Entry & progression
Exam module: Q100. Rules 20–31. Expect to be shown light configurations and asked what you are looking at, and from what angle.
The arcs — learn these first
| Light | Colour | Arc | Where |
|---|---|---|---|
| Masthead | White | 225° | 22.5° abaft the beam on each side — ahead and to both bows |
| Sidelights | Green (stbd), red (port) | 112.5° each | From dead ahead to 22.5° abaft the beam on their side |
| Sternlight | White | 135° | Centred astern — 67.5° from right aft on each side |
| All-round | As specified | 360° | Anchor lights, NUC lights, RAM lights |
| Towing | Yellow | 135° | Same arc as a sternlight, carried above it |
The arithmetic is worth doing once: two sidelights (112.5 × 2 = 225) plus a sternlight (135) equals 360. The masthead light's 225° is exactly the two sidelights combined. That is not a coincidence — it is how the system is built.
Length thresholds
The rules change with the size of the vessel, and the exam is precise about the numbers.
| Length | What changes |
|---|---|
| Under 7 m | International only: if max speed does not exceed 7 knots, an all-round white light alone, with sidelights if practicable (Rule 23(d)(ii)). Inland has no such provision. |
| Under 12 m | May show an all-round white light and sidelights in lieu of masthead light and sternlight (Rule 23(d)). |
| Under 20 m | Sidelights may be combined in a single lantern (Rule 21(b)). Shall not impede a vessel confined to a narrow channel (Rule 9(b)). |
| 50 m and over | A second (after) masthead light becomes mandatory. Below 50 m it is optional (Rule 23(a)(ii)). |
| 100 m and over | Deck lights when at anchor (Rule 30(c)). |
Shapes
- Ball — at anchor (one forward). Three balls in a vertical line: aground.
- Ball–diamond–ball — restricted in ability to manoeuvre.
- Two balls in a vertical line — not under command.
- Cone, point down — a sailing vessel proceeding under sail and power.
- Diamond — towing, where the length of the tow exceeds 200 m.
- Cylinder — International only: constrained by draught. The Inland Rules have no Rule 28 at all — 33 CFR 83.28 is reserved.
The one that is examined most
A sailing vessel proceeding under sail alone shows sidelights and a sternlight. Start the engine and she becomes a power-driven vessel — she must show a masthead light, and by day, a cone with its point down. It is the single most-missed shape on the exam, and the single most-ignored rule on the water.
NOTE — Inland has no vessel constrained by her draught 33 CFR 83.28 is '[Reserved]'. The three all-round red lights and the cylinder simply do not exist inland — and Inland Rule 18(d), which would require you to avoid impeding such a vessel, is reserved with it. If a question offers 'constrained by draught' as an answer to an Inland problem, it is a distractor.