Aids to Navigation - Buoys and Beacons

ATON’s - IALA Buoys and Beacons

Buoys are floating markers that are anchored to the sea bottom with solid concrete blocks, most commonly referred to as “sinkers.”

Beacons are structures that are permanently fixed either to land or to the sea bottom. The most common recognized are single piling type structures, but can also be more complex such as those mounted on the end of breakwaters or built on off lying dangers.

There are basically (6) types of buoys or beacons found under both the IALA-A or IALA-B Buoyage Systems:

While some lists show only (5) different types of buoys or beacons, incorporating "Safe Water Marks" into the Lateral Marks category, I have kept them separated. "Safe Water Marks," in essence have no lateral significance. In other words safe water marks can be safely passed on any side in any direction or circled endlessly without endangering your vessel.

Identifying the Mark

During the hours of daylight, a buoy or beacon’s identification can be easily determined by observation of a number of factors including:

  1. Markings
  2. Shape
  3. Color/s of the NavAid
  4. Topmark
  5. Special Features – such as sound signals (whistle, gong, horn, etc.), and RACON’s or AIS Transponders, if your vessel is so equipped.

During the periods of darkness we have much less information available to rely on in identifying any particular ATON. Namely:

The buoy or beacon’s lighting, if lighted, including:

The (3) items above, when taken together, are often referred to as the "light’s characteristics" and play a large part in identifying any Aid to Navigation at night.

Some Special Features may also be available to aid in identification, such as sound signals, AIS Transponders or RACONS; again, if your vessel is equipped to receive them.

Aids in Identification

Markings - Depending on the type of mark, it may be lettered, named, or numbered and in many cases you may see a combination of both numbers and letters.

Shapes - There are five basic buoy shapes (see below): can, nun, spherical, pillar, and spar. With the exception of pillar and spar buoys, the shape of the buoy indicates the correct side on which to pass.

Regardless of the IALA Region you are sailing in, when referencing lateral marks, can (or cylinder) buoys will always mark the port side of a channel when entering from seaward. Likewise nun (or conical) buoys will always mark the starboard side. The colors will be different between IALA-A and IALA-B, but the shapes will always remain the same.

Can buoys are often referred to as cylindrical, and nun buoys as conical. The term pillar is used to describe any buoy that is smaller than a large navigational buoy (LNB). Lighted or sound producing buoys in the United States are often referred to as "Combination Buoys," but the correct term is "Pillar Buoy."

ATON Shapes

Colors - Aids to Navigation can be solid colors or they may be horizontally or vertically striped of different colors. Regardless, there are only (6) colors used; Red, Green, Yellow, Blue, White, and Black.

ATON Colors

Topmarks - Five topmarks are used in the IALA buoyage system and consist of Cylinders, Spheres, X’s, Crosses, and Cones. When beacons are used as lateral marks, the cone topmark is often replaced with a triangle and the cylinder is often replaced with a square. The cylinder topmark may often resemble a rectangle or square and the cone topmark may resemble a triangle, especially at distances.

ATON Topmarks

Light Characteristics - If lighted, as mentioned before, a lights characteristics are comprised of (3) items; the lights color, the lights phase, and the lights period.

Light’s Color - There are only (5) light colors authorized on marine ATON’s; Red, Green, Yellow, Blue, and White. Red, Green, and White are the colors most often encountered by the mariner. Yellow is seen regularly and Blue rarely.

ATON Light Colors

Light’s Phase - This is probably the most complex and confusing of any of a light’s characteristics. There are (6) primary light phases:


Morse code

The problem is that within the primary phase categories are many sub-categories and that is where the problem lies. Let us take a look at the Flashing light sub-categories, you have:

Single Flashing
Group Flashing
Composite Group Flashing
Long Flashing
Quick Flashing
Group Quick Flashing

Interrupted Quick Flashing
Very Quick Flashing
Group Very Quick Flashing
Interrupted Very Quick Flashing
Ultra-Quick Flashing
Interrupted Ultra-Quick Flashing

And that’s just "one" of the six primary categories of light phases!

Even the most seasoned mariner is going to have trouble remembering all of these, especially some of the more infrequently used light phases.

Light’s Period - This is simply the time, measured in seconds, in which the light’s phase repeats itself.


There are a number of publications available to help in identifying Aids to Navigation and their characteristics, including: The USCG Light List, NGA List of Lights, and Chart #1. These are available and can be downloaded from our website or if you prefer hard copies; they can be found through the many links on our pages. Regardless whether you prefer electronic copies or hard copies the above listed publications should be carried on your vessel as part of your navigation library.

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