COLREGs Navigation Lights & Shapes: Practical Guide to Rules 20–30
In maritime navigation, visual identification by lights and shapes is the primary safeguard against collision during darkness and restricted visibility. Because a vessel's hull outline cannot be distinguished at night, safe collision avoidance under the international framework of the STCW Convention relies strictly on instantaneous recognition of light combinations, relative bearings, and aspect angles. Use the interactive 3D simulator above to inspect these physical optical arcs in real time.
1. Optical Arcs of Visibility (COLREGs Rule 21)
Rule 21 defines the geometric characteristics and statutory cutoff sectors for all lights displayed by vessels on international waters:
| Light Designation | Color | Horizontal Arc of Visibility | Angular Span | Mounting Position on Vessel |
|---|---|---|---|---|
| Masthead Light | White | From right ahead to 22.5° abaft the beam on either side | 225° (20 points) | Over the fore-and-aft centerline. Ships ≥ 50 m carry two (after masthead higher than foremast). |
| Starboard Sidelight | Green | From right ahead to 22.5° abaft the starboard beam | 112.5° (10 points) | Starboard bridge wing fitted with inboard screens to prevent crossing over the bow. |
| Port Sidelight | Red | From right ahead to 22.5° abaft the port beam | 112.5° (10 points) | Port bridge wing fitted with inboard screens to prevent crossing over the bow. |
| Sternlight | White | From right astern over 67.5° to each side of the vessel | 135° (12 points) | As near the stern as practicable. |
| Towing Light | Yellow | Identical to the sternlight: 67.5° to each side of the vessel | 135° (12 points) | Mounted in a vertical line directly above the white sternlight on a towing vessel. |
| All-Round Light | White, Red, Green, Yellow | Unbroken light over an uninterrupted arc of the horizon | 360° (32 points) | Positioned on masts, stays, or yards where it cannot be obscured by rigging. |
The combined arcs of port and starboard sidelights (112.5° + 112.5° = 225°) exactly equal the forward sweep of the masthead light (225°). Adding the 135° sternlight arc completes the full 360° horizon. This geometric harmony ensures an observer sees distinct, non-overlapping light combinations from every relative bearing.
2. Minimum Range of Visibility by Vessel Length (COLREGs Rule 22)
Navigation lights must provide sufficient luminous intensity under clear dark atmospheric conditions to satisfy statutory visibility distances:
| Vessel Length Category | Masthead Light | Sidelights | Sternlight | Towing Light | All-Round Lights |
|---|---|---|---|---|---|
| Vessels of 50 m or more in length | 6 nautical miles | 3 nautical miles | 3 nautical miles | 3 nautical miles | 3 nautical miles |
| Vessels of 12 m to under 50 m | 5 nautical miles (< 20 m: 3 nm) | 2 nautical miles | 2 nautical miles | 2 nautical miles | 2 nautical miles |
| Vessels of less than 12 m in length | 2 nautical miles | 1 nautical mile | 2 nautical miles | 2 nautical miles | 2 nautical miles |
| Inconspicuous towed objects | — | — | — | — | 3 nautical miles (white all-round) |
3. Statutory Lights by Vessel Category and Operational Status
• Vessels ≥ 50 meters in length: Forward masthead light, second (after) masthead light placed higher and further aft, port and starboard sidelights, and a sternlight.
• Vessels < 50 meters in length: Required to carry a single forward masthead light (second masthead light is optional), sidelights, and a sternlight.
• Vessels < 12 meters: May exhibit an all-round white light and sidelights in lieu of separate masthead and sternlights.
• Vessels < 7 meters (speed ≤ 7 knots): May exhibit an all-round white light and, if practicable, sidelights.
• Towing length ≤ 200 meters: Towing vessel displays two masthead lights in a vertical line, sidelights, a white sternlight, and a yellow towing light placed vertically above the sternlight.
• Towing length > 200 meters: Towing vessel displays three masthead lights in a vertical line (measured from the towing vessel's stern to the aft end of the tow). By day, a black diamond shape is exhibited.
• Towed vessel or barge: Exhibits sidelights and a sternlight only. Masthead lights must never be displayed on a towed object.
• A sailing vessel underway carries port and starboard sidelights and a sternlight. White forward masthead lights are strictly prohibited on a vessel operating solely under sail.
• Optional masthead combination: Two all-round lights in a vertical line at or near the top of the mast — Red over Green ("Red over Green, sailing queen").
• If a sailing vessel operates mechanical propulsion while sails are hoisted, it is legally deemed a power-driven vessel and must exhibit a black conical shape (apex pointing downwards) by day.
• Trawling (dragging dredge or net): Two all-round lights in a vertical line — Green over White ("Green over White, trawling at night"). Vessels ≥ 50 m must also exhibit an after masthead light abaft and above the green light.
• Fishing other than trawling: Two all-round lights in a vertical line — Red over White ("Red over White, fishing in sight"). If outlying gear extends > 150 m horizontally, an all-round white light is shown in the direction of the gear.
• When making way through the water, both classes must additionally exhibit sidelights and a sternlight.
• Not Under Command (NUC): Two all-round Red lights in a vertical line (Day shape: two black balls).
• Restricted in Ability to Manoeuvre (RAM): Three all-round lights in a vertical line — Red-White-Red (Day shape: ball-diamond-ball).
• Constrained by Draught (CBD): Power-driven vessel lights plus three all-round Red lights in a vertical line (Day shape: cylinder).
• Vessels at Anchor: Forward white all-round light (higher) and after white all-round light (lower). Vessels ≥ 100 m must illuminate decks with working lights. (Day shape: one black ball forward).
• Vessels Aground: Anchor lights plus two all-round Red lights in a vertical line. (Day shape: three black balls in a vertical line).
A vessel is legally "underway" whenever it is not at anchor, made fast to shore, or aground. It may be drifting with dead engines (underway, making no way). For vessels with special status (NUC, RAM, Pilot, Fishing), sidelights and sternlights must be extinguished when stopped; they are switched on only when the vessel is actively making headway or sternway through the water.
4. Determining Target Aspect and Action to Avoid Collision
Recognizing a target vessel's relative heading (aspect) dictates operational maneuvering requirements under Part B of the COLREGs:
- Masthead Lights in Transit: On vessels ≥ 50 meters, the relative horizontal displacement of the masthead lights reveals target aspect. If the higher after masthead light is seen to the right of the forward light, the ship is heading to the starboard side of the observer. If both masthead lights are aligned vertically in transit and both sidelights are visible, the vessels are in a Head-on Situation (Rule 14): both vessels must alter course to starboard!
- Crossing Situations (Rule 15): If a green sidelight is sighted on your port bow, or a red sidelight is sighted on your starboard bow, risk of collision exists. Sighting a red light on your starboard bow means you are the Give-way vessel and must take early, substantial action (typically an alteration of course to starboard) to keep clear.
- Overtaking Scenarios (Rule 13): When approaching another vessel from a direction more than 22.5° abaft its beam (where only its white sternlight is visible), you are an Overtaking vessel and must keep clear until completely past and clear.