give sea room.
One all-round white light. Vessels under 7 m not required to show anchor light if anchored outside a channel or fairway where vessels navigate.
One all-round white light where best seen. Same exemption for vessels under 7 m in or near a special anchorage area.
All-round white light forward; all-round white light aft (lower than forward). Same as International.
All-round white light in the fore part; second all-round white light at or near the stern — lower than the forward light.
A vessel pushing ahead or towing alongside on inland waters shall show a special flashing yellow light. This is an Inland-only requirement under Inland Annex V.
No flashing yellow light required for push-tow operations. Standard towing lights apply based on composite length.
Forward masthead light: at least 4.5 m above hull. If the vessel is over 50 m, after masthead light must be at least 4.5 m higher than forward light. Inland Annex I varies from COLREGS Annex I in specific height requirements for certain vessel sizes.
Forward masthead light: at least 6 m above hull (for vessels over 20 m). After masthead light: at least 4.5 m higher than forward masthead light.
Two or three masthead lights in vertical line plus yellow towing light above white stern light. Same basic structure as International; push-tow on Inland Waters additionally shows the flashing yellow light.
Two masthead lights (tow under 200 m) or three masthead lights (tow over 200 m) plus yellow towing light above stern light. No additional flashing yellow light.
Rule 14 Head-On — Inland Horn Signals Required
Rule 14 (Head-On Situation) requires both vessels to alter course to starboard under both rule sets. The critical Inland difference: vessels must exchange agreement signals before altering course. Under COLREGS, each vessel sounds one blast and proceeds — no agreement required.
| Aspect | Inland Rules | COLREGS |
|---|---|---|
| Trigger | Two power-driven vessels meeting on reciprocal or nearly reciprocal courses involving risk of collision. | Same — two power-driven vessels on reciprocal or nearly reciprocal courses with risk of collision. |
| Required action | Both vessels alter course to starboard to pass port-to-port. Both must sound the agreement signal before altering. | Both vessels alter course to starboard. Signals are informational — each sounds one blast and proceeds without waiting for an answer. |
| Sound signal | Each vessel sounds 1 short blast (proposal). Other vessel responds with 1 short blast to agree. Then both alter to starboard. | Each vessel sounds 1 short blast to indicate the alteration is occurring or about to occur. No answering blast required. |
| When in doubt | Treat as head-on and alter to starboard. Sound 1 blast and wait for agreement. | Treat as head-on and alter to starboard. Sound 1 blast — no waiting required. |
Rule 15 Crossing — Right-of-Way Under Both Rule Sets
The crossing rule (Rule 15) applies identically in most situations — the vessel with the other on her starboard side is the give-way vessel. Inland Rules add the Western Rivers exception and the proposal-and-agreement signal system.
| Aspect | Inland Rules | COLREGS |
|---|---|---|
| Basic rule | Vessel that has the other on her own starboard side is the give-way vessel — same as International. | Vessel that has the other on her own starboard side shall keep out of the way — Rule 15. |
| Western Rivers exception | On the Western Rivers, a crossing vessel always gives way to the vessel proceeding up or down the main channel, regardless of which side the other vessel is on. | No Western Rivers exception — standard Rule 15 crossing rules apply everywhere. |
| Sound signal | Give-way vessel sounds 2 short blasts (intending to pass on the stand-on vessel's starboard side). Stand-on vessel must answer with 2 short blasts to agree before the maneuver. | Sound signals are informational — 1 or 2 blasts indicate the alteration being made. No agreement system. |
Rule 23 Power-Driven Vessels — Masthead Light Inland Differences
Rule 23 governs lights for power-driven vessels underway. The light arcs are identical (225 degrees for masthead, 112.5 degrees each sidelight, 135 degrees sternlight), but Inland Annex I specifies different height requirements from COLREGS Annex I for certain vessel sizes.
| Aspect | Inland Rules | COLREGS |
|---|---|---|
| Masthead lights — under 50 m | One forward masthead light required. Second masthead light optional but if carried must be aft and higher. | One forward masthead light required. Second masthead light optional but if carried must be aft and higher — same general rule. |
| Masthead light minimum height | At least 2.5 m above sidelights (for vessels under 20 m). Specific Inland Annex I height requirements differ slightly from COLREGS Annex I. | At least 2.5 m above sidelights for vessels under 20 m; at least 4 m for vessels 20 m or more. |
| Air-cushion vessel (hovercraft) | Flashing all-round yellow light in addition to standard power-driven vessel lights — same as International. | Flashing all-round yellow light (at least 120 flashes per minute) in addition to standard power-driven lights. |
| Wing-In-Ground (WIG) craft | Same as International — high-intensity all-round flashing red light when taking off, landing, or near the surface. | High-intensity all-round flashing red light; in addition to power-driven vessel lights when not in those phases. |
Rule 34(d) — Flashing Lights: Inland Waters Only
Inland Rule 34(d) establishes a flashing light system that supplements whistle signals. There is no equivalent provision under International COLREGS. This is a purely Inland Rules feature that the exam tests directly.
| Flash Pattern | Meaning |
|---|---|
| 1 short flash (~1 second) | I am altering my course to starboard |
| 2 short flashes | I am altering my course to port |
| 3 short flashes | My engines are going astern |
| All-round flashing yellow (hovercraft) | Air-cushion vessel in non-displacement mode |
| Flashing yellow light — push tow | Vessel pushing ahead or towing alongside on Inland Waters |
Comprehensive Inland vs. International Comparison Table
This table covers every major difference the USCG exam tests. Study this table until every row is automatic.
| Topic | Inland Rules | International COLREGS |
|---|---|---|
| Jurisdiction | Shoreward of COLREGS demarcation lines (33 CFR Part 80) — harbors, rivers, bays | Seaward of demarcation lines — high seas and ocean-connected waters |
| Sound signal system | Proposal-and-agreement: signals indicate intent; must receive matching signal before maneuvering | Information only: signals indicate action being taken or about to be taken; no answering blast required |
| 1 blast meaning | Intent to leave other vessel on my PORT side | I am altering course to STARBOARD |
| 2 blasts meaning | Intent to leave other vessel on my STARBOARD side | I am altering course to PORT |
| Narrow channel overtaking | Must signal intent (1 long + 1 or 2 short) and receive agreement before overtaking | 2 long + 1 short (starboard) or 2 long + 2 short (port) — informational, no agreement system |
| Vessel constrained by draft | Category does NOT exist under Inland Rules | Recognized under Rule 28: 3 all-round red lights; cylinder day shape; all vessels must give sea room |
| Flashing yellow maneuvering light | Rule 34(d): 1 flash = starboard; 2 flashes = port; 3 flashes = astern | No equivalent flashing maneuvering light provision |
| Push-tow flashing yellow light | Required — Inland Annex V: special flashing yellow light for vessel pushing ahead or alongside | Not required — no Inland Annex V equivalent under COLREGS |
| Western Rivers passing | Downriver vessel has right-of-way over upriver vessel; special crossing rules at bends | No equivalent — standard Rule 13/14/15 apply everywhere |
| Great Lakes | Inland Rules apply with some supplemental provisions (specific to that water body) | COLREGS do not apply on the Great Lakes |
| Constrained by draft day shape | No cylinder shape in Inland Rules — category does not exist | Cylinder displayed when vessel is constrained by draft |
| Masthead light height details | Inland Annex I specifications (slight variances from COLREGS for some vessel sizes) | COLREGS Annex I specifications — stricter height requirements for certain size ranges |
| Anchor light — vessels under 7 m | Not required if anchored outside channel, fairway, or anchorage where vessels normally navigate | Similar exemption — not required in or near special anchorage areas in certain conditions |
USCG Exam Tips — What the Exam Tests on Inland vs. International
These are the specific distinctions that appear most frequently on the USCG OUPV exam. Memorize these before test day — they account for many of the Rules of the Road questions you will face.
Practice Problems with Solutions
Work through these scenarios. Each one mirrors the style of actual USCG exam questions on Inland vs. International differences. Click each problem to reveal the answer and explanation.
1You are navigating on Inland Waters and you wish to pass another vessel port-to-port. You sound one short blast. What does this signal mean under Inland Rules, and what must happen before you maneuver?
Under Inland Rules, one short blast is a proposal meaning you intend to leave the other vessel on your port side (pass port-to-port). The other vessel must answer with one short blast to agree before you may alter course. If she sounds five short blasts, she disagrees and the maneuver must not be executed. This proposal-and-agreement system is the defining characteristic of Inland maneuvering signals and does not exist under COLREGS.
2At night you observe a vessel ahead showing three all-round red lights in a vertical line and a cylinder day shape during daylight. Are you in COLREGS or Inland waters? What type of vessel is this?
You are in International COLREGS waters. The cylinder day shape and three all-round red vertical lights identify a vessel constrained by draft — a category that exists only under COLREGS Rule 28. This vessel has severely restricted ability to maneuver due to her draft relative to the available depth. You must give her the sea room she needs. On Inland Waters, this category does not exist at all, and no vessel shows these signals under Inland Rules.
3You are overtaking a vessel ahead in a narrow channel on Inland Waters. You want to pass on her starboard side. What signal do you sound, and what must you do after sounding it?
Under Inland Rule 34(c)(i), you sound one prolonged blast followed by one short blast to signal your intent to overtake on the vessel ahead's starboard side. You must wait for the vessel ahead to answer with the same signal (one prolonged + one short) to indicate agreement. Only after receiving that agreement signal may you proceed to overtake. If she sounds five short blasts, she disagrees and you must not attempt the maneuver. Under COLREGS, the signal would be two prolonged + one short blast and no agreement is required.
4In a crossing situation on International COLREGS waters, a vessel to your starboard sounds two short blasts. What does this signal mean, and what is your obligation?
Under International COLREGS, two short blasts mean the vessel is altering her course to port. This is an informational signal — no answering blast is required. You should be alert to the other vessel's movement and assess whether the alteration resolves or worsens the collision risk. As the stand-on vessel in this crossing (the other vessel is on your starboard side — wait, if she is to your starboard, you are the give-way vessel under Rule 15). The give-way vessel should be taking early action to keep clear. Hearing two blasts tells you the other vessel is turning to port — monitor carefully and take avoiding action if needed.
5You are underway on a river governed by Inland Rules and you observe another vessel approaching from downstream. Your vessel is proceeding upstream. Which vessel has the right-of-way?
Under the Western Rivers passing convention in the Inland Rules, the vessel going downriver (downstream, with the current) has the right-of-way over the vessel going upriver. Your vessel proceeding upstream is the give-way vessel. You must sound the appropriate maneuvering signals to propose the passing arrangement and wait for agreement from the downriver vessel before altering course.
6A vessel on Inland Waters shows a flashing yellow light from the bow area in addition to masthead lights, sidelights, and sternlight. What type of vessel is this, and under what rule is this light required?
The flashing yellow light on an Inland Waters vessel indicates a pushing vessel (pushing ahead or towing alongside) under Inland Annex V — sometimes called a special flashing light. This light is required by Inland Rules but has no equivalent in International COLREGS. When you see this light, you know you are in Inland Waters and the vessel ahead is engaged in push-tow operations, which are common on rivers and harbors.
7Under Inland Rule 34(d), you see a vessel showing three short flashes of a white light. What does this mean?
Under Inland Rule 34(d), three short flashes of a white all-round light mean the vessel's engines are going astern. This is the visual equivalent of the three short blasts whistle signal. This flashing light provision exists only under Inland Rules — there is no equivalent under International COLREGS. The light supplements the whistle signals to give visual confirmation of maneuvering status, which is especially useful in situations where whistle signals may be harder to hear.
Frequently Asked Questions
Where do US Inland Rules apply versus International COLREGS?
US Inland Rules apply shoreward of the COLREGS demarcation lines established in 33 CFR Part 80. These lines run across the mouths of harbors, bays, and rivers along the US coastline. Seaward of the demarcation line, International COLREGS (72) apply. When a vessel crosses the demarcation line, the applicable rules switch at that point. The Great Lakes and Western Rivers have their own supplemental provisions within the Inland Rules framework.
What is the biggest difference between Inland and International sound signals?
Under International COLREGS, one short blast means the vessel IS altering course to starboard — it is an information signal and no answering blast is required. Under US Inland Rules, one short blast means the vessel INTENDS to leave the other vessel on its port side — it is a proposal that must be answered with the same signal before the maneuver begins. The Inland Rules use a proposal-and-agreement system. If the other vessel disagrees, she sounds five short blasts (the danger signal). This distinction is the most frequently tested difference on the USCG captain exam.
What is the Rule 34(d) flashing light under Inland Rules?
Inland Rule 34(d) establishes a special flashing light signal for vessels in sight of one another. One short flash (approximately one second) means the vessel is altering course to starboard. Two short flashes mean the vessel is altering course to port. Three short flashes mean the vessel has its engines going astern. This is an Inland-only provision with no direct equivalent in International COLREGS. The flashing light supplements the whistle signals and allows visual confirmation of maneuvering intent.
Does Inland Rules recognize vessels constrained by draft?
No. The vessel constrained by draft category is recognized only under International COLREGS Rule 28. Under COLREGS, a vessel constrained by draft shows three all-round red lights in a vertical line at night and a cylinder day shape. US Inland Rules do not recognize this category at all. On Inland waters, a deep-draft vessel is simply a power-driven vessel with no special right-of-way status. This is a critical and frequently tested distinction on the USCG exam.
What are the passing signals for overtaking in a narrow channel under Inland Rules?
Under Inland Rule 34(c), a vessel wishing to overtake another in a narrow channel must propose the maneuver with a sound signal: one prolonged blast plus one short blast (dot-dash-dot) indicates intent to overtake on the starboard side; one prolonged plus two short blasts indicates intent to overtake on the port side. The vessel ahead must respond with the same signal to agree, or sound five short blasts to disagree. The maneuver shall not proceed until agreement is reached. Under International COLREGS, overtaking signals are simply two prolonged plus one short (starboard) or two prolonged plus two short (port) and no formal agreement is required.
How do anchor light requirements differ between Inland and International rules?
Under both Inland and International rules, a vessel at anchor under 50 meters shows one all-round white light where best seen; a vessel 50 meters or more shows an all-round white light forward and a lower all-round white light aft. A key Inland Rules provision: vessels under 7 meters anchored outside a narrow channel, fairway, or anchorage where other vessels normally navigate are not required to show an anchor light. International COLREGS also exempts vessels under 7 meters in certain anchorages. Both sets of rules require the bell fog signal at anchor (5 seconds of rapid ringing per minute for vessels under 100 meters).
What is the Western Rivers passing signal rule under Inland Rules?
On the Western Rivers (Mississippi River system and certain tributaries), Inland Rules include a specific passing convention: a vessel going downriver has the right-of-way over a vessel going upriver when they meet. Additionally, Western Rivers rules designate specific passing sides at certain bends and crossings. The crossing vessel on Western Rivers always gives way to the vessel proceeding up or down the main channel. These provisions appear in the US Inland Rules but have no equivalent in International COLREGS, which apply only on the high seas.
Drill Inland vs. International until it's automatic
1,628+ practice questions including Rules of the Road questions on every Inland vs. International distinction. Track your weak spots by topic. Do not schedule the exam until you score 95%+ consistently.
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