Rule 23 · USCG Navigation Rules
Power-driven vessel under 50 m
Masthead light, sidelights, and sternlight — the baseline pattern every other vessel modifies.
Viewpoint
Display
Drag to orbit freely, scroll or pinch to zoom — or use the viewpoint buttons above (keyboard friendly).
From the port bow you see:
- white — Masthead light
- red — Port sidelight
→ Power-driven vessel under 50 m — Underway, seen from the port bow.
Memory aid
“One masthead light = under 50 m. From ahead you see white over red+green; from astern only white.”
What it means
Underway
Applies to power-driven vessels of 12–50 m. A vessel under 50 m may add the second, higher masthead light but is not obliged to. Under 20 m the masthead light range drops to 3 nm and the sidelights may be combined in one lantern.
Day shapes
No special day shape — by day this vessel is recognised by sight, not by shapes.
Light details
| Light | Colour | Arc | Position |
|---|---|---|---|
| Masthead light | white | 225° forward | Forward masthead position |
| Port sidelight | red | 112.5° to port | Port side, deck level |
| Starboard sidelight | green | 112.5° to starboard | Starboard side, deck level |
| Sternlight | white | 135° aft | Stern |
2D profile
Labelled side view — works without WebGL and in print. Variant shown: Underway.
In fog, she sounds
Don’t confuse with…
Official source
Rule 23, reproduced from the public-domain COLREGs Rule 23 — USCG Navigation Rules. Diagrams and 3D scenes are original to boatrules.com.