Isolated Danger, Safe Water and Special Marks

Isolated danger marks

In short: a black-and-red mark sitting on a small danger. There is good water all round it, but not close to it.

Read first: IALA Buoyage System and Light Characters (light abbreviations), then Lateral Marks and Preferred Channel, then Cardinal Marks. This is the fourth topic of the Buoyage area; Emergency Wreck Marking Buoy comes next.

• Purpose: marks an isolated danger of limited extent — a rock, a shoal, a small wreck — that has navigable water all around it.
• The mark is placed on or above the danger. Unlike a cardinal mark, it does not point you to a safe side.
• Topmark: two black spheres, one above the other. This is the most important day feature.
• Colour: black with one or more broad red horizontal bands. Chart letters: BRB.
• Shape: usually pillar or spar. The shape carries no meaning.
• Light: white, flashing in groups of two: Fl(2).
• Memory hook: two spheres, two flashes.
• On the bridge: plan a clearing distance from the charted danger, not from the buoy. A buoy swings round its mooring and can drag off station.

Safe water marks

In short: a red-and-white striped mark that says “the water is good all round here”. It is a welcome sign, not a warning.

• Purpose: shows safe, navigable water all around the mark.
• Typical uses: mid-channel or channel centreline marks, landfall buoys at the seaward start of a buoyed channel, and the best point of passage under a fixed bridge.
• Colour: red and white vertical stripes — the only IALA mark with vertical stripes. Chart letters: RW.
• Shape: spherical, pillar or spar.
• Topmark (if fitted): a single red sphere.
• Light: always white, one of: Iso (isophase, equal light and dark), Oc (occulting, light longer than dark), LFl 10s (one long flash of at least 2 s every 10 s) or Mo(A) (Morse “A”: short–long, · —).
• Memory hook: one sphere, one long flash.
• On the bridge: a landfall buoy is often the first mark you meet when approaching a port. Use it to confirm your position before you enter the channel.

Sources differ on the light. Older revision flashcards gave only LFl 10s. Other study guides give white Morse A, isophase or occulting. Each is right but incomplete: IALA R1001 allows all four — Iso, Oc, LFl 10s and Mo(A). Learn all four.

Special marks

In short: yellow means “something special here”. Look it up on the chart before you go near it.

• Purpose: not mainly an aid to navigation. It shows a special area or feature.
• Examples: pipelines and cables (including outfall pipes), recreation zones, no-anchoring areas, data buoys (ODAS), spoil grounds, military exercise areas, and traffic separation where normal channel marks could confuse.
• What exactly it marks: check the chart, the sailing directions or the Notices to Mariners.
• Colour: yellow. Chart letter: Y.
• Shape: optional, but it must not conflict with navigational marks.
• Topmark (if fitted): a single yellow X (St Andrew’s cross).
• Light (if fitted): yellow, with any rhythm other than those used for white lights of navigational marks — so it cannot be mistaken for a cardinal, isolated danger or safe water light.
• On the bridge: a special mark near a pipeline or cable often means do not anchor and do not trawl. Check the chart before letting go.

Telling them apart

In short: read the topmark first by day, the colour of the light first by night.

• By night: yellow light → special mark. White light → count it: two flashes in a group = isolated danger; long flash, isophase, occulting or Morse A = safe water; quick or very quick = cardinal.
• Both IALA regions: all three marks are the same in Region A and Region B.
• On the chart: symbols are in INT 1 / Chart 1, Section Q. On ECDIS, pick the object with the cursor to read its full light.