7.1 Fundamentals of Navigation Lights & Day Shapes (Rules 20-22)
Key Takeaways
Masthead lights cover an unbroken forward arc of 225° (from right ahead to 22.5° abaft the beam on each side), sidelights cover 112.5° each, and sternlights cover an after arc of 135° (67.5° each side of right astern).
Rule 20 mandates navigation lights from sunset to sunrise and in restricted visibility by day, prohibiting any non-prescribed lights that could impair lookout or cause confusion.
Minimum luminous ranges under Rule 22 scale with vessel size: vessels ≥50m require 6nm masthead and 3nm sidelights/sternlight; vessels 12m to <50m require 5nm masthead (3nm if <20m) and 2nm sidelights/sternlight; vessels <12m require 2nm masthead/sternlight and 1nm sidelights.
An air-cushion vessel in non-displacement mode adds an all-round flashing yellow light. WIG craft add an all-round high-intensity flashing red light when taking off, landing or flying near the surface.
Both sidelights with masthead lights can indicate a head-on encounter. Rule 14 requires both power-driven vessels to turn to starboard when meeting on reciprocal or nearly reciprocal courses with collision risk.
7.1 Fundamentals of Navigation Lights & Day Shapes (Rules 20-22)
Note
The International Regulations for Preventing Collisions at Sea (IRPCS / COLREGs) Part C (Rules 20 through 31) prescribes the lights and day shapes that all vessels must exhibit. For Coastal Skipper and Yachtmaster Offshore candidates, instant recognition of light sectors, ranges, and aspect is essential for assessing collision risk and determining give-way or stand-on obligations long before entering close-quarters situations.
Rule 20: Application of Navigation Lights & Day Shapes
Rule 20 establishes the mandatory operational windows and legal standards governing navigation lights and day shapes:
- Sunset to Sunrise: Prescribed navigation lights must be exhibited from sunset to sunrise in all weather conditions. During this period, no other lights may be displayed that can be mistaken for prescribed lights, impair their visibility or distinctive character, or interfere with keeping a proper lookout. Deck working lights and cabin illumination must be carefully screened or switched off.
- Restricted Visibility by Day: Prescribed navigation lights must also be exhibited from sunrise to sunset in restricted visibility (such as fog, mist, falling snow, or heavy rainstorms). They may also be exhibited in all other circumstances when deemed necessary by the skipper.
- Daylight Hours: Rules concerning day shapes (black balls, cones, cylinders, diamonds) apply by day between sunrise and sunset.
- Annex I Technical Compliance: All lights and shapes must comply with the technical provisions of Annex I, which governs positioning, vertical separation, horizontal spacing, and optical screening.
Rule 21: Definitions and Angular Arcs of Visibility
Every navigation light projects an unbroken beam through an exact geometric arc measured relative to the vessel's fore-and-aft centerline. Rule 21 defines six distinct categories of lights:
1. Masthead Light (Rule 21(a))
A white light placed over the fore-and-aft centerline showing an unbroken light over an arc of the horizon of 225 degrees (20 points of the compass). It projects from right ahead (000° relative) to 22.5 degrees abaft the beam on either side of the vessel (112.5° port and 112.5° starboard). On power-driven vessels of 50 metres or more in length, a second after masthead light is mandatory.
2. Sidelights (Rule 21(b))
A green light on the starboard side and a red light on the port side, each showing an unbroken light over an arc of 112.5 degrees (10 points). They project from right ahead (000° relative) to 22.5 degrees abaft the beam on their respective sides. On vessels under 20 metres in length, sidelights may be combined in a single lantern carried on the fore-and-aft centerline.
3. Sternlight (Rule 21(c))
A white light placed as nearly as practicable at the stern, showing an unbroken light over an arc of 135 degrees (12 points). It projects from right astern to 67.5 degrees on each side of the vessel (from 112.5° relative to 247.5° relative), abutting the sidelight and masthead sectors.
4. Towing Light (Rule 21(d))
A yellow light having the same characteristics as the sternlight: an unbroken arc of 135 degrees (67.5° on each side of right astern). It is mounted vertically above the sternlight on power-driven vessels towing astern.
5. All-Round Light (Rule 21(e))
A light showing an unbroken light over an arc of the horizon of 360 degrees (white, red, green, or yellow).
6. Flashing Light (Rule 21(f))
A light flashing regularly at 120 flashes or more per minute (at least 2 Hz). Rule 23 requires a yellow flasher for air-cushion vessels in non-displacement mode and a high-intensity red flasher for WIG craft taking off, landing or flying near the surface.
Rule 22: Visibility of Lights (Luminous Ranges)
Rule 22 defines minimum nominal ranges in nautical miles (nm) at which lights must be visible on a dark night with a clear atmosphere:
| Vessel Length Category | Masthead Light | Sidelight | Sternlight | Towing Light | All-Round Light |
|---|---|---|---|---|---|
| Vessels ≥ 50 metres | 6 nm | 3 nm | 3 nm | 3 nm | 3 nm |
| Vessels 12m to < 50m | 5 nm (3 nm if < 20m) | 2 nm | 2 nm | 2 nm | 2 nm |
| Vessels < 12 metres | 2 nm | 1 nm | 2 nm | 2 nm | 2 nm |
| Inconspicuous, partly submerged tow | — | — | — | — | 3 nm (white) |
Important
Power-driven vessels under 20 metres may reduce their forward masthead light range from 5 nm down to 3 nm. Vessels under 12 metres require only 1 nm visibility for sidelights and 2 nm for all other prescribed lights.
Practical Skippering: Aspect & Collision Risk at Night
At night, without visual cues of a vessel's hull or superstructure, the skipper assesses aspect and relative motion from lights, using all other available means appropriate to establish collision risk.
1. Determining Aspect
Aspect is the orientation of an approaching vessel relative to your line of sight:
- Red Sidelight Only: You see the vessel's port side. If both vessels are power-driven and crossing with collision risk, give way when she is on your starboard side (Rule 15).
- Green Sidelight Only: You see her starboard side. Colour alone does not establish stand-on status; confirm vessel categories, relative position and motion.
- Both Sidelights with Masthead Light(s): This can indicate a head-on encounter. If two power-driven vessels meet on reciprocal or nearly reciprocal courses with collision risk, both turn to starboard (Rule 14).
- Single White Light Only: This may be a sternlight, anchor light or small craft's all-round light. Rule 13 applies when coming up with a vessel from more than 22.5° abaft her beam; identify the encounter before assigning duties.
2. Assessing Collision Risk & Watchkeeping
Rule 7 dictates that risk of collision exists if the compass bearing of an approaching vessel does not appreciably change while range decreases (CBDR). Skippers must take successive magnetic compass bearings using a hand-bearing compass. Relying on visual estimation alone risks optical illusions from vessel motion. Annex I requires fitted inboard matt black screens to prevent sidelight beam overlap across the bow, though sea rolling may cause boundary lights to flicker intermittently.
Under Rule 21(a) and (b), what are the exact angular arcs of visibility for a masthead light and an individual sidelight?
Masthead light 180°; sidelight 90°
Masthead light 225°; sidelight 112.5°
Masthead light 270°; sidelight 135°
Masthead light 135°; sidelight 112.5°
What are the minimum statutory visibility ranges required under Rule 22 for a 15-metre sailing yacht's sidelights and sternlight?
Sidelights 1 nautical mile; sternlight 1 nautical mile
Sidelights 3 nautical miles; sternlight 3 nautical miles
Sidelights 1 nautical mile; sternlight 2 nautical miles
Sidelights 2 nautical miles; sternlight 2 nautical miles
Your power-driven yacht is steering 090°T at night in clear visibility. Another confirmed power-driven vessel shows a white masthead light and green sidelight on your port bow, bearing steadily 045°T with decreasing range. In this crossing situation, what is your initial duty?
Keep course and speed as the stand-on vessel under Rule 15, while monitoring the encounter and remaining ready to act under Rule 17
You are looking at the port side of a vessel crossing from your starboard side; you are the give-way vessel
You are overtaking another vessel within her 135° sternlight sector under Rule 13
You are encountering a power-driven vessel head-on and must immediately alter course to starboard under Rule 14
Sections you finish are checked off in the contents.