The lights are required (must be shown) from sunset to sunrise, but may be shown any time deemed appropriate. Note that automobile lights are required from evening twilight to morning twilight, which is a longer period, but less precisely defined.
For now we look at the three basic nav lights categories: sidelights (red and green), masthead lights (white), and sternlight (white). These are defined in Rule 23a. These rules establish the terminology as well: sternlight and sidelight are written as one word.
Sidelights are seen from dead ahead to 22.5º abaft the beam. Sternlight is shown in the aft gap between those two, and the masthead lights are seen from everywhere except the stern sector, ie masthead lights and sidelights are seen from the same sectors.
The seemingly strange choice of 22.5º is a historic accident as this equals two points on the compass and the original Rules from the 1800s were written in terms of compass points. See our note on boxing the compass.
The seemingly strange choice of 22.5º is a historic accident as this equals two points on the compass and the original Rules from the 1800s were written in terms of compass points. See our note on boxing the compass.
For now we consider only power driven vessels (PDV) and sailing vessels (SV). Note that a sailboat is only an SV when it is actually sailing, no engine engaged, else it is a PDV. This fact applies not just to lights but to all aspects of the rules. A sailboat under power is a powerboat.
We consider PDV in only two categories for now: less than 50 m (164 ft), and 50 m or more in length.
Larger vessels require two masthead lights, the forward one lower than the aft one. Thus seeing them on the horizon we know which way the vessel is headed. The above and below pics are from the USCG Navigation Rules Handbook. There is also a printed edition.
For our Radar Course quizzes, we have to determine which lights we see based on the relative motion track of the vessel on the radar screen—but a first step in that analysis is knowing what the lights look like from various perspectives in plain sight.
Referring back to the first figure, from anywhere in the red sector you would see only one red light and one or two white masthead lights; same from the green sector, one green and one or two whites depending on the size of the vessel.
In the aft sector, you would see only one white light, the sternlight. A 15 ft runabout and the Queen Mary look the same from astern.
From dead ahead you would see red and green sidelights. In this case "dead ahead"—as determined by seeing the other vessel's sidelights—is tied to the technical specs of the lights. According to the Rules, they must fade to zero intensity within 3º of dead ahead. In other words, in principle you should never see green from left of the bow, but to account for technical limitations, you should never see green at all beyond 3º to the left of the bow.
The borders at the aft end of the range are ± 5º, meaning the specified intensity (visual range) must apply to at least within 5º of the aft limit, and then must attenuate to 0 by 5º beyond that. See:
The borders at the aft end of the range are ± 5º, meaning the specified intensity (visual range) must apply to at least within 5º of the aft limit, and then must attenuate to 0 by 5º beyond that. See:
COLREGS Annex I— Positioning and Technical Details of Lights and Shapes,
Section 9. Horizontal sectors
(a) (i) In the forward direction, sidelights as fitted on the vessel shall show the minimum required intensities. The intensities shall decrease to reach practical cut-off between 1 degree and 3 degrees outside the prescribed sectors.
(ii) For sternlights and masthead lights and at 22.5 degrees abaft the beam for sidelights, the minimum required intensities shall be maintained over the arc of the horizon up to 5 degrees within the limits of the sectors prescribed in Rule 21. From 5 degrees within the prescribed sectors the intensity may decrease by 50 percent up to the prescribed limits; it shall decrease steadily to reach practical cut-off at not more than 5 degrees outside the prescribed sectors.
The aft boundary (two points abaft the beam) that separates the crossing and overtaking courses as determined by lights has an uncertainty of ± 5º. We can often determine relative courses to well within this accuracy by solving the relative motion diagram on our radar screen.
We can also learn the other vessel's course (COG) from its AIS signal if available. Vessels with a heading sensor programmed into the AIS will also tell us their heading, which is what determines the lights we should see. With no current or leeway, the heading and COG would be the same.
(In the formative days of the Rules, the sidelights were specified to extend 4º across the bow, but that proved dangerous and was ended in the 1890s. Side lights was two words in those days.)
Masthead lights can be seen from farther off than sidelights. Below are the required ranges given in Rule 22, for the three lights we consider here:
(a) In vessels of 50 meters or more in length:
- (i) a masthead light, 6 miles;
- (ii) a sidelight, 3 miles;
- (iii) a sternlight, 3 miles;
(b) In vessels of 12 meters or more in length but less than 50 meters in length;
- (i) a masthead light, 5 miles; except that where the length of the vessel is less than 20
- meters, 3 miles;
- (ii) a sidelight, 2 miles;
- (iii) a sternlight, 2 miles;
(c) In vessels of less than 12 meters (39.4 ft) in length:
- (i) a masthead light, 2 miles;
- (ii) a sidelight, 1 miles;
- (iii) a sternlight, 2 miles;
It is clear why we can see a ship from farther off than a smaller vessel. The lights are higher and indeed brighter. In all cases, we see the masthead lights before the sidelights.
Below are samples of how the aspect of the lights tell us how the vessel is moving.
These two pictures are from our book Fundamentals of Kayak Navigation. All vessels care about understanding navigation lights.
The mariner at location A sees a red turn into red and green, and then into green alone, as the vessel turns toward them and then white as they passed. Vessel B sees a red light the whole time, except possibly at the very last location where it would go white. Study this figure to be sure both views are clear.
Masthead lights on a ship are often referred to as its "range lights," because we can think of them as a navigational range, and use that concept to decide where we are relative to that range. When navigating in the vicinity of a charted navigational range, when the two lights are lined up from our perspective (the more inland one is always higher), we know we are precisely on the charted line that the two lights define. In the case of a vessel, this means we are looking directly at its bow. Recall we do not see masthead lights astern; only on the sides and ahead.
As the vessel turns toward us at the top of the pic, we see the range lights "closing"; as it turns away from us, we see the range lights "opening." This is common terminology in nighttime navigation: Are the masthead lights opening or closing? When the two lights are lined up, one above the other, the ship is headed straight toward us. We often can see these lights with binoculars and identify its heading long before we see its sidelights.
Sailboat lights are summarized in the figure below
When sailing, there are sidelights and a sternlight, but no masthead light. Under 20m (65.6 ft) all underway sailing lights can be in a combined unit at the masthead—but vessels choosing that option must also have deck level sidelights because the tricolor is not legal under power. Thus a sailing vessel can likely have both masthead sidelights and deck side lights but these cannot be run at the same time. There is a rarely used optional all-round red and green masthead light for when sailing.
Under power, sailboat lights are easy; they are same as PDV of the same length. The above figure is from our textbook Inland and Coastal Navigation.
As you study navigation lights, keep in mind the salient fact that every collision involves the violation of at least one of the Navigation Rules by both vessels. For collisions at night, illegal lights are almost guaranteed to buy you some fraction of the liability, even if they or you were not the primary cause of the collision.
Also it takes an effort to ensure that sailboat lights are legal. It is worth taking the time to view your boat at night from another boat at all aspects—or have a friend take a cellphone video. Sometimes sidelights on a bow can reflect from the bow pulpit or a sail on the bow and be seen in many directions. The light used to illuminate the masthead windvane (Windex) can often be seen as a white light, and so on.
A version of the Navigation Rules that is easy to search is our Pocket Navigation Rules Handbook. You can view it online and then save as a PDF and the cross links should work. Then mail it to yourself and open in your phone and save into the library of your favorite ebook reader.
Samples of running lights.
Samples of running lights.
Vessel under sail is very simple. Sidelight or sternligbht. If we see only a sidelight with no masthead, that is likely a sailboat undersail.
A sailboat sternlight, but we only know that because we can see the boat. Seeing only the light, we do not know what kind of vessel it is... which in a sense does not matter if we are overtaking, we have to stay clear.
A ship's sternlight.
A ship's sternlight.
Cruise ships are mixed bag when it comes to lights. The positive is you cannot miss them, they are a great smear of lights. The negative part is it is very difficult to see any of the official lights in the maze of other lights so it can be hard to tell which way they are going... if they are going at all. They could be just drifting gambling casinos.
Here we can see the two sidelights as it turns left in front of us (red, red and green, and green), but note the illegal red light on the starboard side. It can be difficult to identify their masthead lights—but this is not a fair evaluation of that because we are so close.
A cleaner view of a ship's green sidelight and masthead lights. Many ships that do show distracting lighting when near port, do not have these extra lights on at sea. This is true of cargo and tanker vessels, not cruise ships.
Here is a case of many distracting lights but the mastheads (range lights) are still clear, as is the port sidelight.
This screencap is from—believe it or not—a video about using the proper lights. It is recorded from a speaker on the vessel we see here, filmed by another vessel following it around. This is an example mentioned above of unwanted reflections of your sidelights... the strange part is they could watch this and not edit this out.
Here we can see the two sidelights as it turns left in front of us (red, red and green, and green), but note the illegal red light on the starboard side. It can be difficult to identify their masthead lights—but this is not a fair evaluation of that because we are so close.
A cleaner view of a ship's green sidelight and masthead lights. Many ships that do show distracting lighting when near port, do not have these extra lights on at sea. This is true of cargo and tanker vessels, not cruise ships.
Here is a case of many distracting lights but the mastheads (range lights) are still clear, as is the port sidelight.
This screencap is from—believe it or not—a video about using the proper lights. It is recorded from a speaker on the vessel we see here, filmed by another vessel following it around. This is an example mentioned above of unwanted reflections of your sidelights... the strange part is they could watch this and not edit this out.
