IALA buoyage explained

Lateral marks: the two edges of a channel

Lateral marks only make sense with a direction of buoyage. Coming in from seaward to a harbour, river or estuary, you are going with it. Elsewhere the authorities set the direction, in principle clockwise round land masses, and the chart shows it.

Going with the direction of buoyage in Region A, keep red cans to port and green cones to starboard. A port mark is a red can, pillar or spar, with a red can topmark if it has one, and any red light rhythm except Fl(2+1). A starboard mark is green, cone-shaped (or a pillar or spar) with a cone topmark pointing up, and shows a green light. Leaving harbour, it all reverses. Lateral marks are numbered from seaward: even numbers on the red marks, odd on the green.

Preferred channel marks

Where a channel divides, a lateral mark with a broad band of the other colour shows the main route. Red with one broad green band (a can, red topmark) means the preferred channel is to starboard, so keep it to port. Green with a broad red band (a cone, green topmark) means the preferred channel is to port. Both show Fl(2+1) in their main colour.

Cardinal marks: which side is safe

A cardinal mark stands in one of the four quadrants round a danger, north, east, south or west of it. The safe water is on the named side: pass north of a north cardinal, west of a west cardinal.

Read the two black cones on top first, because the cones point to where the black is. Both up: north, black over yellow. Both down: south, yellow over black. Points apart: east, black with a yellow band. Points together: west, yellow with a black band.

At night every cardinal shows a white light, quick (Q, about 60 flashes a minute) or very quick (VQ, about 120). The flashes follow a clock face: north flashes continuously, east in threes, south in sixes, west in nines. South adds one long flash after its six so that, in a swell, it can't be mistaken for a west.

Isolated danger, safe water and special marks

• Isolated danger: on or near a danger that has navigable water all round. Black with one or more broad red bands, two black spheres on top, white light in groups of two, Fl(2).
• Safe water: navigable water all round, such as a fairway, mid-channel or landfall mark. Red and white vertical stripes, a single red sphere, and a white light that is isophase, occulting, one long flash every 10 seconds, or Morse "A".
• Special mark: yellow, with an optional yellow X on top and a yellow light whose rhythm is not one used by the white lights.
• Emergency wreck marking buoy: blue and yellow vertical stripes in equal number, a yellow upright cross, and an alternating blue and yellow light. It marks a new danger that is not yet on the charts, so treat it as a hazard.

The full lesson is Module 06: IALA A buoyage, with a narrated film, a blinking-light drill and a 10-question quiz.