"Red Nun" -channel markers:

any of you power boaters or long time sea going navigators know how they got their names?

I have been curious ever since I’ve been paddling and couldn’t come up with the origin when I did a search.



a guess
is that they resemble the habits nuns used to wear. Green cans, on the other hand, got their name from a failed budweiser advertising campaign,

Nun habits and flat topped cans.


A Related Question

– Last Updated: Aug-26-09 11:27 AM EST –

Why the difference in shape? Both have the 'X' of sheet metal crossing the top, but on the red bouys that 'X' rises to a point. I would assme the sheet-metal 'X' is to reflect radar (a 90-degree inside bend always reflects wave energy straight back at the source), so do the two shapes show up differently on radar?

Sometimes it can
be difficult to discern colors on the water, so the shapes are different. Shouldn’t be much difference with regard to radar.

Book, Chapter 20

Here’s a book you may find interesting about channel design…it speaks to this.

difference in shape
The color red, the nun shape and even numbers on the cahnnel markers are all used on the same side of the channel (usually the right side when returning to port). So “red nun” is actually redundant - all nuns are red.

Cans are even numbered and usually grren. In areas with lots of sea birds, they may have white stripes,

The Red Nun style has the same
basic layout as a corner reflector, so it might return more energy than the can style at certain frequencies.

But I doubt there is an airborne or seaborne radar that cares about tracking the location of red and green buoys. It’s not very practical and would be pretty crude for navigation.

Any radar looking at the sea will get a lot of clutter from the waves and they will generally remove all returns with small velocities (low Doppler frequencies). Buoys and anything else with no velocity becomes invisible to the radar.

The exception is SAR mapping, but you’d be hard pressed to spot a buoy or something similar with most SAR maps. They are most often useful for mapping terrain and finding large, still targets like a building.


Check this out:

– Last Updated: Aug-26-09 5:14 PM EST –

I was trying to imagine a reason that BOTH the red and green bouys would have that right-angle cross of sheet metal, if it were for some reason other than radar reflection, so I googled the subject and here is the first hit I checked. These are exactly the bouys I was talking about in my previous post.


I see there are other radar reflectors that are more efficient (they not only correct for right-left variation in the direction of incidence, but for vertical tilting of the bouy as well), but this page gives a pretty strong implication that the crossed sheet metal at the top of both kinds of bouys really IS a radar reflector.

By the way, it's hard for me to imagine a modern radar system that is NOT able to differentiate between the the powerful return wave from a purpose-built reflector and the highly diffused reflections bouncing off from incidental objects, especially from sloping wave surfaces. I don't find it hard to believe at all that bouys show up on radar. Maybe 'Salty' will check in soon. He will know the answer.

Check this out too! Those lightweight, plastic-covered, foam-filled bouys that you see on small inland waterways are available with radar reflectors inside the foam!


Dopler Comment
You said

Any radar looking at the sea will get a lot of clutter from the waves and they will generally remove all returns with small velocities (low Doppler frequencies). Buoys and anything else with no velocity becomes invisible to the radar.

Are you saying that radar won’t display any signal with the velocity associated with a stationary object, or slower?

It seems to me that if navigation radar has such a doppler-specific setting, it would be turned off during channel navigation, because that’s a time when you would want all stationary objects to show up bright and clear, rather than being electronically erased!

Second, even on the open ocean, it seems that you’d want to see other ships which you were overtaking, but your statement implies that slower vessels that are dead-ahead and on the same heading would also be eliminated from the display, not to mention a ship that is dead in the water. If I had to guess, on the open ocean, the refection from a ship would be so much more powerful than that of the surrounding waves that the radar operator himself could make the distinction.

Once again, we need to hear from Salty!

They look like ships
on marine radar. They have a huge echo and show up very well on the radar. That’s why the top is shaped the way it is. Don’t know about airplanes or satellite radar. :slight_smile:

thanks, ST!
Great resource, thanks!

Some info…take or leave
Cans are ODD numbered, NOT even numbered. Nuns are even. The ATONS are such so as to be differentiated by number, shape, and color. Daymarks in green are square and odd numbered whereas Daymarks in red are a triangle and even numbered.

Their signal on radar is not associated with their shape, but some markers have a definitive signal that shows a certain way on radar. These often found in traffic lanes where there are course changes.

A bit about radar: Sea kayaks barely show on the big expensive radar on the 100ft. vessel I captain for work. Barely if at all if there’s any weather or wave clutter. There’s been much said about this before so I won’t repeat it all other than to say “NEVER” assume you will show up on radar, even sophisticated ship radar. Be extrememly cautious in fog in areas of regular traffic. DON’T paddle in these areas in heavy fog as it’s roulette.

Safe paddling.

It does appear that some are designed to be picked up by radar, but again, they don’t appear to have distinct returns that are useful for navigation. You simply know that there is a buoy in a particular place and need to visually identify which type it is.


This is basic clutter rejection
that is on board every sea-based and airborne radar. When looking at the earth’s surface, you have significant returns from the objects with near zero velocities (relative to the surface). Most of the time, they are not interested in stationary objects, they are interested in planes, missiles, and other moving vehicles. When you filter out the low frequency content, you are left with only the moving objects.

This is not the case when you look above the horizon. You can then look for objects that are stationary or moving cross range (no closing velocity).

Modern clutter rejection techniques are performed with DSP chips so it’s simple to turn it on and off. Yes, you could see a stationary ship with the background clutter because it has a very large RCS. In the right conditions you can see the metal buoy and even more so the buoy with the corner reflector. As the waves get larger and the radar beam gets wider, it gets more difficult to see a particular object and at some point it will drop down into the noise.

It’s the same basic effect as turning on your car radio to where you can just barely hear it while idling in your car. As you start down the interstate the engine noise and road noise increase until you can’t hear it at all.


more good stuuf thanks salty
Your input on this topic is always good.

Radar Navigation

– Last Updated: Aug-29-09 12:03 AM EST –

Apparently that reflector works very well, according to some stuff I've read since, and as stated in the post by "makinwaves" below. If two parallel rows of very bright blips show up on the radar screen, I can't see why visual verification is needed. If you can see a row of bouys on both sides of the channel, mapped out in two-dimensional space on the radar screen, it couldn't be too hard to figure out which side of the channel is the left and which is the right. Tugboats on the Mississippi can see bouys on their radar quite clearly, and I believe they can travel in fog if they have to, though I think they usually tie-up instead.

Navigation by Radar

– Last Updated: Aug-29-09 12:18 AM EST –

That's all well and good, but just how far are ships in narrow channels looking ahead with their radar, and just how rough are the seas in places where they must follow a channel that probably even requires periodic dredging to keep it open. Channel bouys are not an open-ocean navigation aid, and I still seriously doubt that the problems associated with aircraft radar are much of an issue when you are going at the speed of a barge on the Mississippi or a large ship within the confines of a channel so narrow that marker bouys are required. Remember that the bouys they are looking at on radar would be within easy visual sight distance during daylight hours. Radar just lets these slow-moving vessels locate bouys during times when lack of visibility makes it necessary.

In short, I still don't buy your original statement that standard clutter-reduction makes radar-location of bouys impractical, and I don't think there is any practical relationship between the needs of aircraft radar looking down at the earth when going 500 mph and ship radar looking for reflector bouys that might only be between 1/10th and 2 miles away or so when traveling about 10 mph.