Master Inland Rules vs. COLREGS differences, whistle signals, overtaking signals, Western Rivers rules, and Great Lakes provisions — all high-frequency topics on the OUPV exam.
33 CFR 83
The U.S. Inland Navigation Rules statutory citation
Demarcation Lines
33 CFR Part 80 — the boundary between Inland and COLREGS
Agreement Required
Inland whistle signals need reply before maneuvering — COLREGS does not
Key exam concept: COLREGS Demarcation Lines
The exact boundary is the COLREGS Demarcation Line published in 33 CFR Part 80. A vessel inside the line follows Inland Rules; outside the line follows COLREGS. The line generally follows the outer edge of harbors, jetties, and breakwaters. On the exam, assume any scenario set in a U.S. river, harbor, or bay is governed by Inland Rules unless specifically stated otherwise.
These differences are the most heavily tested material on the Rules of the Road section of the OUPV exam. Know each one cold.
Inside COLREGS Demarcation Lines — U.S. harbors, rivers, lakes, inland waters
On the high seas and waters connected to the high seas, outside the demarcation lines
Signals signal INTENT — must be agreed before maneuvering. 1 short = 'I intend port-to-port'; 2 short = 'I intend starboard-to-starboard'
Signals signal ACTION — no agreement required. 1 short = 'I am altering to starboard'; 2 short = 'I am altering to port'
1 prolonged + 1 short = overtake on your starboard side; 1 prolonged + 2 short = overtake on your port side. Agreement required.
2 long + 1 short = overtake on starboard; 2 long + 2 short = overtake on port. No agreement reply required.
Every whistle maneuvering signal must be accompanied by a flashing light of the same duration, visible 5+ miles
No flashing light requirement for maneuvering signals
Not recognized — CBD vessels operate as ordinary power-driven vessels under Inland Rules
CBD is a recognized status under Rule 28: three all-around red lights (night) or cylinder shape (day)
Designated rivers (Mississippi system): vessel crossing a river always gives way to downstream power vessel
No equivalent — COLREGS does not address river-specific crossing rules
Specific provisions for Great Lakes: fog signal differences, whistle requirements for vessels approaching harbor entrances
No Great Lakes-specific provisions — international rules apply on international waters only
1 prolonged blast when leaving a berth or dock; vessels in the channel must give way
Same — 1 prolonged blast leaving a berth (Rule 34(g))
This is where most exam errors happen — the same blast pattern carries different meanings depending on which rule set applies.
| Signal | Inland Rules Meaning | COLREGS Meaning |
|---|---|---|
| 1 short blast | I intend to leave you on my port side (passing port-to-port — each vessel alters to starboard) | I am altering my course to starboard |
| 2 short blasts | I intend to leave you on my starboard side (passing starboard-to-starboard — each vessel alters to port) | I am altering my course to port |
| 3 short blasts | I am operating astern propulsion | I am operating astern propulsion |
| 5+ short blasts | Danger or doubt signal — I do not understand your intentions / I disagree | Danger or doubt signal — same meaning |
| 1 prolonged + 1 short | I intend to overtake you on your STARBOARD side (agreement required) | N/A as overtaking signal |
| 1 prolonged + 2 short | I intend to overtake you on your PORT side (agreement required) | Vessel being towed in restricted visibility (fog signal) |
| 2 long + 1 short | N/A | Overtaking on starboard side |
| 2 long + 2 short | N/A | Overtaking on port side |
| 1 prolonged blast | Leaving a berth or dock; approaching blind bend | Power vessel underway in restricted visibility (fog signal); leaving berth |
Most navigation light requirements are identical between Inland Rules and COLREGS. The critical difference is that Constrained by Draft (CBD) status — and its three vertical red lights — does not exist in inland waters.
| Vessel Type | Light Configuration |
|---|---|
| Power-driven vessel underway (under 50m) | Masthead light (forward), sidelights (red/green), stern light (white) |
| Power-driven vessel underway (50m+) | Two masthead lights (forward lower, aft higher), sidelights, stern light |
| Vessel at anchor (under 50m) | One all-around white light forward |
| Vessel at anchor (50m+) | All-around white light forward + all-around white light aft (lower) |
| Not Under Command (NUC) | Two all-around red lights vertically + sidelights/stern if making way |
| Towing (tow ≤200m composite length) | Two masthead lights (vertical) + sidelights + towing light (yellow, 135°) + no stern light |
| Towing (tow >200m composite length) | Three masthead lights (vertical) + sidelights + towing light |
| Constrained by Draft (CBD) | NOT APPLICABLE in inland waters — CBD not recognized |
Western Rivers rules apply on the Mississippi River system and other rivers specifically designated in 33 CFR Part 89. They supplement the main Inland Rules with river-specific provisions.
On Western Rivers, any vessel crossing the river must keep out of the way of a power-driven vessel proceeding downstream (descending). This applies regardless of which side the crossing vessel is on — even if the crossing vessel has the descending vessel on its port side (which would normally make the crossing vessel the stand-on vessel under general crossing rules).
In plain terms:
Crossing the river = you are always give-way to any downstream power vessel. No exceptions.
The Great Lakes and their connecting waters fall under Inland Rules, with some additional provisions specific to Great Lakes navigation.
Inland Rule 6 (identical in language to COLREGS Rule 6) requires every vessel to proceed at a safe speed at all times, so it can take proper and effective action to avoid collision and stop within a distance appropriate to prevailing circumstances.
For all vessels:
For vessels with radar:
Exam tip: no numerical speed limit
Safe speed is never a fixed number under the rules. If a question gives a specific speed (e.g., 8 knots) and asks whether it is safe, you must evaluate it against conditions — visibility, stopping distance, traffic — not against a speed limit. The correct answer will always depend on context.
Inland Rules questions appear frequently on the USCG OUPV exam. The most-tested categories are whistle signals, overtaking procedure, Western Rivers priority, and the CBD trap.
Inland 1 short blast ≠ altering to starboard
Inland 1 short blast means 'I INTEND to pass port-to-port' — it is a proposal, not an action. COLREGS 1 short blast means 'I AM altering to starboard' — an action already underway. Students who study COLREGS first frequently get this wrong.
CBD lights do not appear in inland waters
Three all-around red vertical lights are CBD lights under COLREGS. If you see a question about inland waters and three red lights — it's a trick. CBD status does not exist under Inland Rules.
Western Rivers: crossing always gives way to downstream
On Western Rivers (Mississippi system), the vessel crossing the river gives way to any power-driven vessel proceeding downstream. Bearing doesn't matter. If you are crossing, you are give-way to the downstream vessel — period.
Agreement required before maneuvering inland
Under Inland Rules, a vessel must receive an agreement reply before it maneuvers. Under COLREGS, no agreement is needed — you signal and maneuver. This agreement requirement is what makes the danger signal (5 blasts) mandatory when the other vessel does not respond or disagrees.
1 prolonged + 2 short means different things
In inland waters, 1 prolonged + 2 short = 'I intend to overtake you on your PORT side.' Under COLREGS fog signals, 1 prolonged + 2 short = towing vessel in restricted visibility. Same sound, completely different meaning based on context (in-sight vs. fog).
Flashing light accompanies every inland whistle signal
Every maneuvering whistle signal under Inland Rules must be accompanied by a flashing light of the same duration and number, visible at least 5 miles. International COLREGS does not require this. It is commonly tested on the exam.
COLREGS Demarcation Lines define the boundary
The line between Inland Rules and COLREGS is the COLREGS Demarcation Line, published in 33 CFR Part 80. Generally: inside harbors and rivers = inland; outside breakwaters and in open ocean = COLREGS. Learn which rule set applies by knowing which side of the demarcation line you are on.
Great Lakes: vessels meeting in a fairway
Under Great Lakes Pilot Rules, vessels approaching a harbor entrance from different directions have specific whistle requirements. Power vessels must sound whistle signals when approaching bends in rivers. The Great Lakes rules are supplementary to the main Inland Rules — they add specificity, not contradiction.
Safe speed has no fixed number — it is situational
Rule 6 safe speed is frequently tested with scenario questions. 'A vessel traveling at 12 knots in thick fog — is this safe speed?' The answer depends on the vessel's stopping distance, radar range, traffic, etc. — not the number alone. Always evaluate safe speed against stopping ability and conditions.
The U.S. Inland Navigation Rules apply on all inland waters of the United States, including rivers, harbors, and lakes that lie inside the COLREGS Demarcation Lines. These lines are published by the U.S. Coast Guard in 33 CFR Part 80. West of the demarcation lines (on the high seas and in international waters) COLREGS applies. East of the lines, Inland Rules apply. Specific rule sets also apply on Western Rivers and the Great Lakes as designated.
The key difference is intent vs. action and the requirement for agreement. Under Inland Rules, a whistle signal signals intent and must be answered with an agreement signal before the vessel maneuvers. Under International COLREGS, the signal announces an action already taken or being taken — no agreement reply is required. For example, inland 1 short blast means 'I intend to leave you on my port side (passing port-to-port)' and the other vessel must reply with 1 short blast before either alters course. Under COLREGS, 1 short blast means 'I am altering my course to starboard' — it is reporting action, not requesting agreement.
Under Inland Rules: 1 short blast = 'I intend to leave you on my port side' (passing port-to-port — each vessel alters to starboard). 2 short blasts = 'I intend to leave you on my starboard side' (passing starboard-to-starboard — each vessel alters to port). The other vessel must respond with the same signal to confirm agreement. If the other vessel disagrees, 5 or more short blasts (the danger signal) must be sounded.
Under Inland Rules, an overtaking vessel must sound 1 prolonged + 1 short blast to indicate intent to overtake on the overtaken vessel's starboard side, or 1 prolonged + 2 short blasts to overtake on the port side. The vessel being overtaken must reply with the same signal to agree, or sound 5 short blasts if it disagrees. Under International COLREGS the signals are different: 2 long + 1 short (starboard overtake) or 2 long + 2 short (port overtake), and no agreement reply is required.
Western Rivers rules apply on the Mississippi River system and other designated rivers in 33 CFR Part 89. The key rule: on Western Rivers, a vessel crossing a river must keep out of the way of a power-driven vessel descending (going downstream) regardless of which side the crossing vessel is on. The crossing vessel is always the give-way vessel to a downstream vessel. This is different from the general inland crossing rule where the vessel with the other on its starboard side gives way. Western Rivers rules also have specific provisions for passing in narrow channels.
No. 'Constrained by Draft' (CBD) is recognized only under International COLREGS Rule 28. Under U.S. Inland Rules, there is no equivalent status. A vessel that would claim CBD status on the high seas must operate as a normal power-driven vessel under Inland Rules. This is a common exam trap — students who learn CBD lights and shapes must remember they do not apply in inland waters.
Five or more short blasts of the whistle is the danger (or doubt) signal under both Inland Rules and International COLREGS. Under Inland Rules, it is specifically used when: (1) a vessel does not understand the other vessel's signal or intentions, or (2) a vessel disagrees with a proposed passing arrangement after an agreement signal has been exchanged. When the danger signal is heard, both vessels must slow down, stop, or reverse as necessary until the situation is clarified and a safe passing agreement is reached.
Inland Rule 6 mirrors COLREGS Rule 6 — every vessel must proceed at a safe speed at all times so that it can take proper and effective action to avoid collision and be stopped within a distance appropriate to the prevailing circumstances. Factors include visibility, traffic density, maneuverability of the vessel, state of wind and sea, proximity of navigational hazards, and whether radar is in use. There is no specific numerical speed limit — safe speed is determined by conditions. In narrow channels, rivers, and congested harbors, a safe speed is typically lower than open water.
Complete Rules of the Road reference: right-of-way hierarchy, sound signals, lights, and Inland vs. International differences
Maneuvering signals, fog signals by vessel type, distress signals — Rules 32–37 complete reference
Navigation light arcs, vessel-type light requirements, day shapes — Rules 20–31 complete reference
Part B steering and sailing rules, right-of-way hierarchy, restricted visibility procedures
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