Rule 25 · USCG Navigation Rules
Sailing vessel underway
Sidelights and a sternlight only — no white masthead light, which is exactly how you tell sail from power at night.
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:
- red — Port sidelight
→ Sailing vessel underway — Under sail alone, seen from the port bow.
Memory aid
“Sail runs dark up top: sidelights + sternlight, and NO white masthead light.”
What it means
Under sail alone
Under 20 m the sidelights and sternlight may be combined in one tricolor lantern at the masthead (see the tricolor entry). The moment the engine drives her, she becomes a power-driven vessel.
Day shapes
No special day shape — by day this vessel is recognised by sight, not by shapes.
Light details
| Light | Colour | Arc | 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: Under sail alone.
In fog, she sounds
Don’t confuse with…
Official source
Rule 25, reproduced from the public-domain COLREGs Rule 25 — USCG Navigation Rules. Diagrams and 3D scenes are original to boatrules.com.