1The rule in plain words
In one sentence…
A vessel constrained by her draft may, in addition to Rule 23 lights for power-driven vessels, exhibit three all-round red lights in a vertical line or a cylinder.
What may she exhibit?
In addition to the lights prescribed in Rule 23 for power-driven vessels, she may show three all-round red lights or a cylinder.
Where and how?
The three lights must be exhibited where they can best be seen, in a vertical line.
Who can be constrained by draft?
Rule 28 uses lights for power-driven vessels; see Rule 3(h): only a power-driven vessel can be constrained by draft.
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.
Constrained by draft → an extra display is permitted.
- 2.
Rule 23 lights for power-driven vessels remain the reference.
- 3.
Red signal: three all-round lights · vertical line.
- 4.
By day: a cylinder where it can best be seen (day shape).
4Frequent mistakes
Wrong: "The red lights replace the Rule 23 lights."
Right: No: they are exhibited in addition to the lights prescribed in Rule 23.
Wrong: "The three lights are side by side."
Right: They form a vertical line and are shown where they can best be seen.
Wrong: "A sailing vessel can be constrained by her draft."
Right: Rule 3(h) identifies a power-driven vessel as the vessel that can be constrained by her draft.
5Exam keys & interpretation
Exam keys
"In addition to" means the Rule 23 lights stay; the three red lights do not replace them.
Spot all three cues: three, all-round red, vertical line; missing one changes the answer.
"Or a cylinder": three red lights at night, the cylinder by day (Rule 20).
How to read it
Read "may" as permission, not an instruction that every vessel constrained by draft must always display the three lights.
Use Rule 3(h) to identify the vessel; Rule 28 tells you what additional display she may exhibit.
6Official text
IMO COLREG 1972U.S. Coast Guard — International Navigation Rules (2024)
Full wording of the rule, reproduced from the official source below.
A vessel constrained by her draft may, in addition to the lights prescribed for power-driven vessels in Rule 23, exhibit where they can best be seen three all-round red lights in a vertical line, or a cylinder.7Test your knowledge
What may a vessel constrained by her draft exhibit in addition to the Rule 23 lights?
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