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Understanding Aids to Navigation

Know how to steer your boat by interpreting buoys and markers


Green can

Green can - contemporary shape

"Aids to navigation" is the official term for the wide range of buoys and markers positioned in navigable waters to assist boaters when navigating. It is important to understand different kinds of aids to navigation, and their meanings, to avoid hazards and remain in the safety of a channel or deep water.

In the U.S., the Coast Guard places and maintains navigational aids according to a systematic federal system. That system is described here. In the waters of other countries, sailors often must learn other systems or variations.

The Lateral System

Most important is the understanding that in the lateral system used in the U.S., buoys and other markers are positioned (and color coded) to the side of shallow water or obstructions. Beginning boaters sometimes confuse such markers with waypoints, the term used on chartplotters and navigational apps to indicate a point to which the boat plans to move. In other words, when navigating a channel, for example, you go between navigational aids marking both sides of the channel - you do not go from one marker to another as you would with waypoints entered into your navigational system.

Theoretically, as long as you stay on the correct side of a buoy or other navigational aid, you could try to navigate from one buoy to the next, but that could easily get you in trouble if a current or the wind affected your position. Far safer is the understanding that you should try to stay well inside the channel whose edges are marked by the navigational aids.

Channel Markers

Channel markers are a common type of navigational aid and the easiest for new boaters to understand. Imagine a channel coming into a harbor from open water, with rocks and sandbars on both sides of the channel. In the U.S., as you approach the harbor, the aids on your starboard (right) side will be red, and those on your port (left) side will be green. Hence the easily remembered saying "red right returning" - keep the red ones on your right side as you return into the harbor.

The most common types of navigational aids are:

  • Cans - which look just like giant tin cans floating upright - usually but not always painted green
  • Nuns - look like a can in which the top half tapers in a cone shape almost to a point
  • Lighted buoys - floating buoyed structures usually with a red or green flashing light on the top - sometimes also with an audible signal such as a bell or whistle
  • Daymarks - which look like signs at the top of posts (red triangles or green squares)

The shape of the aid is important because in the dark or conditions of reduced visibility, or from some distance away, it can be difficult to judge an aid's color. Remembering that a nun is usually red helps you identify that shape ahead as a mark to keep to the right as you return to harbor. Since most cans are green, you can tentatively assume that dark can-shape ahead may be green and to be kept on your left as you return. But you must confirm your assumptions with a clear view of the navigational aid as you approach.

When navigating by visual navigation aids, also use your printed or electronic chart to confirm both your position and the nature of an aid ahead. The chart shows the color of the aid as well as its number and shape. "C" stands for can, "N" for nun. The individual aid's number is given in quotation marks following the C or N. Green aids have odd numbers, and red aids have even numbers.

The numbers on aids get higher as you proceed through a channel into the harbor. For example, the first aid out in open water may be a green can C"1" - with a red nun N"2" off to starboard. You pass between them, looking ahead to C"3" to the left and N"4" to the right. The numbers continue to rise as you advance in the channel.

Sometimes red and green markers are positioned in pairs to clearly show the way to go between them. In other circumstances they simply mark the edges of the channel on both sides and may be relatively far apart. For example, you might pass green C"5" and not see red N"6" for several hundred years farther into the channel. As a rule, aids are positioned nearer each other in narrow or winding channels and farther apart when there is more open water.

Not always Red, Right, Returning! Not all channels are simple pathways from open water into a harbor. A channel may traverse laterally along a shoreline, between the mainland and a series of islands or shoals - such as between two different harbors. You might be going one way in the channel returning to your harbor, but a boat may also be going the opposite direction as it returns to a different harbor. Obviously you both can't keep the red markers to your right - so how do you know which side of the channel is colored which way? Use your chart!

Navigational Aids Outside Channels

Often there is an aid positioned in open water some distance out from the entrance to a channel. Typically this is a red, lighted buoy, and your chart may show that it is safe to pass the buoy on either side, if it does not mark the actual channel yet (except for a large ship that cannot safely stray to its shallower side).

A red and white buoy is used in deep water where it is safe to approach close to the mark on any side, such as when coming closer to land from the open sea). You might want to come close enough to read the lettering on the buoy, for example, in order to confirm your position on the chart. These aids that are not part of a channel numbering system usually are named by letters, such as RW"MR", and may be followed by additional annotations such as BELL or WHIS for sound signals or FlG for a flashing green light.

In other open water areas you may see a single aid to navigation such as a red nun or a green can, indicating not a channel edge but an underwater obstruction such as a rock or shoal. Typically you will not be able to safely interpret your passage around this obstruction without consulting your chart.

Check Your Chart!

This article is only an introduction to the basic types of aids to navigation. You should always know how to interpret navigational aids as they appear on your chart or chartplotter. To be sure, carry a chart symbol app or a standard seamanship book that explains chart symbols such as navigational aids.

Important related articles:

Basic Navigation with a Chartplotter
Nautical Charts: Raster vs. Vector Charts
Navigational Apps

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