Red and green are the standard sidelights on a boat: red marks the vessel’s port (left) side and green marks its starboard (right) side. A flashing blue light has a different purpose under the U.S. Inland Navigation Rules: it identifies a law-enforcement vessel engaged in direct law-enforcement or public-safety activity. Blue is an identification signal, not a navigation light that gives the vessel automatic right of way.
To decide what an approaching boat is doing, never rely on color alone. Check whether the light is steady or flashing, which part of the vessel’s light arc you can see, any accompanying white masthead or stern light, the vessel’s movement, and whether U.S. Inland Rules or the 72 COLREGS apply where you are operating.
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What red, green and blue mean at a glance
| Color | Typical pattern | What it identifies | Mounting and visible arc | Where the cited rule applies |
|---|---|---|---|---|
| Red | Steady sidelights | The vessel’s port (left) side | Port side; each sidelight shows an unbroken 112.5-degree arc forward and toward its side | Standard navigation-light system under the applicable navigation rules |
| Green | Steady sidelight | The vessel’s starboard (right) side | Starboard side; each sidelight shows an unbroken 112.5-degree arc forward and toward its side | Standard navigation-light system under the applicable navigation rules |
| Flashing blue | Flashing | A qualifying U.S. law-enforcement or public-safety vessel | The cited Inland rule does not prescribe a universal viewing arc or recreational-boat mounting position; installation and local requirements control | U.S. Inland Navigation Rules, including 33 CFR 88.05(a), with possible additional local requirements |
The U.S. Coast Guard defines a sidelight as “a green light on the starboard side and a red light on the port side each showing an unbroken light over an arc of the horizon of 112.5 degrees.” Those lights describe which side or aspect of the vessel is visible; they do not, by themselves, tell you which vessel must maneuver.
Red means port (left)
When you see a red sidelight on another vessel, you are seeing that vessel’s port side. Port is the nautical term for the vessel’s left side when a person faces forward toward the bow. The red lamp is fitted on that side and shines through the prescribed forward-facing arc.
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“Left” and “right” can be confusing when two boats face different directions. Port and starboard are fixed to each vessel, so use those terms rather than trying to translate them into your own left and right. A red light does not necessarily mean the other boat is turning left, entering a channel, or giving way. It only identifies the side of the vessel that is presenting its sidelight to you.
What a red light can look like in an encounter
- Red and green together: You are generally looking toward the bow, because both forward sidelight arcs overlap in your direction. Determine whether the situation is head-on or crossing by checking bearing, movement and any white masthead light.
- Red without green: You are seeing the vessel’s port-side aspect, but the exact encounter still depends on relative course and the white lights visible with it.
- No red or green, only a white light from behind: You may be looking at the vessel’s stern rather than its bow or side. Confirm the applicable rule for an overtaking situation before maneuvering.
Green means starboard (right)
A green sidelight marks the vessel’s starboard side. Starboard is the right side when facing the bow. Like the red sidelight, the green light is steady and covers a 112.5-degree horizontal arc under the U.S. Coast Guard definition.
Green is not a universal “safe to pass” or “stand-on” indication. It tells you which side of the other vessel is visible. The steering-and-sailing rules, not the color by itself, determine whether you must keep clear, maintain course, or take early and substantial action.
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Using green with the vessel’s white lights
Green becomes useful when combined with white lights. A white masthead light can show that you are viewing the forward part of a power-driven vessel, while a white stern light indicates the rear aspect. The number, placement and visibility of white lights depend on the vessel and the governing rule set. Treat the color as one input in an aspect-and-motion assessment, not as a complete maneuvering instruction.
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What a flashing blue boat light means
Under the U.S. Inland Navigation Rules, “Law enforcement vessels may display a flashing blue light when engaged in direct law enforcement or public safety activities.” The cited provision is 33 CFR 88.05(a). The blue light identifies the vessel’s role while it is carrying out that qualifying activity.
The same rule makes an important limitation clear: the blue light is an identification signal and does not create a special privilege or right of way. You must still comply with the applicable steering-and-sailing rules, maintain a proper lookout, proceed at a safe speed and avoid collision. If a law-enforcement boat needs you to maneuver, follow a lawful instruction from its crew; do not assume that the blue lamp alone changes the collision rules.
Does flashing blue always mean “police”?
In the context of the cited U.S. Inland rule, it indicates a law-enforcement vessel engaged in direct law-enforcement or public-safety activity. That is narrower than “any boat operated by police” and does not automatically describe every blue light used in every country or waterway. States, municipalities, port authorities and other jurisdictions can impose additional requirements, and local rules may govern the color, flash pattern, mounting or permitted users.
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How red, green and blue differ
| Question | Red sidelight | Green sidelight | Flashing blue light |
|---|---|---|---|
| Does it identify a side of the vessel? | Yes: port | Yes: starboard | No |
| Is it normally steady or flashing? | Steady | Steady | Flashing under the cited U.S. provision |
| Does it have the 112.5-degree sidelight arc? | Yes | Yes | Not as a sidelight; no universal arc is specified by the cited provision |
| Does it identify a vessel’s activity? | No | No | Yes, a qualifying law-enforcement or public-safety activity in U.S. Inland waters |
| Does it grant right of way? | No | No | No |
| Should a recreational operator install it as an ordinary navigation light? | Only as a compliant port sidelight | Only as a compliant starboard sidelight | No, unless specifically authorized for the vessel and activity |
When navigation lights must be shown
U.S. Coast Guard Rule 20 states that the prescribed lights apply in all weathers from sunset to sunrise. The required lights must also be displayed from sunrise to sunset when visibility is restricted, such as in fog, heavy rain or other conditions that prevent you from seeing vessels clearly.
The rule is about visibility and collision prevention, not merely darkness. A clear afternoon does not require you to switch on every night light solely because a clock has reached a particular time, but restricted visibility can require the prescribed lights during daylight. Follow the exact equipment and display requirements for your vessel type and operating area.
Inland Rules versus the 72 COLREGS
The governing regime depends on where you are operating. U.S. waters use either the U.S. Inland Navigation Rules or the international 72 COLREGS according to the Coast Guard’s demarcation framework. The two systems are closely related but are not interchangeable in every detail. Check the demarcation lines, local notices and the rules carried aboard your vessel before relying on a light interpretation.
Outside U.S. waters, the 72 COLREGS and the implementing law of the coastal state control. A blue-light practice that is valid in one jurisdiction may not have the same meaning elsewhere. If you cross a boundary, learn the local requirements rather than carrying over an assumption from home waters.
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How to read an approaching boat step by step
- Check whether the light is steady or flashing. Steady red and green are standard sidelights. A flashing blue light is a different identification signal under the cited U.S. rule. Do not classify a rapidly blinking or irregular light by color alone; it may be an approved special-purpose light, a work light or a malfunction.
- Identify the color and side. Red means the other vessel’s port side; green means its starboard side. Keep the vessel’s orientation in mind, not your own left and right.
- Look for the visible arc. Sidelights cover the forward 112.5-degree arcs on their respective sides. Seeing one color, both colors or neither can indicate whether you are viewing a bow, side or stern aspect.
- Find accompanying white lights. A masthead light, stern light or other prescribed white light helps establish whether the vessel is power-driven, sailing, anchored, towing or engaged in another operation. The required combination varies with vessel category and activity.
- Assess movement and bearing. Watch whether the bearing stays nearly constant while range closes. A constant bearing with decreasing distance is a collision-risk warning even if the color seems familiar.
- Apply the correct rule set. Use the Inland Rules or 72 COLREGS that govern your location, then apply the relevant head-on, crossing, overtaking, restricted-visibility and special-activity provisions.
- Take clear, early action when required. Maneuver in a way that is substantial enough to be recognized by the other vessel, while maintaining a safe speed and monitoring the result. A blue identification light does not remove your duty to avoid collision.
Common encounter patterns
Both red and green are visible ahead
Both sidelights generally mean you are seeing toward the vessel’s bow. If the vessel is a power-driven boat and the courses are reciprocal or nearly reciprocal, the situation may be a head-on encounter. Do not decide from color alone: verify motion, relative bearing and any masthead light, then apply the head-on rule that governs your waters.
Only a green light is visible
You are seeing the other vessel’s starboard side. That can occur in a crossing situation, during a turn or when the vessel’s course places its starboard arc toward you. Determine which vessel is the give-way vessel under the applicable rule; green does not authorize you to pass automatically.
Only a red light is visible
You are seeing the other vessel’s port side. As with green, this is an aspect clue rather than a maneuvering command. Keep watching for course changes and white lights, and use the correct crossing or overtaking analysis.
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A white light appears without a sidelight
A white light alone can represent a stern aspect, an anchored vessel or another prescribed configuration, depending on the vessel and activity. Identify the complete light combination and the vessel’s movement before choosing a course.
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A flashing blue light approaches
Treat it as a possible U.S. law-enforcement or public-safety vessel engaged in a qualifying activity. Keep clear of the vessel and any scene it is managing, obey direct lawful instructions, and continue applying collision-avoidance duties. Do not attempt to “claim” right of way because the blue light is flashing, and do not imitate the signal.
Why the 112.5-degree arc matters
The sidelight arc is designed to reveal a vessel’s forward side while reducing ambiguity about whether you are looking at its bow, beam or stern. The red and green lamps are not intended to illuminate the entire horizon. Their defined sectors work with the vessel’s white lights to create a recognizable pattern at night and in restricted visibility.
Because the arc is horizontal, the apparent color can change quickly as a vessel turns or as your relative bearing moves across the sector boundary. A momentary glimpse of red or green is not enough to establish the encounter. Observe the light over time, compare it with the white lights and use radar, AIS where fitted and other available information without abandoning a visual lookout.
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The International Maritime Organization identifies Rule 22 of the COLREGs as the rule for minimum visibility ranges. The exact range is not one universal number: it varies with vessel category and the prescribed light. Rule 25 addresses sailing vessels underway and establishes their relevant light arrangements. Consult the complete rules for the vessel type rather than applying a range remembered from another class of boat.
Light performance also depends on correct installation, unobstructed lenses, electrical supply, darkness, weather and the observer’s height and distance. A compliant lamp that is dirty, corroded, aimed incorrectly or hidden by canvas can be functionally unsafe. Test lights before departure and replace damaged lenses or bulbs with equipment approved for the vessel and jurisdiction.
Fitting and checking red and green sidelights
Port and starboard placement
Install the red sidelight on the port side and the green sidelight on the starboard side, in positions that produce the prescribed forward sectors without being blocked by rails, antennas, cabins, cargo or deck gear. Follow the vessel’s certification, manufacturer instructions and local inspection requirements; color alone does not make a fitting compliant.
Quick Recap
Pre-departure inspection
- Switch on each sidelight separately and confirm red is on port and green is on starboard.
- Check that each lamp is steady, bright and free from cracks, salt film, condensation and paint overspray.
- Inspect wiring, fuses, switches and battery voltage; intermittent lights can be mistaken for a flash signal.
- View the lights from ahead and from each side to make sure the intended sectors are not masked.
- Verify that masthead, stern and other required lights operate with the sidelights as specified for your vessel.
- Carry the applicable navigation rules and know which demarcation regime covers your route.
Blue-light cautions for recreational boaters
- Do not use blue as a substitute for navigation lights. A blue lamp cannot replace compliant red, green, white or yellow lights required for your vessel.
- Do not infer an exemption. The U.S. provision identifies qualifying activity but does not erase speed, lookout or collision-avoidance duties.
- Expect local variations. Harbor, state and municipal authorities may add requirements for emergency, patrol, pilot or public-safety craft.
- Keep distance from an active scene. A flashing blue light may mark an enforcement stop, rescue, accident response or restricted area. Slow down safely and follow instructions rather than passing close aboard.
- Report uncertainty through normal channels. If a light configuration appears unsafe or an unmarked boat is impersonating an authority vessel, avoid confrontation and notify the appropriate maritime authority when it is safe to do so.
Fast decision checklist for the helm
- Am I seeing a steady red or green sidelight, or a flashing blue identification light?
- If it is red or green, which side of the other vessel does that color represent?
- Can I see both sidelights, one sidelight, a stern light or a masthead light?
- Is the bearing changing, or is the range closing on a nearly constant bearing?
- Are visibility, weather or darkness triggering the navigation-light requirements?
- Am I in waters governed by the U.S. Inland Rules or by the 72 COLREGS?
- Which steering-and-sailing rule applies, and what early, obvious action keeps us clear?
- If blue is flashing, have I avoided assuming right of way, kept clear of the operation and followed lawful instructions?
Key takeaways
- Red is the port (left) sidelight; green is the starboard (right) sidelight.
- Each standard sidelight shows an unbroken 112.5-degree horizontal arc under the U.S. Coast Guard definition.
- A flashing blue light identifies a qualifying U.S. law-enforcement or public-safety vessel under 33 CFR 88.05(a).
- Blue is not an ordinary recreational navigation light and does not grant right of way.
- Navigation lights are required from sunset to sunrise in all weather and during restricted visibility in daylight, subject to the applicable rule set.
- Color must be interpreted with flash pattern, visible arc, white lights, vessel activity, movement and the Inland or international rules that govern your location.
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