Rule 23 · USCG Navigation Rules
Power-driven vessel 50 m or more
Two masthead lights — the after one higher — plus sidelights and sternlight; the second white tells you she is a big ship and which way she points.
Viewpoint
Display
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From the port bow you see:
- white — After masthead light (higher)
- white — Forward masthead light (lower)
- red — Port sidelight
→ Power-driven vessel 50 m or more — Underway, seen from the port bow.
US Inland difference
Inland Rule 23: a power-driven vessel operating on the Great Lakes may carry an all-round white light in lieu of the second masthead light and the sternlight, carried in the position of the second masthead light.
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Memory aid
“Two masthead lights = 50 m or more. Lower light forward, higher light aft — the whites 'lean' the way she is going.”
What it means
Underway
The forward masthead light is lower, the after one higher — reading the two whites tells you the ship's heading long before you can see her sidelights.
Day shapes
No special day shape — by day this vessel is recognised by sight, not by shapes.
Light details
| Light | Colour | Arc | Position |
|---|---|---|---|
| Forward masthead light (lower) | white | 225° forward | Forward masthead position |
| After masthead light (higher) | white | 225° forward | Aft masthead position, higher |
| 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.