Rule 27 · USCG Navigation Rules
Vessel engaged in mine clearance
Three all-round green lights (or three balls) — one near the foremast head and one at each end of the fore yard — warn you off by 1000 m.
Situation
Viewpoint
Display
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From the port bow you see:
- All-round green — foremast head
- All-round green — port fore-yard end
- All-round green — starboard fore-yard end
- white — Masthead light
- red — Port sidelight
→ Vessel engaged in mine clearance — Underway, seen from the port bow.
Memory aid
“Three greens in a triangle — mines below: stay 1000 m away.”
What it means
Underway
The three greens are shown IN ADDITION to normal power-driven lights. They mean: dangerous to approach within 1000 m of the mine clearance vessel.
At anchor, still working
At anchor she shows the three greens in addition to the Rule 30 anchor lights/ball — and the 1000 m danger zone still applies.
Day shapes
Underway
Ball — foremast head
One of three balls forming a triangle: this one near the foremast head.
Ball — port fore-yard end
One of three balls: at the port end of the fore yard.
Ball — starboard fore-yard end
One of three balls: at the starboard end of the fore yard.
At anchor, still working
Ball — foremast head
One of three balls forming a triangle near the foremast head.
Ball — port fore-yard end
One of three balls: port end of the fore yard.
Ball — starboard fore-yard end
One of three balls: starboard end of the fore yard.
Anchor ball
The Rule 30 anchor ball, forward.
Light details
Underway
| Light | Colour | Arc | Position |
|---|---|---|---|
| All-round green — foremast head | green | 360° all-round | Mast, vertical line (top) |
| All-round green — port fore-yard end | green | 360° all-round | Port yardarm |
| All-round green — starboard fore-yard end | green | 360° all-round | Starboard yardarm |
| 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 |
At anchor, still working
| Light | Colour | Arc | Position |
|---|---|---|---|
| All-round green — foremast head | green | 360° all-round | Mast, vertical line (top) |
| All-round green — port fore-yard end | green | 360° all-round | Port yardarm |
| All-round green — starboard fore-yard end | green | 360° all-round | Starboard yardarm |
| Anchor light (forward) | white | 360° all-round | Forward, centreline |
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 27, reproduced from the public-domain COLREGs Rule 27(f) — USCG Navigation Rules. Diagrams and 3D scenes are original to boatrules.com.