The mark in plain words
- What does it show?It is on or near an isolated danger, with navigable water around it. The danger has a limited extent.
- How do I recognise it?It is black with one or more red horizontal bands and carries two black spheres vertically as its topmark.
- What light does it show?If fitted, it is white and gives a group of 2 flashes. That combination helps distinguish it from other aids.
- How do I pass?You may pass on any side, but check the chart and nautical publications for the danger's extent and safe passing distance.
See it in 3D
Turn the mark to inspect its colours, shape and topmark, then switch to night to see its light.
An interactive 3D illustration is shown here. The same content is described in the rule text and key takeaways below.
How you recognise it
- Colour
- black with one or more red horizontal bands
- Shape
- normally pillar or spar
- Topmark
- two black spheres vertically
- Light
- white, group flashing (2)
Frequent mistakes
Wrong: "If there is water around it, I can pass close to the mark."
Right: Check the danger's extent and keep the safe clearance shown by the chart.
Wrong: "Two black spheres mean a cardinal mark."
Right: Two black spheres identify an isolated danger mark.
Wrong: "The danger is far away because the mark is over navigable water."
Right: The danger is directly below or very close to the mark.
Exam keys & interpretation
- If you see two black spheres vertically, look for an isolated danger; do not confuse them with cardinal cones.
- "Navigable water all around" does not mean passing close: check the chart, extent and safe clearance.
- Black with red bands + white group flashing (2) = isolated danger mark.
How to read it
Picture it over a danger of limited extent: water surrounds it, but the centre remains dangerous.
Identify the mark first; then use the chart to decide how far clear you should stay.
Key points
- The danger is below the mark or very close to it.
- Navigable water is all around, but check the chart before closing in.
- Two black spheres vertically: the main visual clue.
- White light: group flashing (2), if fitted.
Test your knowledge
You see a black mark with red bands and two black spheres vertically. What does it indicate?
Full lesson & sources
An isolated danger mark is on or near a danger with navigable water around it. Consult charts and nautical publications for the extent of the danger and safe passing distance.
- Colour: black with one or more red horizontal bands
- Topmark: two black spheres (balls) vertically
- Light (if fitted): white, group flashing (2) — two flashes
- Shape: normally pillar or spar
Reviewed by Ramon Curto, Nautical teacher at IECNPC · Last reviewed: