The isolated danger mark: a danger with navigable water all round.

What is an isolated danger mark?

A mark placed on or near a danger that has navigable water all around it. R1001 defines it as a mark 'erected on, or moored on or above' an isolated danger.

It is common to Region A and Region B.

Which side do I pass an isolated danger mark?

R1001 doesn't name a side: the danger has navigable water all around it. As we read it, you can pass on any side, but not close: the mark may be near the danger rather than on it, and R1001 gives no passing distance. Take the clearance from the chart.

How far off should I pass an isolated danger mark?

R1001 doesn't give a distance. Because the size of the danger and the safe passing distance can't be specified for every case, it says the mariner 'shall consult the relevant charts and nautical publications'. In practice: check the chart for the extent of the danger before you decide how close to go.

How is it different from a cardinal mark?

A cardinal mark tells you which side of it the safer water is; an isolated danger mark is on or near the danger itself, with navigable water all round. R1001 picks out the double black sphere topmark and the group flashing (2) white light as what distinguishes the isolated danger mark from cardinal marks — whose topmarks are cones and whose lights are very quick or quick flashing.

What colour is an isolated danger mark?

Black with one or more broad horizontal red bands.

What is the topmark?

Two black spheres, one above the other. R1001 calls the double sphere topmark 'a very important feature' of the mark by day: it should be used wherever practicable, as large as possible, with a clear separation between the spheres. Where weather or ice make a topmark impractical, the authority may decide not to fit one.

What shape is the buoy?

Optional, as long as it doesn't conflict with lateral marks; a pillar or spar is preferred.

What light does an isolated danger mark show?

If one is fitted: white, group flashing (2) — flashes in groups of two. It may also carry an AIS AtoN or a racon.

A memory aid of ours: two spheres, two flashes.

Which marks show a white light?

Of the six types of mark, three: cardinal marks, the isolated danger mark and the safe water mark. Cardinals flash very quick or quick: with no grouping (north), in groups of three (east) or nine (west), or six followed by a long flash (south). The isolated danger mark shows group flashing (2). The safe water mark shows isophase, occulting, one long flash every 10 s, or Morse 'A'. R1001 says each of the latter two has a rhythm that can't be confused with a cardinal's.

Isolated danger or safe water — both have water all round?

Yes, and that is the trap. Both marks have navigable water all around them, but the isolated danger mark is on a danger and the safe water mark 'does not mark a danger'. By day: black and red bands with two black spheres, against red and white vertical stripes with one red sphere. At night: group flashing (2), against isophase, occulting, a long flash every 10 s or Morse 'A'.

Can an isolated danger mark mark a new danger?

Yes. R1001 lists isolated danger marks among those that can mark a newly discovered danger, alongside lateral and cardinal marks and the emergency wreck mark — see the emergency wreck mark page for how new dangers are marked and when a mark is duplicated.

What might they ask about the isolated danger mark?

Examples of the kind of question to be ready for (our wording, not the MCA's):

“Describe an isolated danger mark by day and at night.”

“At night you see a white light flashing in groups of two. What is it, and how do you pass it?”

“How close can you pass an isolated danger mark? Where would you find out?”

“What is the difference between an isolated danger mark and a safe water mark?”

The common slips: giving a passing distance that R1001 doesn't, and mixing up the two black spheres with the safe water mark's single red sphere.