Rule 27 · USCG Navigation Rules
Dredging / underwater operations (obstruction)
RAM lights plus two reds over the fouled side and two greens over the side you may pass.
Situation
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:
- All-round red (top)
- All-round white (middle)
- All-round red — obstructed side (upper)
- All-round green — clear side (upper)
- All-round red (bottom)
- All-round red — obstructed side (lower)
- All-round green — clear side (lower)
→ Dredging / underwater operations (obstruction) — At work, not making way / at anchor, seen from the port bow.
Memory aid
“Pass on the greens: two greens = go side, two reds = dead side.”
What it means
At work, not making way / at anchor
Drawn with the obstruction to PORT: two all-round reds on the obstructed side, two all-round greens on the side another vessel may pass. When at anchor she shows THESE lights and shapes instead of anchor lights.
Making way through the water
Making way she adds masthead light(s), sidelights and a sternlight to everything above.
Day shapes
At work, not making way / at anchor
Ball – diamond – ball
Ball, diamond, ball in a vertical line — the RAM shape.
Two balls — obstructed side (port here)
Two balls in a vertical line marking the side the obstruction is on.
Two diamonds — clear side (starboard here)
Two diamonds in a vertical line marking the side another vessel may pass.
Making way through the water
Ball – diamond – ball
Ball, diamond, ball in a vertical line — the RAM shape.
Two balls — obstructed side (port here)
Two balls in a vertical line marking the obstructed side.
Two diamonds — clear side (starboard here)
Two diamonds in a vertical line marking the side to pass on.
Light details
At work, not making way / at anchor
| Light | Colour | Arc | Position |
|---|---|---|---|
| All-round red (top) | red | 360° all-round | Mast, vertical line (top) |
| All-round white (middle) | white | 360° all-round | Mast, vertical line (2nd from top) |
| All-round red (bottom) | red | 360° all-round | Mast, vertical line (3rd from top) |
| All-round red — obstructed side (upper) | red | 360° all-round | Port yardarm (top) |
| All-round red — obstructed side (lower) | red | 360° all-round | Port yardarm (2nd from top) |
| All-round green — clear side (upper) | green | 360° all-round | Starboard yardarm (top) |
| All-round green — clear side (lower) | green | 360° all-round | Starboard yardarm (2nd from top) |
Making way through the water
| Light | Colour | Arc | Position |
|---|---|---|---|
| All-round red (top) | red | 360° all-round | Mast, vertical line (top) |
| All-round white (middle) | white | 360° all-round | Mast, vertical line (2nd from top) |
| All-round red (bottom) | red | 360° all-round | Mast, vertical line (3rd from top) |
| All-round red — obstructed side (upper) | red | 360° all-round | Port yardarm (top) |
| All-round red — obstructed side (lower) | red | 360° all-round | Port yardarm (2nd from top) |
| All-round green — clear side (upper) | green | 360° all-round | Starboard yardarm (top) |
| All-round green — clear side (lower) | green | 360° all-round | Starboard yardarm (2nd from top) |
| 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: At work, not making way / at anchor.
In fog, she sounds
Don’t confuse with…
Official source
Rule 27, reproduced from the public-domain COLREGs Rule 27(d) — USCG Navigation Rules. Diagrams and 3D scenes are original to boatrules.com.