Special marks
Special marks show an area or feature whose nature you can look up on the chart or in other nautical publications — a spoil ground, a cable, an exercise area, a moving hazard. They are the same in Region A and Region B. Summarised in our own words from IALA Recommendation R1001.
A mark for a special area or feature whose nature you can find from the chart or another nautical publication. R1001 says special marks are not generally meant to mark channels or obstructions where the system already has a suitable mark.
Special marks are common to Region A and Region B.
Among R1001's examples: Ocean Data Acquisition Systems (ODAS), spoil grounds, military exercise zones, cables and pipelines, recreation zones and aquaculture — and moving objects or hazards (see MAtoN below).
Mooring buoys are not usually counted as aids to navigation, but where one needs to be more conspicuous — in or right beside a channel, for example — R1001 says it should be marked as a special mark.
An important use of special marks: a channel or area of interest to one class of vessel, such as a channel dredged for deep-draught ships through water that is already deep enough for most. The limits of safe navigation for vessels in general stay marked by lateral (or cardinal) marks; the special channel is marked by special marks with the appropriate daymark shape.
Yellow.
A single yellow 'X' — a diagonal cross — preferably three-dimensional so that it can be seen from all directions. Where weather or ice make a topmark impractical, the authority may decide not to fit one. Special marks may also be lettered or numbered and carry a pictogram, using IHO symbols where appropriate.
Optional, as long as it doesn't conflict with lateral marks. R1001's example: a special mark on the port-hand side of a channel may be cylindrical but will not be conical.
As we read it: beside a channel, a special mark may share the shape of the lateral marks on its side, but never takes the shape of the other side's.
If one is fitted: yellow, with any rhythm other than those reserved for cardinal, isolated danger, MAtoN and safe water marks. To avoid yellow being mistaken for white in poor visibility, a special mark's yellow light does not use any of the rhythms of the white lights. It may also carry an AIS AtoN or a racon.
A Mobile Aid to Navigation: a special mark attached to a floating object that may be a collision hazard. It is yellow, and its shape is optional so long as it doesn't conflict with lateral marks. If practical, it should carry the special mark's yellow 'X' topmark and a radar reflector, to make it easier to see and to detect by radar.
For a low-lying or partly submerged hazard watched mainly from the air, an extra mark such as a circular symbol may be added if practicable. R1001 also points to IALA Recommendation R1016, which asks authorities to liaise with vessel traffic services before a MAtoN is deployed in a VTS area.
Its own defined character, which an ordinary special mark may not use: three flickering flashes, then two ordinary flashes. Each flicker and each flash lasts 1 second, the flickers at 5 Hz, and the dark gaps after them are 0.7, 0.7, 0.5, 0.5 and 3 seconds.
Examples of the kind of question to be ready for (our wording, not the MCA's):
“You see a yellow buoy with an X topmark. What is it, and how do you find out what it marks?”
“Why don't special marks use the same light rhythms as white lights?”
“A special mark sits on the port-hand side of a channel. What shape can it be, and what can it not be?”
“What is a MAtoN, and what does its light look like?”
The common slips: confusing the special mark's diagonal 'X' with the emergency wreck mark's upright cross, and treating a special mark as a channel mark.
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