Can buoys are green, cylindrical navigation markers placed on the left (port) side of a waterway as you travel upstream. They carry odd numbers, and their flat-topped drum shape is the key visual feature that distinguishes them from the cone-topped “nun” buoys on the opposite side. If you’re studying for a boating safety exam, those three facts (green, odd-numbered, port side) are the essentials. But there’s more to know about how these markers work in practice.
Shape, Color, and How to Spot Them
A can buoy looks like a cylinder or drum floating on its axis, with a flat top. The name “can” comes directly from this shape. They must be at least 9 inches in diameter, though most navigation buoys in open waterways are considerably larger for visibility at distance.
In the United States and other IALA Region B countries, lateral can buoys are green. This is the opposite of IALA Region A (used in Europe, Africa, and most of Asia), where port-side markers are red. The color-and-shape pairing helps boaters identify them even in poor visibility or at a distance where numbers aren’t readable. If you can see a flat top, you know it’s a port-side marker without needing to read anything on it.
Odd Numbers and the Upstream Rule
Can buoys always carry odd numbers: 1, 3, 5, 7, and so on. Red nun buoys on the starboard (right) side carry even numbers. Both sets increase as you travel upstream, so if you pass green can “3” and then green can “5,” you’re heading in the upstream direction. This numbering system gives you a constant check on your position and direction of travel.
“Upstream” can be confusing in coastal areas where there’s no obvious river current. The U.S. Coast Guard defines the conventional direction of buoyage as moving from the open sea into a harbor, or traveling clockwise around a landmass. So when you’re entering a port from the ocean, green cans should be on your left.
The “Red Right Returning” Memory Aid
The classic mnemonic “red, right, returning” tells you to keep red nun buoys on your right when returning from sea. Can buoys are the mirror image of that rule: keep green cans on your left when returning. If you’re heading out to sea, the system flips, and green cans will be on your right side. The buoy numbers will decrease as you move seaward, giving you a second confirmation of direction.
Can Buoys vs. Nun Buoys
The two buoy types form a matched pair in the lateral marking system. Here’s what separates them:
- Can buoys: Cylindrical with a flat top, green, odd-numbered, mark the port (left) side when heading upstream.
- Nun buoys: Cylindrical with a cone-shaped top (pointed or slightly rounded), red, even-numbered, mark the starboard (right) side when heading upstream.
The shape difference exists so boaters can tell them apart even when color is hard to judge, such as at dusk or in fog. A flat top means port. A pointed top means starboard.
Lighting and Visibility at Night
Not all can buoys are lit, but when they are, the light matches the buoy’s color. Green can buoys display green lights. The flash pattern can vary, but it won’t use composite group flashing (a distinctive 2+1 pattern), which is reserved for other marker types. Some modern buoys use solar-powered lanterns that charge during the day and flash through the night.
Increasingly, navigation buoys of all types are being fitted with AIS (Automatic Identification System) transmitters. These broadcast the buoy’s position and purpose to any vessel with an AIS receiver, allowing boaters to identify a port or starboard marker on their electronics before it’s even visible on the water or detectable on radar.
Regulatory Can Buoys Are Different
There’s a second type of can-shaped buoy that serves a completely different purpose. Regulatory can buoys are white with orange bands at the top and bottom, and they carry orange symbols with black text to communicate rules: speed limits, no-wake zones, restricted areas, or hazard warnings. If a regulatory can buoy is lit, it shows a white light rather than green. These are common on inland lakes and rivers managed by state agencies. The cylindrical shape is the same, but the white-and-orange color scheme immediately tells you it’s conveying information rather than marking a channel edge.
What They’re Made Of
Modern can buoys are built from a range of materials depending on where they’re deployed. Inland and nearshore buoys are typically made from polyethylene plastic or closed-cell foam coated with polyurethane to resist UV damage and saltwater corrosion. These foam buoys generally last 5 to 10 years. Offshore buoys in harsher conditions often use steel or stainless steel construction, which can withstand heavy seas and last 15 to 20 years, though they require anti-corrosion coatings to prevent rust. Cheaper plastic buoys used in sheltered waters may only last 2 to 3 years before needing replacement.
The interior of foam buoys is designed to keep them afloat even if the outer shell is damaged. Closed-cell foam doesn’t absorb water, so a cracked buoy won’t sink, an important safety feature for markers that take regular hits from waves, ice, and the occasional boat hull.