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SESK: Stuff Everyone Should Know · sesk.info/boat-isolated-danger-mark

A black buoy with red horizontal bands and two black balls on top marks what?

An isolated danger you can pass on any side, carefully.

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  1. An isolated danger you can pass on all sides ✔
  2. The middle of a safe channel
  3. A mooring spot for visitors

Why: Isolated danger marks are black with red bands and a topmark of two black spheres. They sit on or near a danger like a rock, so pass on any side but don't get close.

Huh, didn't know that: At night an isolated danger mark shows a white light flashing in groups of two.

The dad joke
Why did the black buoy look lonely? It was an isolated case.
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References

Answer: An isolated danger you can pass on all sides. Isolated danger marks are black with red bands and a topmark of two black spheres. They sit on or near a danger like a rock, so pass on any side but don't get close.

  1. 33 CFR 62.29, Isolated danger marks www.ecfr.gov/current/title-33/part-62
    Isolated danger marks indicate an isolated danger which may be passed on all sides.
    Checked 2026-10-09.

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