1.12 Lights & Shapes — General (Rules 20–22)

Opening Part C — Why General Light Rules Matter First

Rules 20–22 open Part C — Lights and Shapes: when to show lights and shapes, definitions of each light type, and how far they must be visible by length. Later rules (23–30) hang specific light pictures on these foundations. On Master 100 Q100 (90%), definition and range numbers are free points — and they unlock every night-geometry stem built on sternlight or sidelight aspect.

Rule 20 — Application

Rule 20 sets timing and weather rules for the entire lights-and-shapes part:

1. All weathers. The lights-and-shapes rules must be complied with in all weathers.
2. Sunset to sunrise. Rules concerning lights shall be complied with from sunset to sunrise. During those hours, no other lights shall be exhibited except lights that cannot be mistaken for the prescribed lights, do not impair their visibility or distinctive character, and do not interfere with a proper lookout.
3. Restricted visibility by day. The prescribed lights shall, if carried, also be exhibited from sunrise to sunset in restricted visibility and may be exhibited in all other circumstances when it is deemed necessary.
4. Shapes by day. Rules concerning shapes shall be complied with by day.
5. Annex I compliance. Lights and shapes must meet Annex I technical standards (placement, intensity, chromaticity).

Master Operational Reading

• At night: required navigation lights on; decorative or deck lights must not wash out sidelights or confuse other vessels.
• In daytime fog: if you carry the lights, show them — do not wait for sunset.
• On a bright day with haze that is not true restricted visibility, you may show lights if needed for safety (for example, hard-to-see fiberglass hull against glare).
• Shapes (ball, diamond, cylinder, cone) are the daytime language for anchor, towing, CBD, sailing under power, etc. Night uses lights; day uses shapes (with some vessels showing both as required).

Rule 20 Timing Table

Rule 21 — Definitions (Arcs and Colors)

These definitions are pure exam currency. They also explain why Rule 13 uses "only the sternlight" as the night overtaking test.

Arc Arithmetic You Must Own

• Masthead 225° + sternlight 135° = 360° around the horizon for white centerline lights (with sidelights providing the colored aspect forward).
• Each sidelight 112.5° = half of the masthead's half-breadth logic: from ahead to 22.5° abaft the beam.
• 22.5° abaft the beam is the shared boundary used in both sidelight/masthead sectors and the Rule 13 overtaking definition. If at night you see only white sternlight, you are in the 135° stern sector → overtaking. If you see a red or green sidelight, you are forward of that boundary.

Flashing (Exam Trap)

Rule 21's flashing light means ≥120 flashes per minute. Ordinary navigation lights are fixed (steady). Do not confuse this Rules definition with ATON buoy rhythms from the Light List.

Rule 22 — Visibility of Lights (Length Bands)

Rule 22 sets minimum ranges so other vessels can see you in time. Ranges increase with vessel length. These numbers are frequently tested verbatim.

(a) Vessels 50 meters or more in length

(b) Vessels 12 meters or more but less than 50 meters

(c) Vessels less than 12 meters

(d) Inconspicuous, partly submerged vessels or objects being towed

Master 100 Length Reality Check

Most Master LT 100 passenger vessels are under 50 meters and many inspected small passenger vessels are under 20 meters. That means your exam-relevant band is often:

• masthead 3 miles (if <20 m) or 5 miles (if 20–50 m),
• sidelights 2 miles,
• sternlight 2 miles.

A ~27 m dinner boat needs a 5-mile masthead; a ~14 m water taxi (<20 m) needs 3 miles. Do not apply the ship-sized 6-mile masthead figure to a sub-50 m passenger vessel.

Master Scenario: Night Departure Checklist

You are Master of a 55-foot (≈17 m) passenger vessel departing at dusk into haze:

1. Rule 20: lights required at sunset and appropriate in restricted visibility — turn them on early.
2. Rule 21: masthead white (225°), red/green sidelights (or combined lantern if lawful), white sternlight (135°); keep deck lights from washing out sidelights.
3. Rule 22: 12–20 m → masthead 3 miles, sidelights 2, sternlight 2.
4. A vessel showing only white sternlight is in the overtaking sector (Rule 13 geometry built on these arcs).

Exam Framing for Q100 (90%)

Highest-yield Rule 20–22 facts: lights sunset to sunrise; lights in daytime restricted visibility; shapes by day; masthead 225° white; sidelights 112.5° red/green; sternlight 135° white; towing light yellow with sternlight characteristics; all-round 360°; flashing ≥120/min; visibility tables 6/3/3 (≥50 m), 5 (or 3 if <20 m)/2/2 (12–50 m), 2/1/2 (<12 m). Master stems often place you on a 12–50 m passenger vessel — answer with the correct band, not the ship-sized 6-mile masthead by default.