Isolated danger marks are placed on or moored over an isolated danger with navigable water all around. The danger has a limited extent.
- Colour: black with one or more red horizontal bands
- Topmark: two black spheres (balls) vertically
- Light: white, group flashing (2) — two flashes
- Shape: pillar or spar
Vessels may pass on any side but should keep clear of the mark itself as the danger is directly below or very close.
An interactive 3D illustration is shown here. The same content is described in the rule text and key takeaways below.
Buoyage Reading Order
Read the mark in a fixed order: topmark, colour pattern, light rhythm, charted meaning, then the safe side or action required.
Confirm the conventional direction of buoyage from the chart or pilotage plan before deciding port-hand or starboard-hand treatment.
Treat every mark as one aid among several.
Cross-check with charted depth, position, radar, visual bearings and the planned track.
Exam Focus
For cardinals, use the cones first, then the colour bands, then the flash mnemonic.
If those three agree, the answer is usually secure.
For lateral marks, the region only changes the colours, not the core idea: the mark still identifies the side of the channel in the conventional direction of buoyage.
Key Takeaways
Danger is directly below the mark
Navigable water exists all around the mark
Two black balls as topmark
White light: group flashing (2)
Common Mistakes
Passing too close to the mark — the danger is right below it
Confusing with cardinal marks which indicate direction of safe water
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