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Rules of the Road – Interpreting Lights Correctly

Source: Mariners Learning System, by Captain Bob Figular

During daylight hours, when visibility is good, it is fairly easy to recognize a situation that exists between two boats while underway. But at night, with only navigation lights to indicate the relative position of vessels, it is a little more difficult. This article is designed to teach you how to interpret and identify correctly the lights found on motorboats and read them to determine what situation exists.

Before we go into the Lights and their configurations it is first necessary to understand a few simple definitions as they pertain to lights covered in this writing.


Navigation Lights are required on most vessels operating upon the high seas and the inland waterways of the United States. The rules for these lights are covered under either the International or the Inland Rules of the Road. The Great Lakes are covered under the Inland Rules, with a few exceptions covered by special rules made by local authorities. Certain rivers and tributaries in the United States abide by rules set by local authorities and deviate slightly from the Inland Rules of the Road.


The masthead light is a white light placed over the fore and aft centerline of a vessel showing an unbroken light over an arc of the horizon of 225°, and so fixed as to show the light from right ahead to 22.5° abaft the beam on either side of the vessel.


The sidelights of a power-driven vessel are green on the starboard side and red on the port side, each showing an unbroken light over an arc of the horizon of 112.5° and so fixed as to show the light from right ahead to 22.5° abaft the beam on its respective side. The memory aid: “Port wine is red” can be used to help remember that the red sidelight is found on the port side. If a vessel is less than 20 meters (65.5 ft) in length, the sidelights may be combined in one lantern carried on the fore and aft centerline of the vessel.


The stern light is a white light placed as near the stern as possible, showing an unbroken light over an arc of the horizon of 135° and so fixed as to show the light 67.5° from right aft on each side of the vessel.

It would be impractical to try to describe or show every situation that could arise while running your boat at night, or in restricted visibility. However, we can focus on a couple of the typical situations you will encounter while operating on our local waterways. By doing this, you should learn to recognize a situation quickly, estimate the relative course of the other boat and by your knowledge of Rules of the Road, and be able to determine the proper course of action.

For example, suppose you saw lights as shown in view A of Figure 1-1 bearing down on you from dead ahead. The white light is about midway between the sidelights; therefore, the boat is heading straight at you.

By understanding their viewable arc you know that both sidelights are not visible across the bow and are not visible from aft. Your conclusion must be that this is a meeting situation, and the rules say that both of you must alter course to starboard and pass port to port.

The boat in view B is showing sidelights in a combination lantern, which is allowed for boats less than 20 meters (65.5 ft) in length, and is also bearing down on you from dead ahead, making this a meeting situation. We know this because the combination light is directly in line with the white light.

View A and B

Figure 1-1 – Motorboat lights-meeting situation

Now assume that you saw lights as shown in view A of Figure 1-2 about 20° off your starboard bow. When looking at the sidelights, you can see only the port sidelight that is red in color, so you know the boat is approaching port-side-to. If the bearing does not change appreciably, you are on collision courses. The other boat is on your starboard side and therefore, is the stand-on vessel. You must alter course and/or speed to pass astern of it; consequently, a change of course to starboard is in order. Make the change great enough to bring your course parallel to, or slightly away from, the course of the other boat.

Figure 1-2

Figure 1-2 Motorboat lights - Crossing situation

If you saw lights as they are shown in view B of Figure 1-2 and they were on your port side, your boat would be the stand-on vessel, and you would be required to maintain your course and speed. Do not, however, go on your way forgetting about the other boat.

You know the situation; you know your boat is the stand-on vessel; you know the Rules of the Road, and must follow them. But what about the other boat operator? Do they know the rule governing this situation? Will he/she change course if necessary?

According to the Rules of the Road you must take action if you think that a collision is possible and that the person running the other boat is not going to act in time.

In this case, you might be tempted to alter your course to port, reasoning that this would involve the smallest change and, hence, the least loss of time. Resist this impulse. DO NOT swing to port in this circumstance. Why not? First, the Rules of the Road state that in a crossing situation, as the stand-on vessel you are not to alter your course to port if the other vessel is on your own port side, if the circumstances of the case admit. Second, even if you suspect that the other vessel operator does not know the Rules of the Road, you must assume that he/she does know them and that they will act accordingly. Next, consider the situation as seen by the other boat operator. You and your vessel are on their starboard side. They know that makes you the stand on vessel and that he must reduce his speed, stop, reverse, or change course and pass astern of you.

These examples are just the basics with regards to understanding Lights and the Rules of the Road. However, mastering these fundamentals will significantly improve your ability to navigate a vessel safely in restricted visibility or while operating at night.

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