Aids To Navigation

Chris Riley by Chris Riley Updated on June 28, 2020. In nauticalknowhow

Boat Safe is a community supported site. We may earn commission from links on this page, but we have confidence in all recommended products.

Chapter VI – Operations

Section 3 – Aids To Navigation

Aids To Navigation

In order to find our way safely from place to place on the water we must depend on road signs just as we do on land. The aids to navigation are the road signs of the water. Learning to understand them requires experience and practice. However, the benefit to be derived in being able to navigate safely by using the system is well worth the effort.

There are two systems of marking the waterways in the United States; the U.S Aids to Navigation System (ATONS) which includes the Western River System, and the Uniform State Waterway Marking System. Both systems are described below. The Uniform State Waterway Marking System is being phased out in favor of the U.S. ATONS. This change will be gradual and should be complete by 2003. In the meantime, you may see either system in use on state waterways.

U.S. Aids to Navigation (U.S. ATONS)

The buoys and beacons in this system conform to the International Association of Lighthouse Authorities (IALA) guidelines and are located in IALA region B. They are sometimes referred to as the IALA-B system. In this system, there are lateral and non-lateral markers. The lateral markers indicate the navigable channel by their position, shape, coloring, numbering and light characteristics. The non-lateral markers are informational and regulatory markers.

To navigate safely using the lateral markers, you should pass between the red and green. Returning from sea, the red markers are on your right (red, right, returning) and the green are on your left.

boating safety course navigation aids graphic

Lateral Buoys and waterway markers

In the International system, navigation aids mark the edges of channels to tell which way open water is. They are called day beacons if unlighted, lights if lighted at night, or buoys if they are floating. Some buoys are also lighted for identification at night.

“Red, Right, Returning” tells you to leave the red markers to your right, or starboard, when returning from sea. The green markers are then left on your port side and between is the channel. Be sure to look behind you when navigating a narrow channel to make sure you are not being pushed out by wind or current.

Floating Red markers are called nuns and are triangular in shape. They are numbered with even numbers. Floating Green markers, on the other hand, are called cans and are square or shaped like a large can and carry odd numbers.

boating safety course navigation aids graphic

Preferred channel markers or junction markers indicate the preferred channel at points where the waterway splits or branches off in another direction. If you are returning from sea and see a preferred channel marker that is red over green, to stay in the main channel you would consider the marker as if it were a red marker and keep it on your right. However, if your intended destination is the waterway branch consider the marker a green marker and leave to your port side. (See example below.)

boating safety course navigation aids graphic


boating safety course navigation chart graphicWhen looking at a chart you read the markers as follows:


boating safety course navigation aidboating safety course navigation aidboating safety course navigation aid The first marker you see entering from sea is the open, or safe water, marker. The RW “G” means that it is a Red and White striped marker with, in this case, the letter “G” on it.
boating safety course navigation aid A lighted marker flashes at night and during restricted visibility. The next marker you see as you enter this waterway is the G “1” Fl G 4 sec. This tells you it is Green, has the number 1 on it, and it Flashes Green once each 4 seconds.
boating safety course navigation aidboating safety course navigation aid R N “2” plus the red symbol indicates a Red Nun buoy with the even number 2 on it. Conversely G C “3”, is a Green Can buoy with the odd number 3 on it.
boating safety course navigation aid When entering from seaward you may find two channels which separate and go in different directions. A junction marker referred to as RGN “C” uses its top color to show you the major or primary channel. This is represented by a Red and Green Nun buoy with (in this case) a “C” on it. You use the top color and shape as if it were a stand alone marker if you want to follow the primary channel. In this case leave the Red Nun to the right. Otherwise use the bottom color.


Intracoastal Waterway System

The Intracoastal Waterway (ICW) runs parallel to the Atlantic and Gulf Coasts from Manasquan Inlet on the New Jersey shore to Brownsville, Texas at the Mexican border. Aids to navigation marking these waterways have some portion of them marked with yellow. Otherwise, the colors and numbering of buoys and day marks, and lights follow the same system as that observed in all other waterways.

Red, right, returning also applies to the ICW markers. However, how do you know which way is returning? Generally south, from New Jersey to Brownsville, Texas is considered returning. So, going generally south on the ICW you would have red right. There are some areas of the ICW such as the West Coast of Florida where you are actually going north. Still, if you are going from New Jersey to Texas you are returning.

Aids to navigation marking the Intracoastal Waterway (ICW) display unique yellow symbols to distinguish them from aids marking other waters. Yellow triangles indicate aids should be passed by keeping them on the starboard side of the vessel. Yellow squares indicate aids should be passed by keeping them on the port side of the vessel. A yellow horizontal band has no lateral significance, but simply identifies aids as marking the ICW.

boating safety course navigation aids in the intracoastal
Dual Purpose Markings

Red Buoy with yellow square — marks left side of channel when traveling the intracoastal waterway (south), but right side of channel when coming in from sea.

Green Buoy with yellow triangle — marks right side of channel when traveling the intracoastal waterway (south), but left side of channel when coming in from sea.

Red Buoy with yellow triangle — marks right side of channel when traveling the intracoastal waterway (south), and right side of channel when coming in from sea.

Green Buoy with yellow square — marks left side of the channel when traveling the intracoastal waterway (south), and left side of channel when coming in from sea.

Non-Lateral Informational and Regulatory Markers

Regulatory or informational markers are used to advise you of situations, dangers, or directions. They may indicate shoals, swim areas, speed zones, etc. They can be easily identified by the orange bands on the top and bottom of each buoy.


boating safety course navigation aid
Boat Exclusion Area:
Explanation may be placed outside the crossed diamond shape, such as dam, rapids,
swim area, etc.

boating safety course navigation aid
The nature of danger may be indicated inside the diamond shape, such as rock, wreck, shoal, dam, etc.

boating safety course navigation aid
Controlled Area:
Type of control is indicated in the circle, such as slow, no wake, anchoring, etc.

boating safety course navigation aid
For displaying information
such as directions, distances, locations, etc.

boating safety course navigation aid
Buoy used to display
regulatory markers.
boating safety course navigation aid
May show white light
May be lettered


Other Non-Lateral Markers

Safe Water Marker: These are white with red vertical stripes and indicate unobstructed water on all sides. They mark mid-channels or fairways and may be passed on either side.

Inland Waters Obstruction Marker: These are white with black vertical stripes and indicate an obstruction to navigation. You should not pass between these buoys and the shore.

Mooring Buoys: These are white with a blue horizontal band. They are usually placed in marinas and other areas where vessels are allowed to anchor. These are the only buoys you may legally tie up to.

Western Waterway Marking System

Found only on the Mississippi River and its tributaries as defined by the U.S. Coast Guard in Rule 3 of the COLREGs. These aids to navigation are not numbered as in IALA-B system. Numbers indicate mileage from a fixed point. Lights on green aids show a single-flash which may be green or white. Lights on red aids show a group-flash which may be red or white. Diamond-shaped crossing dayboards, red or green as appropriate, are used to indicate where the river channel crosses one bank to the other.

boating safety course navigation aid

Uniform State Waterway Marking System (USWMS)

On bodies of water wholly within state boundaries and not navigable to the sea, each state is responsible for establishing and maintaining aids to navigation. A uniform system of aids and regulatory markers has been agreed to by all states

Some informational/warning flags you might see include:

Divers’ Flag indicates that scuba divers or snorkelers are operating in the area. Stay well clear of this flag. Many states have specific distance-away requirements from 100 to 300 feet. Check your state regulations.

Rigid Alpha Flag is flown by a vessel engaged in diving operations. This flag does not substitute for the diver-down flag above in states that require the diver-down flag. You will usually see both flags in use. The alpha flag indicates that the vessel is restricted in ability to maneuver, in this case due to the nature of its work.

What are all those red and green things?

1. 1. Preferred Channel Marker , Red and Green Nun labeled with the letter “C” in this example . Because top color is red you would treat the marker as a red nun if you want to take the preferred channel.
2. 2. Lighted MOA (morse code alpha) fairway or safe water marker labeled with the letter G in this example. This marks center of channel and can be passed on either side. Light will flash morse code alpha one short and one long (di dah)
3. 3. USWMS regulatory buoy . In a danger area the danger would be printed in the diamond.
4. 4. Lighted green ” Can Buoy “, port side channel marker returning from the sea. Flashing Green light every 4 seconds. Labeled with the number 1 in this example.
5. 5. Unlighted ” Nun Buoy ” starboard channel marker when returning from the sea. Labeled with the number 2 in this example.
6. 6. Unlighted Can Buoy marks the left side of the channel returning from sea. Labeled with the number 3 in this example.

About Chris

Outdoors, I’m in my element, especially in the water. I know the importance of being geared up for anything. I do the deep digital dive, researching gear, boats and knowhow and love keeping my readership at the helm of their passions.



  • Rudy Shappee on May 7, 2020

    What does a lighted nun buoy look like?


  • W f Jones on September 6, 2020

    What does a square green sign with the letters in NS mean?


  • Dalton Bourne on August 22, 2022

    If you encounter a red buoy and a green buoy on your way back to the home port from seaward, your boat is entering a channel. Both red buoys and green buoys are known as lateral markers and assist you in determining which side you should pass on to stay in a channel: Based on general guidelines, regardless of whether you are moving from waterways to inland water or a river’s upstream in a river, keep the red buoys to your right and the green markers to your left.


Leave a Reply

Your email address will not be published. Required fields are marked *

More in nauticalknowhow

How to Tie a Boat to a Dock

How to Clean a Boat Cover

Everything You Need to Know About Your Boat’s Bilge Pump

Rockport Perth Mens Boat Shoes

4 Ways to Tie Your Boat Shoes

All content is © Copyright 2020. All rights reserved.