IALA Buoyage and Light Characteristics

Prerequisites

• RYA Competent Crew or equivalent experience. Competent Crew introduces red and green lateral marks and the idea of cardinal marks. Here you need to identify every IALA mark instantly, by day and by night, and use them to navigate.
• Day Skipper chartwork basics: reading a chart, chart symbols and abbreviations, taking a bearing with a hand-bearing compass, and plotting a position.
• Familiarity with the Day Skipper pilotage lesson (transits, clearing bearings, pilotage plans). Buoyage is one of the main tools of pilotage.
• For the practical course, ideally 4 night hours already logged. Recognising lit marks is assessed during night cruising.

Learning Objectives

This lesson covers the RYA Day Skipper shorebased syllabus topic "Buoyage Systems" (IALA System A, and awareness of System B) and the light characteristics, sector lights and leading lights used in pilotage, which are assessed on the practical course. By the end of this lesson you will be able to:

• Explain the IALA buoyage system, Regions A and B, and the conventional direction of buoyage (syllabus: IALA buoyage system A and B).
• Identify lateral, preferred channel, cardinal, isolated danger, safe water, special and emergency wreck marks by colour, shape, topmark and light (syllabus: identification of marks).
• Say which side to pass each mark and why (syllabus: using buoyage in pilotage).
• Decode light characteristics as written on charts and in the almanac (Fl, LFl, Q, VQ, Oc, Iso, F, Al, Mo, groups, period, colour, elevation and range) (syllabus: light characteristics).
• Calculate the geographic range of a light and the dipping distance, and compare it with the nominal range.
• Use sector lights and leading lights, and understand racons and fog signals (syllabus: lights and electronic aids to pilotage).
• Identify a lit mark or lighthouse at night by timing its period and counting its flashes.
• Use buoyage safely: cross-checking it with chart, depth and position, and treating marks as aids, not guarantees.

Why a System?

Buoys and beacons are signposts on the water. They only work if every mariner reads them the same way. The International Association of Marine Aids to Navigation and Lighthouse Authorities (IALA) agreed a single system in 1980 that uses five main types of mark (plus a sixth, the emergency wreck mark, added in 2006):

1. Lateral marks: the sides of a channel
2. Cardinal marks: where the safe water lies relative to a danger, by compass direction
3. Isolated danger marks: a small danger with safe water all around
4. Safe water marks: safe water all around, such as mid-channel or a landfall buoy
5. Special marks: features or areas, not primarily navigational
6. Emergency wreck marks: new wrecks, pending permanent marking

Every mark can be identified by day from its colour, shape and topmark, and by night from its light colour and rhythm. A good skipper can name a mark from any one of these clues and confirm it with the others.

IALA Regions A and B

Only lateral marks differ between the two regions. Every other mark is the same worldwide.

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The American mnemonic "red right returning" is Region B. In UK waters it puts you on the wrong side of every channel mark. Notice that the shapes are the same in both regions: cans are always to port and cones to starboard. Only the colours swap. Use the Caribbean with special care, because some islands are A and some are B. The RYA Training Almanac deliberately makes its Neptune Islands a Region B area next to a Region A mainland, so that exam questions can test whether you have read the port notes: an entrance there is marked Q.G and Q.R, with red to starboard entering.

The Direction of Buoyage

Lateral marks are laid relative to the conventional direction of buoyage:

• In a harbour, river or estuary, it is the direction from seaward towards the harbour (going in, or "returning").
• Along a coast it is set by the buoyage authorities. Around the UK it generally runs clockwise around Great Britain: northward up the west coast, eastward along the north coast of Scotland, southward down the east coast and westward along the English Channel.
• On a chart, where it is not obvious, it is shown by a large magenta arrow with two dots, one red and one green.

When you are leaving a harbour, everything is reversed: red marks are then on your starboard side and green on your port side.

Lateral Marks

Lateral marks mark the edges of a channel. In Region A, going with the direction of buoyage:

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Points to note:

• Many lateral buoys in the UK are pillar or spar buoys. With these the topmark gives the shape, and the colour is the main clue.
• Lateral marks are often numbered: even numbers on port marks, odd on starboard, counting up from seaward. Names on the buoys (for example "Bar", "No 4") appear on the chart.
• Where the chart shows a light, it gives the characteristic, for example Fl(2)R.5s for a red port mark flashing twice every 5 seconds. Neighbouring marks have different rhythms so you can tell them apart.
• A lateral mark marks the edge of the dredged or deep channel, not necessarily the edge of all danger. Small craft can often go outside it, but only after checking the chart. Big ships must stay inside, so keep clear of their path (COLREGs Rule 9).

Preferred channel marks

Where a channel divides, a modified lateral mark shows which is the preferred (main) channel.

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Rule of thumb: read the main body colour as if it were an ordinary lateral mark. The band is the minor channel. The Fl(2+1) light (a group of two flashes followed by one) is the night signature of a junction mark.

Cardinal Marks

A cardinal mark is placed in one of the four quadrants (north, east, south or west) of a danger and tells you that safe water lies on the side named. A north cardinal has safe water to its north, so you pass north of it. The danger lies on the opposite side, to the south.

Cardinal marks are used to show the deepest water, the safe side on which to pass a danger, a bend or junction in a channel, or the end of a shoal. They are always black and yellow, with two black cones as topmark. Lights, when fitted, are always white, quick (Q) or very quick (VQ).

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Memory aids

• The cones point to where the black is. North cones point up and the black is on top. South cones point down and the black is underneath. East cones point outward, so the black is top and bottom. West cones point inward, so the black is in the middle.
• The flashes follow a clock face. East is 3 o'clock, so 3 flashes. South is 6 o'clock, so 6 flashes. West is 9 o'clock, so 9 flashes. North is 12, which you can think of as continuous.
• The long flash after the south's six stops a south being mistaken for an east or west when you lose count. It also makes it unique.
• West's topmark looks like a Women's waist. East's looks like an Egg.

Using a cardinal: worked example

You are heading 090°T along a coast. Ahead, slightly to port, you see a buoy showing Q(6)+LFl every 15 seconds. That is a south cardinal. Safe water lies to its south. The danger is to its north. You must pass south of it: keep it on your port side and give it a sensible berth, checking the chart for the extent of the danger.

Isolated Danger, Safe Water and Special Marks

Isolated danger mark

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• Black with one or more broad red bands, topmark two black spheres, light white Fl(2).
• Placed on or above a small danger, such as a rock or wreck, with navigable water all around.
• You may pass on any side, but not close. The danger is directly under the mark, and its extent may not be exactly known. Check the chart for the charted depth and the size of the hazard.
• Memory aid: two spheres, two flashes. Think of it as a "danger, danger" mark.

Safe water mark

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• Red and white vertical stripes, spherical or pillar shape, topmark single red sphere, light white Iso, Oc, LFl 10s, or Morse "A" (short-long).
• Marks safe water all around: a fairway, a mid-channel point or a landfall buoy at the approach to a harbour (often the first mark you make for).
• No danger is implied. You may pass on either side, but in a fairway it is good practice to keep it to port, as it marks the middle of the channel.
• It is the only mark with vertical stripes. Vertical stripes mean safe. Horizontal bands mean danger.

Special mark

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• Yellow, any shape that doesn't conflict with navigational marks, optional topmark yellow X, light yellow, any rhythm not used for white lights.
• Marks a special area or feature: ODAS data buoys, outfalls, cables and pipelines, spoil grounds, military exercise zones, water-ski or swimming areas, racing marks, traffic separation where conventional marks could be confused.
• The meaning is on the chart, in the almanac or in local notices. Check it. Anchoring over a cable or entering a firing range can be expensive or dangerous.
• If a special mark is can-shaped or conical, pass it as you would a lateral mark of that shape.

Emergency wreck marking buoy

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• Blue and yellow vertical stripes (an equal number of each, at least four and no more than eight), topmark an upright yellow cross, light alternating blue and yellow flashes: Al.Bu.Y, each about 1 second on with half a second dark, over a 3-second period.
• Laid quickly over a new wreck that is not yet charted, so that it may not be on your chart or in your plotter. A racon (Morse D) or AIS signal may also be fitted.
• Treat it as you would the cardinal or isolated danger mark that will eventually replace it, and give it a wide berth. Expect it to be backed up by navigational warnings (Securité) on VHF.

Light Characteristics

At night a buoy is just a point of light. You identify it from three things: colour, rhythm and period. The chart and almanac write these in a standard abbreviated form.

Reading a chart light description

Take Fl(3)G.10s12m5M as an example:

Types of rhythm

Two things often confuse beginners:

• Occulting vs flashing. Ask "is it mostly on or mostly off?" An occulting light is a steady light that blinks off. A flashing light is a dark that blinks on.
• Q vs VQ. Both flash rapidly. Q is about one flash per second and VQ about two per second. On a cardinal, either may be used. What matters is counting the group and timing the period.

Identifying a light step by step

1. Pre-plan. Before dark, list the lights you expect to see on each leg, with their characteristics and expected bearings, in your pilotage notes.
2. Note the colour. Red or green suggests a lateral mark, white a cardinal, isolated danger, safe water or lighthouse, yellow a special mark.
3. Count the flashes in each group, several times.
4. Time the period with a stopwatch (or by counting "one thousand, two thousand..."), from the start of one group to the start of the next.
5. Compare with the chart. Is there a light with that characteristic on roughly the expected bearing?
6. Confirm with a bearing and the depth before relying on it.

Range of Lights

A light has three different ranges, and the Day Skipper syllabus expects you to know the difference.

• Nominal range is the luminous range in a meteorological visibility of 10 nautical miles, as printed on the chart and in the almanac (for example 5M). It depends on the strength of the light.
• Luminous range is the real range of the light in the visibility on the night. In poor visibility it falls well below the nominal range; in very clear air it can exceed it. Tables in the almanac convert nominal range to luminous range for a given visibility.
• Geographic range is how far the light can be seen before the curve of the Earth hides it. It depends on the height of the light and the height of your eye. The visible range is the smaller of the luminous range and the geographic range.

Geographic range

The formula, with heights in metres and the answer in nautical miles, is:

Geographic range = 2.03 x (square root of the height of eye + square root of the height of the light).

Worked numbers: a lighthouse with a light 36 m above MHWS and an observer with eye height 4 m. The square root of 36 is 6 and of 4 is 2, so geographic range = 2.03 x (6 + 2) = 16.2 nautical miles. If the charted nominal range is 18M, you will see the light at about 16M in clear weather, since the curvature of the Earth, not the brightness, limits it. If the nominal range is 10M, the light's strength is the limit, and visibility will cut this further in haze.

Dipping distance

When you first see a light rise above the horizon, you are at its dipping distance, and you can fix a position circle: the distance from the light is the geographic range using your eye height. Using the example above, if you see the light just appear, you are 16.2 miles away. Combine this with a bearing to fix the position. Heights of lights on charts are given above MHWS, so correct for the state of tide if you need accuracy: at low tide you are higher above the water, relative to the datum.

The almanac light lists and tables

Each almanac prints an alphabetical list of lights for every stretch of coast (the Training Almanac lists those with a range of 5 miles or more), giving the structure ("white round tower, black top"), the characteristic, the elevation and the range, for example Fl(2+1)30s87m22M. Use it to identify an unfamiliar light from what you observe: filter by colour, then by the number of flashes in the group, then by the period, and confirm with the bearing. The reference pages also tabulate the distance off when rising and dipping for combinations of height of light and height of eye, which is the geographic range formula below worked out for you.

Planning use

When you plan a night passage, check each light's nominal range against the visibility forecast and calculate the geographic range at your eye height. A low light (3 m) with a nominal range of 5M will only be seen at about 2.03 x (2 + 1.7) = 7.5 miles in perfect conditions, so 5M is the working number. A high lighthouse on a cliff may show 20M or more, so if the first one you see is further away than you expect, check whether you have misidentified it.

Sector Lights and Leading Lights

Sector lights

A sector light shows different colours (or a white and a coloured light) over different arcs of the horizon. They are used to guide a boat along a safe channel or to warn of a danger. On the chart a sector light is drawn with coloured arcs and printed bearings, and the bearings are always given from seaward, as you would see them from the boat looking toward the light.

In UK waters, the normal convention is:

• White sector: the safe channel or approach.
• Red sector: danger on the port side as you enter, or a shoal side.
• Green sector: danger on the starboard side as you enter.

Steering in the white sector keeps you in safe water. If the light turns to red you have moved to port of the line, because the red sector lies on the port side as you enter, and you should alter course towards the white. Combine the light with a depth check, since the edges of a sector are not exactly sharp: there is an intermediate sector of a few minutes of arc where the colour fades from white to red or green.

Consider a harbour entrance with a sector light at the head of a pier: Fl.WRG.4s 11m 10/8M. The light flashes every 4 seconds, shows white, red or green depending on the bearing, has a height of 11 m, and its white range is 10M and its red and green are 8M. The chart shows the bearings of each sector, for example 340 to 350 degrees white, 350 to 010 degrees red and 330 to 340 degrees green. Steering to keep the light white (bearing 340 to 350 degrees) gives a safe track of about 345 degrees true.

Leading lights and transits

A pair of lights, one higher and further back, are placed on a line so that when one appears directly above the other you are on the leading line. They are often fixed or isophase, and sometimes in red or green. You steer so that the lights remain in line. If the rear light appears to the right of the front light, you are to the right of the line, and you steer left until they stack. In general, the rear light appears on the same side as you are off the line, so steer away from that side. Leading lights are used for narrow channels and harbour entrances, and in daylight leading marks (beacons, towers or painted boards) do the same job.

Always check the leading line against the compass bearing in your notes and the depth. If the line is true, the leading bearing should match the charted one.

Fog Signals, Racons and Electronic Marks

• Fog signals: some lighthouses and large buoys are fitted with a horn, bell, gong or whistle. The chart shows the type and the period (for example, Horn(2)30s). Use the sound only as a warning that a mark is near, since fog distorts direction. Never rely on hearing alone to find a mark, and note that many fog signals are only operated on request.
• Racon: a radar beacon that returns a Morse code letter on the radar screen when it is triggered by a ship's radar. It makes a buoy or lighthouse easy to find on radar. The chart shows Racon with a letter, such as Racon (T).
• AIS aids to navigation (AtoN): a real mark can broadcast its position on AIS, and a virtual mark can appear on a plotter with no physical buoy. Check that the chart plotter updated the mark and use your own lookout.
• Ramark and radar reflectors: a good radar reflector on your own boat matters. Many marks have radar reflectors, but in rough seas a small yacht's echo can be lost among clutter.

Marks by Day: Putting It Together

Use this order when you meet an unfamiliar mark by day: colour, shape, topmark, number or name.

1. Colour tells you the type: red or green is lateral, black and yellow cardinal, red and white vertical stripes safe water, black and red banded isolated danger, yellow special, blue and yellow wreck.
2. Shape then helps: can, cone, pillar, spar or sphere.
3. The topmark confirms: cone or can for a lateral, two cones for a cardinal, two spheres for isolated danger, one sphere for safe water, X for special, a cross for a wreck.
4. A number or name identifies the individual mark on the chart.

The practical value of tide on a buoy

Watch how the water streams past a buoy. A buoy leaning heavily and with a bow wave shows a strong stream, and the angle shows its direction. On a long leg across a tidal estuary, passing from buoy to buoy lets you estimate the set and the rate.

Second Worked Example: Day Pilotage with a Cardinal Mark

You are heading 270 degrees (T) in a yacht at 5 knots along a coast with a tidal stream setting south at 1.5 knots. A north cardinal appears ahead. The chart shows a shoal, the Ridge, extending 0.6 miles south of the mark, with a charted depth of 1.2 m at its shallowest and 4 m on the edge. Your yacht draws 1.7 m and the height of tide is 2.0 m.

• The mark has a north cardinal topmark and safe water lies to the north of it. You should pass to the north.
• The tide will set you south, towards the shoal side, at 1.5 knots. Within 10 minutes you are carried about 0.25 mile south of your track.
• The shoal edge: at a height of tide 2.0 m, the depth at the shallowest point is 1.2 + 2.0 = 3.2 m: still 1.5 m under the keel, but dropping as the tide falls. Plan to keep to the north.
• You set a course that allows for tide so that your track passes 2 cables north of the mark. In practice, aim up-tide of the mark (towards the north side) so the stream sets you onto the intended track. Check the echo sounder, and expect a minimum depth of about 6 m on the safe side.

What would you see at night? A white light, Q or VQ continuous. If the light had been Q(3) you would have an east cardinal and passed to the east.

Exam and Practical Tips

• Be able to draw each mark with colours and topmarks from memory, and state the light that goes with it.
• State which side to pass first, then the colour: "Pass south of a south cardinal, safe water is to the south."
• When asked about a light, give the colour, rhythm, period and what the mark means. For Fl(3)G.10s say "three green flashes every 10 seconds, a starboard-hand lateral mark in Region A."
• Use the almanac light list to identify an unnamed light from observed characteristics: look at the colour, the group and the period.
• Say what you will check before relying on any mark: the chart, depth and a second fix.

Using Buoyage Safely

• Buoys move. They can drag in storms, be dislodged by ships, or be repositioned as sandbanks shift. Lights fail. Check Notices to Mariners and local information, and confirm your position by other means.
• Never steer straight for a buoy, especially in a tidal stream. You can be set onto it. Aim off and pass at a sensible distance on the correct side.
• Watch the tide on the buoy. The way it leans and the water flowing past it show the strength and direction of the stream. This is very useful when crossing a channel.
• Shipping channels. Small craft can often sail just outside the line of lateral buoys, in shallower water, to keep clear of large vessels. Check the chart depth first.
• Racons and AIS. Some important marks have a racon, which shows a Morse letter on radar (for example, the code letter appears as a line of dashes behind the mark). Some broadcast an AIS position, and some are only "virtual" AIS marks with no physical buoy. Electronics are aids, not substitutes for a lookout.
• Unexpected or missing lights: stop and verify. An expected light that does not appear may mean you are not where you think you are.

Worked Example

Night entry to a Region A harbour. You are approaching from the south at 2200. Your pilotage notes say:

• Landfall buoy: safe water mark, LFl.10s
• Outer channel: Fl.G.5s (No 1, starboard), Fl.R.5s (No 2, port)
• Junction: Fl(2+1)R.10s (preferred channel to starboard)
• Shoal east of the channel marked by a west cardinal, Q(9)15s

What you see:

1. Dead ahead, a white light: on for about 2 seconds, then dark for about 8 seconds, repeating. This is LFl.10s, the landfall buoy. You head for it and pass it close but not directly over it.
2. Next you see a green light flashing once every 5 seconds fine on your starboard bow and a red light flashing once every 5 seconds to port. These are Nos 1 and 2. You steer to pass between them, green to starboard and red to port, because you are entering harbour.
3. A red light shows two flashes, a pause, and one flash, every 10 seconds. This is the junction mark, red with a green band. Treat it as a port-hand mark: keep it on your port side for the main channel. The main channel lies to its starboard side (your starboard as you enter).
4. To starboard you see a white light giving a burst of rapid flashes. You count nine, then darkness, total period 15 seconds. This is the west cardinal. Safe water lies to the west of it, so you keep west of it. Since you are heading north, it should stay on your starboard side.
5. At every step, the depth matches the chart and your bearings confirm the position.

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Common Mistakes

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• Using "red right returning" in Region A. In UK and European waters, red is on your port side when going in.
• Forgetting to reverse the rule when leaving harbour. Going out, red is to starboard.
• Confusing east and west cardinals at night. Three and nine flashes are easy to miscount in a seaway. Count several cycles and time the period: 10 seconds for Q(3), 15 seconds for Q(9).
• Passing a cardinal on the wrong side. The name tells you where the safe water is, not where the danger is.
• Passing close to an isolated danger mark because "you can pass on any side". The danger is directly below it.
• Ignoring yellow special marks. They may mark cables, firing ranges or no-anchoring zones.
• Steering directly at a buoy in a tidal stream and being set onto it.
• Trusting the buoy over everything else. Marks can be off station or unlit. Confirm with depth, bearings and GPS.

Summary

• IALA has six types of mark: lateral, cardinal, isolated danger, safe water, special and emergency wreck. Only lateral marks differ between Region A and Region B.
• In Region A going with the direction of buoyage (into harbour, or clockwise round Britain): red cans to port, green cones to starboard. Lights match the colour.
• Preferred channel marks: read the main colour as a normal lateral mark. Light Fl(2+1).
• Cardinal marks: black and yellow, two black cones, white Q/VQ lights. Pass on the side named. Cones point to the black. Flashes follow the clock: N continuous, E 3, S 6 plus long, W 9.
• Isolated danger: black and red, two black spheres, white Fl(2). Pass any side but not close.
• Safe water: red and white vertical stripes, red sphere, white Iso, Oc, LFl.10s or Mo(A).
• Special: yellow, yellow X, yellow light. Check the chart.
• Emergency wreck: blue and yellow vertical stripes, yellow cross, alternating blue and yellow light.
• Identify lights by colour, number of flashes and period. Confirm with bearings and depth.

Check Your Understanding

1. You are leaving a UK harbour. On which side should you leave a red can buoy?

Answer: On your starboard side. The direction of buoyage is into harbour, so going out everything is reversed. Red port-hand marks are passed on your starboard side.

1. A buoy is yellow over black with both cones pointing down. What is it, which way do you pass, and what light would it show?

Answer: A south cardinal. Safe water lies to the south, so pass south of it. Its light is white Q(6)+LFl every 15 seconds or VQ(6)+LFl every 10 seconds.

1. At night you see a white light giving three quick flashes every 10 seconds. What is it, and where is the safe water?

Answer: An east cardinal mark, Q(3)10s. Safe water lies to the east of the mark.

1. What does Fl(2)R.6s mean, and what kind of mark might carry it?

Answer: A red light flashing in groups of two, repeating every 6 seconds. It would be on a port-hand lateral mark in Region A.

1. Describe an isolated danger mark by day and by night. How should you pass it?

Answer: Black with one or more broad red horizontal bands and a topmark of two black spheres. At night it shows a white Fl(2). It sits on top of a small danger with navigable water all round. You may pass on any side, but should give it a good berth.

1. You approach a junction and see a green buoy with a broad red band. Which way is the preferred channel?

Answer: To port. Treat the buoy as a starboard-hand mark, keeping it on your starboard side, and the main channel will be on your port side.

1. What is the difference between an occulting and an isophase light?

Answer: Occulting: the light is on for longer than it is off (steady light with brief eclipses). Isophase: equal periods of light and darkness.

1. You see a buoy with blue and yellow vertical stripes and a yellow cross topmark. It is not on your chart. What is it and what should you do?

Answer: An emergency wreck marking buoy, marking a new, uncharted wreck. Give it a wide berth, treat it as a danger, and listen for navigational warnings (Securité broadcasts) for details.

1. Why is it unwise to steer directly at a buoy in a strong tidal stream?

Answer: The stream can set you onto it. Buoys are steel and heavy, and collisions can cause serious damage. Aim off up-tide and pass at a safe distance on the correct side, using the buoy's tide wake to judge the set.

1. Your pilotage plan expects a white LFl.10s light ahead, but after 10 minutes you have not seen it. What do you do?

Answer: Stop or slow down and verify your position by other means (depth compared with the chart, bearings of other lights, GPS). Check Notices to Mariners and local warnings for a light outage. Do not assume the mark is there and press on. You may not be where you think you are.

1. The light of a lighthouse is 25 m above sea level and your eye height is 4 m. What is the geographic range?

Answer: 2.03 x (square root of 4 + square root of 25) = 2.03 x (2 + 5) = 14.2 nautical miles. The visible range is the smaller of this and the luminous range, which depends on the light's power and the visibility.

1. You are steering to follow leading lights and the rear light appears to the right of the front light. What does this tell you and what should you do?

Answer: You are off the leading line, to the right of it. Alter course to port until the two lights are vertically in line, and check the compass bearing and depth against the chart.

1. A sector light shows white over the safe channel, with red to port and green to starboard as you enter. You see a red light. What does it mean?

Answer: You have moved out of the white sector into the red sector, which marks the danger on the port side of the safe line. Alter course to starboard until the light shows white, and check your depth and position.

1. What is the difference between nominal range and geographic range?

Answer: Nominal range is the luminous range in 10 miles meteorological visibility and depends on the brightness of the light. Geographic range is the distance at which the curvature of the Earth hides the light, and depends on the heights of the light and the observer's eye.