An isolated danger mark is a black buoy or beacon with one or more red horizontal bands and two black balls on top. It sits on or above a small danger that has navigable water all round it, so keep well clear of the mark itself.
An isolated danger mark is placed on, or moored above, a hazard of limited size, such as a single rock or a small wreck, where there is safe water on every side. Unlike a cardinal mark, it does not tell you which side to pass: any side will do, provided you give it room.
How to recognise it:
- black with one or more broad red horizontal bands;
- a topmark of two black spheres, one above the other;
- if lit, a white light showing groups of two flashes, written Fl(2) on the chart.
One way to remember the light is that two flashes go with the two balls.
The danger is right at the mark, so do not pass close to it. The chart shows how big the danger is and how much water there is around it: check it and choose a safe distance, particularly at low tide.
A different mark, the safe water mark, is easily confused with it because both have red on them, but it means the opposite:
- red and white vertical stripes, often a spherical or pillar buoy;
- a topmark of a single red sphere;
- if lit, a white light that is isophase (equal periods of light and dark), occulting, one long flash every ten seconds, or Morse code "A".
A safe water mark shows navigable water all round it. It is often used as a landfall buoy or at the start of a channel, and you can pass close to it on any side.
Isolated danger and safe water marks are the same in both IALA A and IALA B regions.
More in the book: Chapter 14, Buoys, charts and tides