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IALA Isolated Danger Marks

Isolated danger mark: black with red bands and two black spheres, navigable water all around.

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

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

1

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.

2

Navigable water exists all around the mark

3

Two black balls as topmark

4

White light: group flashing (2)

Common Mistakes

Passing too close without checking the charted extent of the danger and safe clearance

Confusing with cardinal marks which indicate direction of safe water

Test Your Knowledge

Practise this topic and review the explanation for each answer.

More rules in this Part

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