IALA buoyage · Special marks
Special mark
Identical in A and B
All yellow with a yellow ✕ topmark: marks a special area or feature — check the chart for what and which side to pass.
Fl Y 4sOne flash every 4 s — dark longer than light. Colour: yellow.One flash every 4 s — dark longer than light. Colour: yellow.
Pillar buoy; solid colour: yellow; topmark: a yellow X. Light: Fl Y 4s.
What it means
Indicates a special area or feature rather than a side of the channel: anchorages, spoil grounds, cables and pipelines, military exercise zones, recreation zones, racing marks, data-collection (ODAS) buoys. Identical in both IALA regions.
What you do
The mark itself does not tell you which side to pass — consult the chart for the area it protects, and keep clear of any activity it marks.
Memory aid
Yellow with a yellow ✕: “X marks the special spot — go check the chart.” Its light is always yellow, in any rhythm not used by white lights.
Source: IALA Maritime Buoyage System — USCG Navigation Center, U.S. Aids to Navigation System