IALA Buoyage System and Light Characters

The IALA system and its two regions

In short: one worldwide system, six kinds of mark, two regions that differ only in the colours of the lateral marks.

Start here. This is the first topic of the Buoyage area. Read it before Lateral Marks and Preferred Channel, Cardinal Marks, Isolated Danger, Safe Water and Special Marks, and Emergency Wreck Marking Buoy.

• IALA is the international body of lighthouse and aids-to-navigation authorities. Its Maritime Buoyage System is set out in IALA Recommendation R1001 (Edition 2.0, revised 2023).
• SOLAS V/13 asks governments to take IALA recommendations into account when they set up aids to navigation.
• The UK introduced System A in 1977 (Trinity House). Systems A and B were merged into one system with two regions in 1980.
• On the bridge, the reference book is NP735 IALA Maritime Buoyage System (UKHO). Use it to check any mark you are unsure of. It sits with the recommended, not compulsory, bridge publications.

Six kinds of mark

1. Lateral marks — port and starboard sides of a channel (plus preferred channel marks).
2. Cardinal marks — where the safe water is: N, E, S or W of the mark.
3. Isolated danger mark — a small danger with navigable water all round.
4. Safe water mark — navigable water all round: landfall, fairway or mid-channel.
5. Special marks — an area or feature, not a navigation danger (cables, pipelines, recreation zones, ODAS, firing ranges).
6. Emergency wreck marking buoy — a new wreck not yet on the charts.

By day you identify a mark by colour, shape and topmark. By night, by light colour and rhythm. The system is built so a mark can still be identified if its light fails or its topmark is lost.

Region A and Region B

• Same in both regions: the shapes of lateral marks (can to port, cone to starboard) and every other kind of mark.
• The only difference: lateral colours, and so lateral light colours, are reversed.
• On the bridge: before arriving in a Region B country (for example a yacht crossing from the Med to Florida), brief the bridge team: from now on, red marks are left to starboard when entering.

Direction of buoyage

In short: lateral marks only make sense with a direction. Find it on the chart, then leave the marks on the correct side.

• Port and starboard lateral marks are named as seen by a ship going with the direction of buoyage (IALA R1001).
• Local direction: the direction a ship takes when approaching a harbour, river or estuary from seaward.
• General direction: elsewhere it is set by the authorities, generally clockwise round land masses.
• Where to find it: on the chart, shown by a magenta arrow symbol where the direction is not obvious, and in sailing directions.
• Why it matters: it tells you which side to leave every lateral mark. Get it wrong and you leave the marks on the wrong side, straight onto the danger they mark.
• On the bridge: when you leave harbour you are going against the buoyage. In Region A the red marks are then on your starboard side.

Light characters — reading the rhythm

In short: every light is described by its colour, its rhythm and its period. Learn the abbreviations once and you can read any chart.

• Colour: no letter = white. R = red, G = green, Y = yellow, Bu = blue.
• Period: the time in seconds from the start of one sequence to the start of the next. “Fl(2)R.6s” = 2 red flashes, repeating every 6 s.
• Lighthouses add the height of the light and its nominal range, e.g. “Fl(3)10s 25m 18M”. Buoys usually show only colour, rhythm and period.
• On the bridge at night: note the colour first, then count the flashes and time the period with a stopwatch. Only then compare with the chart. Never “make the light fit” what you expect.

The light of every mark

In short: the colour tells you the family — red or green = lateral, white = cardinal, isolated danger or safe water, yellow = special, blue and yellow = new wreck. The rhythm tells you which mark.

North and South are easy to swap. Some revision material has written North as “6 + 1 long” and South as “6”. That is wrong. IALA: North = continuous quick or very quick, South = 6 + one long flash. The long flash is there so you can tell the 6 apart from the 3 and the 9 when counting fast flashes.

• Lateral lights: “continuous or flashing” — any rhythm the authority chooses, except the 2+1 reserved for preferred channel marks.
• Safe water: revision guides often give only one or two rhythms (one says LFl 10s; another says Mo(A), Iso or Oc). IALA allows all four — learn all four. A safe water mark can be passed on either side; a common habit is to keep it on your port hand, so you stay on the starboard side of the fairway.
• Special marks: the yellow light uses a rhythm that cannot be mistaken for a white mark. Uses — mnemonic CROFTS: channel, recreation zones, ODAS, firing ranges, submerged cables (and pipelines).
• Wreck marks: blue is the colour only this mark uses. Full detail in the Emergency Wreck Marking Buoy topic.

Recognising marks on a paper chart and on ECDIS

In short: the MCA syllabus asks you to recognise IALA A and B marks and state their meaning on both a paper chart and an ECDIS. The mark is the same; only the symbol changes.

Paper chart

• The buoy symbol is drawn in its shape (can, cone, pillar, spar, sphere), with the topmark on top.
• Colour letters under the symbol read top to bottom: BY = black over yellow (North cardinal) · YB = South · BYB = East · YBY = West · BRB = isolated danger · RW = safe water · Y = special.
• Cardinal colours follow the topmark cones: the black band is where the cone points go (both up = black on top; both down = black at the bottom).
• A lit buoy carries a magenta light flare, with the light character written beside it, e.g. “Q(6)+LFl15s” or “Fl(2)R.6s”.
• Symbols and abbreviations are listed in the chart key, INT 1 (section Q for buoys and beacons).

ECDIS

• An ENC can show buoys with “paper chart” (traditional) symbols or “simplified” symbols. The operator chooses; the meaning does not change.
• Lights are shown with light flares. To read the full character (colour, rhythm, period), pick the object with the cursor.
• Do not assume the region: read the colour of the lateral mark on the chart, then apply the direction of buoyage.
• On the bridge: if a mark on the water does not match the chart or ECDIS, slow down, fix your position by other means and check NP735 and the latest Notices to Mariners.