Lesson 18 of 40 · Module 3 — Buoyage, lights and the rules of the road

Isolated danger, safe water and special marks

The remaining IALA marks, including the emergency wreck marking buoy.

Day Skipper15 minVideo · Notes · Interactive · Exercises

0:00 / 4:10

0:00 / 4:10Chapter 1 of 9: Completing the IALA system

Transcript

In the last two lessons you met lateral marks, which show the sides of a channel, and cardinal marks, which show where the safe water lies around a danger. Four more marks complete the IALA buoyage system. They are the isolated danger mark, the safe water mark, the special mark and the emergency wreck marking buoy. Each one tells you what is there, and what you should do about it.
An isolated danger mark stands on, or is moored above, a small danger with navigable water all around it, such as a single rock. It is black, with one or more broad red bands. Its topmark is two black spheres, one above the other. At night it shows a white light, flashing in groups of two. Two spheres, two flashes: an easy way to remember it.
On the practice chart, Mew Rock beacon is an isolated danger mark south-west of Port Kerran. Its light flashes twice every five seconds. The rock itself dries one point two metres. At a low spring tide it stands clear of the water, but for most of the tide it is covered and out of sight. Navigable water all around means you may pass on either side, not that you may pass close, so give it a respectful berth.
A safe water mark tells you there is navigable water all around it. It has red and white vertical stripes. Its topmark, if it has one, is a single red sphere. At night its light is white, with one of four rhythms: isophase, occulting, one long flash every ten seconds, or Morse code A. You will often find one at a landfall, at the seaward end of a channel, or in the middle of a channel.
On the practice chart, the Kerran Approach buoy is the safe water mark south of Port Kerran. Its light shows one long flash every ten seconds. It marks the start of the approach: the channel entrance, between the number one and number two buoys, lies one point six miles due north. There is no danger at the buoy itself, so you can pass it on either side.
Special marks are yellow. If there is a topmark, it is a single yellow X, and any light is yellow too. They are not mainly there to guide you: they mark a special area or feature, such as an outfall, a cable, a spoil ground, a recreation zone or a data buoy. On the practice chart, the Outfall buoy, two point two miles north of Gull Point, flashes yellow every five seconds. When you see one, look on the chart to find out what it marks, and keep clear of whatever that is.
In December 2002, the car carrier Tricolor sank after a collision near the Dover Strait. Within weeks, two more ships had run into the wreck, despite the warnings. So IALA, the body behind the buoyage system, created a new mark for exactly this job, one that cannot be mistaken for any other.
The emergency wreck marking buoy has blue and yellow vertical stripes, and an upright yellow cross on top. Its light flashes blue and yellow, alternately. It is laid on or around a new wreck, and stays until the wreck has been surveyed, charted and, where needed, permanently marked, so keep well clear and check the navigational warnings. Any new danger can also be marked with ordinary marks, and if it is especially serious, one of them may be duplicated, with a second, identical mark close by.
That completes the IALA buoyage system. By day, read the colour first, then the shape and the topmark. At night, the colour of the light narrows it down, and the rhythm tells you which mark it is. White lights belong to cardinal, isolated danger and safe water marks, red and green to lateral marks, yellow to special marks, and blue and yellow to a new wreck. For every mark, ask two questions: what is it, and what do I do?

In this lesson you will learn

Recognise an isolated danger mark by day and night, and pass it with a good berth
Recognise a safe water mark and its light rhythms, and know where you will find one
Recognise a special mark and find out what it marks
Recognise the emergency wreck marking buoy, and know how new dangers are marked
Identify any IALA region A mark from its colour, shape, topmark or light, and decide what to do

Completing the IALA system

In Lesson 16 you met the lateral marks that show the two sides of a channel, and in Lesson 17 the cardinal marks that show which side of a danger the safe water lies. Four more marks complete the IALA region A buoyage system used in UK and European waters:

Isolated danger mark: a small danger with navigable water all round it.
Safe water mark: no danger at all; navigable water all round. Often a landfall or channel-entrance mark.
Special mark: a special area or feature, such as an outfall or a cable, rather than an aid to finding your way.
Emergency wreck marking buoy: a new wreck, not yet on the charts.

Isolated danger mark

A small danger, navigable water all round

Safe water mark

Navigable water all round

Special mark

A special area or feature

Emergency wreck marking buoy

A new wreck, not yet charted

The four marks in this lesson, as you would see them from a boat.

Every mark answers two questions: what is it, and what should I do about it?

Isolated danger marks

An isolated danger mark is placed on, or moored on or above, a danger of limited extent (a single rock, a small shoal, a wreck) that has navigable water all round it. Rather than surrounding the danger with cardinal marks, one mark sits right on top of it.

	Isolated danger mark
Colours	Black with one or more broad red bands (bands run across the mark)
Shape	Usually a optional; pillar or spar preferred, or a beacon on the danger itself
Topmark	Two black spheres, one above the other: the most important feature by day
Light	White, group flashing two: Fl(2). Two flashes, then darkness, repeated

Some marks are buoys, floating on a mooring; others are beacons, fixed structures standing on the seabed or on the danger itself. On the practice chart, Mew Rock beacon stands on Mew Rock, 2.7 M south-west of Port Kerran. Its light is Fl(2)5s: two white flashes every 5 seconds.

white, group flashing, 2 flashes every 5 seconds0.0 s of 5 s

An isolated danger mark by day, and Mew Rock beacon’s light at night (Fl(2)5s). Flash lengths are representative.

Give it a respectful berth

“Navigable water all round” means you may pass on either side. It does not mean you may pass close. The mark shows where the danger is, not how far it extends, and a danger you can’t see is the worst kind. Mew Rock shows why.

Worked example

How much water is over Mew Rock?

1Mew Rock’s drying height is 1.2 m: it stands 1.2 m above chart datum. Over a drying height, depth = height of tide − drying height (Lesson 8).
2At an average spring low water at Port Kerran the tide is about 0.4 m above chart datum: 0.4 − 1.2 = −0.8 m. Negative, so the rock stands 0.8 m out of the water. You can see it.
3At an average neap low water, about 1.6 m: 1.6 − 1.2 = 0.4 m. The rock is just covered: invisible, but shallow enough to stop any yacht.
4At an average spring high water, about 5.2 m: 5.2 − 1.2 = 4.0 m over it.

For most of the tide Mew Rock is covered, so you can’t see it. Only the beacon tells you it is there.

The tide levels are Port Kerran’s averages, used here as a guide; Module 4 shows you how to find the height of tide at any time.

Safe water marks

A safe water mark tells you there is navigable water all round it. It doesn’t mark a danger: it marks a place. You will find them as landfall marks (the first mark to look for when arriving from sea), at the seaward end of a buoyed channel, and on the centre line of a channel.

	Safe water mark
Colours	Red and white vertical stripes: the stripes run up and down
Shape	Spherical, pillar or spar
Topmark	Single red sphere (if it has one)
Light	White: isophase, occulting, one long flash every 10 seconds, or Morse code A

The light rhythms

Lesson 19 covers every light rhythm in detail. For now, the four you may see on a safe water mark:

Isophase (Iso): equal periods of light and dark.
Occulting (Oc): the light is on for longer than it is off; it is briefly eclipsed.
Long flash (LFl 10s): one flash lasting at least 2 seconds, every 10 seconds.
Morse A (Mo(A)): a short flash then a long one, the Morse code for the letter A (· —).

Iso

Isophase

Equal light and dark

Occulting

More light than dark

LFl 10s

Long flash

One flash of 2 s or more, every 10 s

Mo(A)

Morse A

Short, then long: · —

0 s5 s10 s

Ten seconds of each rhythm on the same time scale. Only the long flash has a set period (10 seconds); the others are examples.

None of these is a short group of flashes or the rapid flashing of a cardinal mark, so at night a safe water light is easy to tell from the other white lights on buoys.

On the practice chart, Kerran Approach buoy (LFl10s) lies 3.1 M south of Port Kerran. It is the usual starting point for the harbour approach: from it, the channel entrance between No 1 and No 2 buoys lies 1.6 M due north (000°T).

Check your understanding: ahead you see a pillar buoy with two black spheres on top. Can you pass close to it?

Special marks

Special marks are yellow. They are not mainly there to help you find your way: they mark a special area or feature, which the chart or the sailing directions explain. For example:

outfall pipes, and cables or pipelines on the seabed;
spoil grounds, where dredged material is dumped;
military exercise areas;
recreation zones, such as water-ski or swimming areas;
data buoys that measure wind, waves or tide.

	Special mark
Colour	Yellow
Shape	Optional, but it must not conflict with the navigational marks: for example, a special mark on the port side of a channel may be can-shaped, but not conical
Topmark	A single yellow X (if it has one)
Light	Yellow (if lit), in any rhythm not used by the white lights of cardinal, isolated danger or safe water marks

On the practice chart, the Outfall buoy (Fl Y 5s: one yellow flash every 5 seconds) lies 2.2 M north of Gull Point lighthouse. It marks the end of an outfall pipe: not a danger to navigation as such, but a good place not to anchor.

The three marks on the practice chart

The practice chart has one of each of these three marks. Zoom in on each one, see it by day and by night, and check what it tells you.

InteractiveTour the marks

Pick a mark: the chart zooms in, and you see the mark, its light flashing in its real rhythm, and what to do. Use + and − to zoom further.

Mew Rock beacon: 49°44.0'N 004°15.0'W

white, group flashing, 2 flashes every 5 seconds0.0 s of 5 s

Mew Rock beacon

Isolated danger mark · Fl(2)5s

• Rock dries 1.2 m
• Neap low water (1.6 m): only 0.4 m over it
• Spring low water (0.4 m): 0.8 m of rock showing

An isolated danger mark: a beacon standing on Mew Rock, 2.7 M south-west of Port Kerran. The rock is a danger of limited extent, with navigable water all around it.

What to do: Pass on either side, but give it a respectful berth: for most of the tide the rock is covered and out of sight.

New dangers and the emergency wreck marking buoy

In December 2002 the car carrier Tricolor sank after a collision near the Dover Strait. The wreck lay in one of the busiest stretches of water in the world, and within weeks two more ships had run into it, despite the warnings. In response, IALA (the body behind the buoyage system) created a new mark for exactly this job, one that cannot be mistaken for any other: the emergency wreck marking buoy.

	Emergency wreck marking buoy
Colours	Blue and yellow vertical stripes (four to eight stripes)
Shape	Pillar or spar
Topmark	Upright yellow cross
Light	Alternating blue and yellow flashes. No other mark in Lessons 16–18 shows blue

Alternating blue and yellow flashes.0.0 s of 3 s

The emergency wreck marking buoy, and its light: blue and yellow flashes, alternately. The timing shown is representative.

It is laid as close to a new wreck as possible (a large wreck may have several around it) and stays until the wreck is well known: surveyed, published in official nautical information and, where needed, permanently marked with ordinary IALA marks.

New dangers

A new danger is a newly discovered hazard that is not yet shown on charts or in sailing directions: a wreck, but also a rock or a shoal. New dangers can be marked with ordinary lateral, cardinal or isolated danger marks, or with emergency wreck marking buoys. If the danger is considered especially serious, at least one of the marks may be duplicated: a second, identical mark laid close by.

What is it, and what do I do?

You have now met every IALA region A mark. Identifying one follows the same order every time:

By day, colour first, then shape, then topmark. Colour sorts the marks into families: red or green (lateral), black and yellow (cardinal), black and red (isolated danger), red and white (safe water), yellow (special), blue and yellow (new wreck).
At night, colour first, then rhythm. White lights belong to cardinal, isolated danger and safe water marks; red and green to lateral marks; yellow to special marks; alternating blue and yellow to emergency wreck marking buoys.
Check the chart: find the mark, read its characteristic, and make sure it is the one you think it is.
Decide what to do before you reach it.

Worked example

A white light at night

1Approaching Port Kerran from the west at night, you see a white light ahead: two flashes, then darkness, repeating every few seconds.
2White: so a cardinal, isolated danger or safe water mark (or a lighthouse, which Lesson 19 covers).
3Two flashes in a group: Fl(2). Cardinal marks flash quickly or very quickly, and safe water lights are isophase, occulting, a long flash or Morse A. Fl(2) is the isolated danger light.
4On the chart: Mew Rock beacon, Fl(2)5s. Time the light: two flashes every 5 seconds confirms it.
5What to do: there is a danger right beneath it and navigable water all round. Pass on either side, well clear.

Mew Rock beacon, an isolated danger mark. Give it a good berth: the rock dries 1.2 m and is hidden for most of the tide.

InteractiveWhat is it, and what do I do?

You see a mark from Lessons 16 to 18, by day or at night. Name it, then choose what to do. Switch between day, night and mixed at the top.

Marks from Lessons 16–18

By day: colour first, then shape and topmark.

1. What is it?

Every IALA region A mark

Mark	By day	At night	What to do
Port-hand lateral mark Lesson 16	Red. Can (flat-topped), pillar or spar; or a pillar or spar. Topmark: single red can.	Red, any rhythm except Fl(2+1) except Fl(2+1)	Keep it to port (on your left) going with the buoyage
Starboard-hand lateral mark Lesson 16	Green. Cone (pointed), pillar or spar; or a pillar or spar. Topmark: single green cone, point up.	Green, any rhythm except Fl(2+1) except Fl(2+1)	Keep it to starboard (on your right) going with the buoyage
Preferred channel to port Lesson 16	Green with one broad red band. Cone; or a pillar or spar. Topmark: green cone.	Green, Fl(2+1)	Main channel to port: keep it to starboard (on your right) going with the buoyage
Preferred channel to starboard Lesson 16	Red with one broad green band. Can; or a pillar or spar. Topmark: red can.	Red, Fl(2+1)	Main channel to starboard: keep it to port (on your left) going with the buoyage
North cardinal mark Lesson 17	Black above yellow. Pillar or spar. Topmark: two black cones, both pointing up.	White, VQ or Q (continuous)	Pass to the north of it
East cardinal mark Lesson 17	Black with one broad yellow band. Pillar or spar. Topmark: two black cones, base to base.	White, VQ(3) 5s or Q(3) 10s	Pass to the east of it
South cardinal mark Lesson 17	Yellow above black. Pillar or spar. Topmark: two black cones, both pointing down.	White, VQ(6)+LFl 10s or Q(6)+LFl 15s	Pass to the south of it
West cardinal mark Lesson 17	Yellow with one broad black band. Pillar or spar. Topmark: two black cones, point to point.	White, VQ(9) 10s or Q(9) 15s	Pass to the west of it
Isolated danger mark Lesson 18	Black with one or more broad red bands. Optional; pillar or spar preferred. Topmark: two black spheres.	White, Fl(2)	Pass either side, giving it a good berth
Safe water mark Lesson 18	Red and white vertical stripes. Spherical, pillar or spar. Topmark: single red sphere.	White: Iso, Oc, LFl 10s or Mo(A)	Pass either side: there is no danger at the mark
Special mark Lesson 18	Yellow. Optional, but must not conflict with navigational marks. Topmark: yellow X (if any).	Yellow, any rhythm not used by cardinal, isolated danger or safe water marks	Check the chart for what it marks, and keep clear of that
Emergency wreck marking buoy Lesson 18	Blue and yellow vertical stripes. Pillar or spar. Topmark: upright yellow cross.	Alternating blue and yellow flashes	Keep well clear: it marks a new wreck not yet on the charts

Lesson 19 decodes full light characteristics such as Fl(3)10s28m15M, so you can identify lighthouses as well as buoys at night.

Key terms

Isolated danger mark: Black with red bands, two black spheres, white Fl(2): a small danger with navigable water all round.
Safe water mark: Red and white vertical stripes, a red sphere, a white Iso, Oc, LFl 10s or Mo(A) light: navigable water all round.
Special mark: Yellow, optional yellow X topmark and yellow light: marks a special area or feature, not mainly a navigational aid.
Emergency wreck marking buoy: Blue and yellow vertical stripes, upright yellow cross, alternating blue and yellow flashes: a new wreck.
New danger: A newly discovered hazard not yet shown on charts or in sailing directions.
Duplicated mark: A second, identical mark laid close by when a new danger is considered especially serious.
Beacon: A mark fixed to the seabed or to the danger itself, rather than a floating buoy.
Landfall mark: The first mark you look for arriving from sea: often a safe water mark.
Isophase (Iso): A light with equal periods of light and dark.
Occulting (Oc): A light that is on for longer than it is off.
Long flash (LFl): A flash lasting at least 2 seconds.

Exercises

Check your understanding. You get a second try at each question, then the worked answer. Score 70% or more to complete the lesson — your progress is saved in this browser.

What is this mark?

A buoy has red and white vertical stripes and a single red sphere on top. What does it tell you?

At night you see this white light ahead. Which mark is it?

A light seen at night.0.0 s of 10 s

Which of these lights could you see on a safe water mark?

A yellow buoy with a yellow X-shaped topmark is most likely to be marking…

You see this buoy ahead. What should you do?

Sailing along the south of the island, you see Mew Rock beacon (Fl(2)5s) ahead. How should you pass it?

On the practice chart, tap the safe water mark.

Mew Rock dries 1.2 m. At an average neap high water, the height of tide at Port Kerran is 4.0 m. How much water is there over the rock?

Two identical north cardinal marks have been laid close together near a wreck that is not on your chart. What does this tell you?

0 of 10 answered · 0 correct

Every IALA mark: practice

Five fresh questions each time, across all the marks from Lessons 16–18: name a mark by day and by night, match a mark to its light or its description, decide what to do, and find one on the chart.