Lights, Shapes and Sounds I: Rules 20 and 21 and Annex I
Overview, arcs and ranges
Part C of the Collision Regulations could have been included as part of our COLREGS series, however we’ve chosen to write a separate series of articles. These will cover Rules 20 to 35, though not in strict order; sometimes it makes sense to group rules together, or include part of one rule when learning another.
Relevant documents and M-notices: MSN 1781
When we can see other vessels, lights and shapes give us information that we can then use to make decisions. Some sound signals do the same thing, when we can’t see other vessels. Other sound and light signals tell us what a vessel is doing, or alert us to something. Quickly and accurately interpreting the lights, shapes and sound signals given in the Collision Regulations is key to understanding a situation, allowing you to make decisions on what action to take. If you can’t identify whether a vessel is RAM, NUC or PDV then you can’t make a decision as to which rules apply.
Part C - Lights and Shapes
Rule 20 - Application
(a) Rules in this Part shall be complied with in all weathers.
(b) The Rules concerning lights shall be complied with from sunset to sunrise and during such times no other lights shall be exhibited, except such lights as cannot be mistaken for the lights specified in these Rules or do not impair their visibility or distinctive character, or interfere with the keeping of a proper look-out.
complied with in all weathers
Don’t switch off your lights just because it’s poor visibility and you think that nobody will be able to see them.
lights…sunset to sunrise and during such times no other lights shall be exhibited
Don’t get sunset and sunrise the wrong way round. Don’t show any other lights, as these will potentially confuse other vessels.
except such lights as cannot be mistaken for the lights specified in these Rules or do not impair their visibility or distinctive character, or interfere with the keeping of a proper look-out.
So a small, low-down blue light to allow you to walk safely around the deck might be OK, as would weak LEDs on a control panel, provided they didn’t interfere with the keeping of a proper look-out (by affecting night-vision, for example).
(c) The lights prescribed by these Rules shall, if carried, also be exhibited from sunrise to sunset in restricted visibility and may be exhibited in all other circumstances when it is deemed necessary.
shall, if carried
Not all vessels are obliged to carry all of the lights described in these rules, but if you do have them, you have an obligation.
also be exhibited from sunrise to sunset in restricted visibility
This makes sense; having lights showing can only make it easier to see ships in restricted visibility.
may be exhibited in all other circumstances when it is deemed necessary.
Remember that ‘may’ means you have a choice. You may, for example, show your lights during an eclipse, or on an extremely overcast and dull day, or in any other circumstance when you deem it necessary.
(d) The Rules concerning shapes shall be complied with by day.
The shapes described in these rules are mostly black spheres, back diamonds or black cylinders. They’d be nearly invisible at night (which is why, of course, we have the lights).
(e) The lights and shapes specified in these Rules shall comply with the provisions of Annex I to these Regulations.
Annex I gives positioning and technical details of lights and shapes. For example, it specifies the vertical and horizontal positioning and spacing of lights, the designs for screens for lights, the sizes and vertical spacing of shapes, and the colour spectrum and intensities of lights.
Rule 20 can be fairly easily summarised:
Between sunset and sunrise (regardless of the weather), and between sunrise and sunset when in restricted visibility, display lights as per the rules in Part C.
Also display lights in any other circumstances where you deem it necessary.
During the day, display shapes
The lights and shapes are as per Annex I
Rule 21 - Definitions
Rule 21 defines the different lights that a vessel may display.
(a) “Masthead light” means a white light placed over the fore and aft centreline of the vessel showing an unbroken light over an arc of the horizon of 225 degrees and so fixed as to show the light from right ahead to 22.5 degrees abaft the beam on either side of the vessel.
(b) “Sidelights” means a green light on the starboard side and a red light on the port side each showing an unbroken light over an arc of the horizon of 112.5 degrees and so fixed as to show the light from the right ahead to 22.5 degrees abaft the beam on its respective side. In a vessel of less than 20 metres in length the sidelights may be combined in one lantern carried on the fore and aft centreline of the vessel.
(c) “Sternlight” means a white light placed as nearly as practicable at the stern showing an unbroken light over an arc of the horizon of 135 degrees and so fixed as to show the light 67.5 degrees from right aft on each side of the vessel.
We’ll cover vertical positioning etc. later. For now, we’ll focus on the horizontal arcs of the lights, and the best way to visualise this is an aerial view of a ship.
Masthead light
The white light in the middle of the image is the masthead light. Ships will have either one or two (or three or four) of these, depending on their length and what they are doing - we’ll cover that later in the series. As you can see, masthead lights are visible from ahead of the vessel to 22.5 degrees on both sides.
Why 22.5 degrees?
Think of the points of the compass. There are 360 degrees in a circle. Dividing that circle into four (using the North, East, South and West points) gives us four 90 degree arcs. Dividing each of these arcs in half, then half again (so 32 total segments) gives us 11.25 degrees. This is one point.
22.5 degrees, then, is two points.
Points are often used to give bearings relative to a vessel’s head (for example, a lookout informing you of a ‘flashing light, three points to starboard’). We use points because it is easier to measure an angle in ‘chunks’ like a point than it is to try and do so using numbers. Also, with numbers there is a danger of confusing a bearing relative to the ships head (‘thirty degrees to starboard’) with a true bearing ‘bearing zero-three-zero’.
Remember that the rules are based on aspect. Rule 13, for example, includes the phrase…’from a direction more than 22.5 degrees abaft her beam’. This could as easily have been ‘from a direction more than two points abaft her beam’.
The lights are designed to help you work out what aspect of the other vessel you are looking at.
If we can see the masthead light we know that we are in the sector that is forward of 22.5 degrees abaft the beam of the vessel. So (unless we’ve already come from more than 22.5 degrees abaft her beam and are in the process of overtaking her) we are not overtaking her. But what is her heading relative to yours? This brings us to the sidelights.
Sidelights
These, taken together, show the same arc of visibility as the masthead light, except that on the starboard side one shows green and on the port side one shows red.
How to remember port and starboard, red and green?
Port is a sweet red wine.
If you can see a vessels red sidelight, you are on her port side and forward of 22.5 degrees abaft her beam to dead ahead of her. If you can see a vessels green sidelight, you are on her starboard side, and forward of 22.5 degrees abaft her beam to dead ahead of her. Sometimes, when a vessel is heading straight towards you, you will see both sidelights.
Sternlight
This is placed as nearly as practicable at the stern of the vessel. It shows a white light, from 22.5 degrees abaft the beam on both sides of the vessel through the stern. If you can see just the sternlight of the vessel, you are possibly overtaking her.
Towing light
This has the same characteristics as the sternlight except that it is yellow. It is placed above the sternlight.
All-round light
All-round lights, as you can imagine, show an unbroken arc of the horizon of 360 degrees. That means that, wherever you are relative to the vessel (ignoring distance), you can see the all-round light. It doesn’t give you information on aspect like the masthead, sternlight and sidelights but it does tell you about the status of a vessel - more on this later.
Flashing light
This is an all-round light flashing at regular intervals, with a frequency of 120 or more flashes per minute. These are displayed by hovercraft when operating in non-displacement mode (a yellow flashing light) and WIG craft when taking off, landing or flying near the surface (a high-intensity red flashing light).
Signals for purse seiners hampered by their gear
These exhibit two flashing yellow lights in a vertical line. These flash alternately every second.
Now that we’ve covered the types of lights that vessels can display, let’s look at their ranges.
Ranges of lights
The minimum visible ranges of lights are given in Rule 22. Have a read of Rule 22 now, either in MSN 1781 or in your COLREGS booklet. Instead of going through the rule, I’ve always found that the best way to learn them is using a table:
One way to remember this is using a poem.
If she’s big like a tree, sixes and threes,
Fifty to twenty (into ten) goes five and two,
Twelve to twenty still twos but three up high,
Less than twelve gets twos and one on the side,
And all-round three if you’re trying to hide
The minimum luminous intensity of lights is calculated using a formula in COLREGS Annex I (8). Note that the formula specifies luminous range. For a reminder about luminous range, visible range and nominal range see the linked article.
Read of COLREGS Annex I (9). Note that there are slight margins where lights may show outside of their prescribed arcs. Don’t worry about committing much from this Annex I memory, but do have an understanding of 9 (a) and (b). To summarise:
Sidelight intensity shall decrease to reach practical cut-off between 1 and 3 degrees outside the prescribed sector in the forward direction. If it is impossible to comply with this then multiple lights shall be fitted and screened to appear as one.
Sternlights, masthead lights, and sidelights at the aft end of their prescribed sectors reach practical cut-off not more than 5 degrees outside their prescribed sectors.
All-round lights cannot be obscured in their arc by more than 6 degrees, except anchor lights.
MCAQs
What are the ranges of the following lights:
Masthead light for a vessel of 14m?
Sidelights for a vessel of 55m?
Towing light for a vessel of 34m?
All-round light for a vessel of 45m?
Masthead light for a vessel of 200m?
Sidelights for a vessel of 200m?
Explain the arcs of visibility of the following:
Masthead light
Sidelights
Stern light
Towing light
All-round light
Flashing light
Explain how the arcs of the lights covered in this article help you work out the aspect of another vessel.
What frequency does a flashing light flash at?
What kind of vessel exhibits a flashing light, and what colours of flashing light?
You can see a yellow light and a white light; the yellow light is above the white one. What does that indicate?