Instructor Modules / Navigation Lights
IYT Module 23 · DOK Complete
Instructor & student module

Navigation Lights

At night, a vessel's lights are all you have to identify her — they show her size, propulsion and aspect before you can see anything else. This module covers IYT Module 23 at Complete depth: when lights are required, each light and its arc, the exact configurations for powerboats (Rule 23), sailboats and human-powered craft (Rule 25) and vessels at anchor (Rule 30) — capped with a night-identification trainer on a dark screen, the way it actually looks.

Sunset to sunriseMasthead 225°Sidelights 112.5°Sternlight 135° Towing lightRule 23Rule 25Rule 30Night identification
Why it matters afloat: the arcs aren't trivia — they're geometry that answers the only question that matters at night: which way is she going, and am I in her path? Pair this module with the give-way logic in Module 22 and the two become one skill.
Lesson 1

When Lights Are Required

Navigation lights are mandatory between sunset and sunrise and in any period of restricted visibility — fog, heavy rain, snow. Even a craft with no fixed lights at all — a kayak, a rowboat — must carry a watertight flashlight or lantern showing a white light.

The rules of the road themselves don't change after dark. What changes is your information: at night you determine another vessel's speed, position and size from her lights alone. Three factors decide what a vessel must show:

1 · Size
The length breakpoints: 7 m, 12 m, 20 m, 50 m
2 · Power or sail
Machinery on = power lights, whatever the sails are doing
3 · Underway or at anchor
Different suites entirely — Rule 23/25 vs Rule 30
Lesson 2 · Interactive

The Six Lights & Their Arcs

First, two words the arcs use. The beam is the sideways direction — straight out to the side of the boat, at 90° to the way she points. Forward of the beam means anywhere ahead of that line; abaft the beam means anywhere behind it, toward the stern. Each light shines through a fixed slice of the circle around the boat, so which lights you can see tells you which way she is facing relative to you (her aspect). Every configuration is built from these six lights — click each one and study its arc:

The arithmetic that makes it work: masthead 225° = the two sidelights' 112.5° + 112.5° combined; add the sternlight's 135° and you close the full 360°. See a sidelight and you're forward of her beam; see only white astern and you're the overtaking vessel.
Lesson 3

Powerboats Underway — Rule 23

Under 12 m — two options
  • Masthead light + sidelights + sternlight, or
  • One all-round white light + sidelights
Many small boats carry the all-round white on a stern pole — it must sit above the boat's structure so it truly shows all round, acting as combined masthead + sternlight.
12 m to under 50 m
  • Masthead light (forward)
  • Sidelights
  • Sternlight
  • A second masthead light is optional
50 m and over
  • All of the above, plus the second masthead light becomes required — abaft of and higher than the forward one
Two white steaming lights = big ship; the lower one is the bow end. Read her heading from which is left or right.
Lesson 4

Sailboats & Human-Powered Craft — Rule 25

Sail, under 7 m — three options
  1. Sidelights + sternlight
  2. One combined lantern (sidelights + sternlight) at the masthead
  3. If neither is possible: a torch or lantern showing white, used far enough in advance to prevent collision
Sail, 7 m to under 20 m
  1. Sidelights + sternlight, or
  2. The combined tricolour lantern at the masthead
The tricolour saves battery and shows better from a distance — but it is for sailing only.
Sail, 20 m and over
Separate sidelights + sternlight — the combined lantern is no longer an option.

Human-powered boats
Either the sailboat lights for their length, or a white lantern/torch shown in time to prevent collision.
Motor-sailing at night = power lights. The moment the engine drives her, a sailboat shows a masthead light with sidelights and sternlight — and the tricolour goes OFF. Tricolour plus masthead burning together is the classic charter-boat error: it reads as two conflicting vessels.
Lesson 5

At Anchor — Rule 30

Small craft
  • Under 50 m: one all-round white light where it can best be seen; by day, one black anchor ball forward
  • In or near a channel, fairway, anchorage or anywhere others navigate, showing it isn't optional — that's exactly where you'll be run down
Ships
  • 50 m and over: a forward all-round white plus a second near the stern, lower than the forward one
  • Over 100 m: working/deck lights on as well
Teaching moment for the Exumas: an anchorage full of masthead anchor lights looks like a field of low stars — brief your crew that a single steady white that doesn't move against the land is a boat, not a beacon, and give the swing circles room.
Lesson 6 · Interactive

Night Identifier — What Are You Looking At?

Lights on a black horizon, exactly as the watchkeeper sees them. Work the IYT sequence — aspect → propulsion → length → anything special — then make the call:

Knowledge Check

Navigation Lights — Knowledge Check

10 questions at IYT exam standard (75% to pass), drawn from the IYT Module 23 knowledge review.

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