1The rule in plain words
In one sentence…
COLREG changes the lights and shapes you show when towing, pushing ahead or towing alongside.
What does the towing vessel show?
It carries two masthead lights in a vertical line, sidelights, a sternlight and a towing light above it; when the tow exceeds 200 metres, it carries three masthead lights and a diamond shape.
What happens when pushing ahead or towing alongside?
With a rigid connection forming a composite unit, the vessels are regarded as a power-driven vessel and show the lights in Rule 23. Without a composite unit: two masthead lights in a vertical line, sidelights and a sternlight.
What does the vessel or object being towed show?
Except those in paragraph (g), sidelights and a sternlight; when the tow exceeds 200 metres, also a diamond shape where it can best be seen.
What special cases matter?
An inconspicuous, partly submerged tow uses all-round white lights and shapes according to its dimensions; if they cannot be shown, its presence is indicated. A vessel not normally engaged in towing may assist one in distress, but indicates the relationship.
2See it in 3D
Turn the vessel or pick bow, stern, port or starboard: this is how her lights look at night.
An interactive 3D illustration is shown here. The same content is described in the rule text and key takeaways below.
3Key points
- 1.
Tow exceeding 200 metres → three masthead lights and a diamond shape.
- 2.
Rigid composite unit = lights prescribed in Rule 23.
- 3.
Group of towed vessels → lighted as one vessel.
- 4.
Inconspicuous tow → all-round white lights and shapes based on breadth and length.
4Frequent mistakes
Wrong: "A tow of exactly 200 metres gets the third light and diamond shape."
Right: The text requires the length of the tow to exceed 200 metres.
Wrong: "A composite unit shows two masthead lights and a towing light."
Right: With a rigid connection, it is regarded as a vessel and shows the lights in Rule 23.
Wrong: "An inconspicuous tow is lit like any other tow."
Right: It has all-round white lights and additional shapes according to its dimensions.
5Exam keys & interpretation
Exam keys
"Exceeds 200 metres" triggers the third masthead light and diamond shape; exactly 200 does not.
"Composite unit" is decisive: rigid connection → lights prescribed in Rule 23.
If you read "inconspicuous and partly submerged", check breadth, length and the maximum 100-metre spacing.
How to read it
Read the rule by the connection: composite unit, separate pushing or towing, or an inconspicuous tow.
"All possible measures" does not erase the duty: if a light is missing, at least indicate the presence or relationship.
6Official text
IMO COLREG 1972U.S. Coast Guard — International Navigation Rules (2024)
Full wording of the rule, reproduced from the official source below.
(a) A power-driven vessel when towing shall exhibit:
(i) instead of the light prescribed in Rule 23(a)(i) or (a)(ii), two masthead lights in a vertical line. When the length of the tow, measuring from the stern of the towing vessel to the after end of the tow, exceeds 200 meters, three such lights in a vertical line;
(ii) sidelights;
(iii) a sternlight;
(iv) a towing light in a vertical line above the sternlight; and
(v) when the length of the tow exceeds 200 meters, a diamond shape where it can best be seen.
(b) When a pushing vessel and a vessel being pushed ahead are rigidly connected in a composite unit they shall be regarded as a power-driven vessel and exhibit the lights prescribed in Rule 23.
(c) A power-driven vessel when pushing ahead or towing alongside, except in the case of a composite unit, shall exhibit:
(i) instead of the light prescribed in Rule 23(a)(i) or 23(a)(ii), two masthead lights in a vertical line;
(ii) sidelights; and
(iii) a sternlight.
(d) A power-driven vessel to which paragraph (a) or (c) of this Rule applies shall also comply with Rule 23(a)(ii).
(e) A vessel or object being towed, other than those mentioned in paragraph (g) of this Rule, shall exhibit:
(i) sidelights;
(ii) a sternlight;
(iii) when the length of the tow exceeds 200 meters, a diamond shape where it can best be seen.
(f) Provided that any number of vessels being towed alongside or pushed in a group shall be lighted as one vessel:
(i) a vessel being pushed ahead, not being part of a composite unit, shall exhibit at the forward end, sidelights;
(ii) a vessel being towed alongside shall exhibit a sternlight and at the forward end, sidelights.
(g) An inconspicuous, partly submerged vessel or object, or combination of such vessels or objects being towed, shall exhibit:
(i) if it is less than 25 meters in breadth, one all-round white light at or near the forward end and one at or near the after end except that dracones need not exhibit a light at or near the forward end;
(ii) if it is 25 meters or more in breadth, two additional all-round white lights at or near the extremities of its breadth;
(iii) if it exceeds 100 meters in length, additional all-round white lights between the lights prescribed in subparagraphs (i) and (ii) so that the distance between the lights shall not exceed 100 meters;
(iv) a diamond shape at or near the aftermost extremity of the last vessel or object being towed and if the length of the tow exceeds 200 meters an additional diamond shape where it can best be seen and located as far forward as is practicable.
(h) Where from any sufficient cause it is impracticable for a vessel or object being towed to exhibit the lights or shapes prescribed in paragraph (e) or (g) of this Rule, all possible measures shall be taken to light the vessel or object towed or at least to indicate the presence of such vessel or object.
(i) Where from any sufficient cause it is impracticable for a vessel not normally engaged in towing operations to display the lights prescribed in paragraph (a) or (c) of this Rule, such vessel shall not be required to exhibit those lights when engaged in towing another vessel in distress or otherwise in need of assistance. All possible measures shall be taken to indicate the nature of the relationship between the towing vessel and the vessel being towed as authorized by Rule 36, in particular by illuminating the towline.7Test your knowledge
A power-driven vessel tows a tow of more than 200 metres. What does it add?
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