Three all-round red lights in a vertical line; cylinder by day.
Rule 28: Constrained by draught
A vessel constrained by her draught 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 by day a cylinder.
Bow / head-on
Port side
Starboard side
Stern / overtaking
Rule 28 Constrained by Draught (CBD)
Wording follows the International Regulations as printed in the US Coast Guard Navigation Rules handbook (2024), with British spelling, and is shortened in places. Download the rules
Read the whole of Rule 28 on one page
Rule 28: Constrained by draught
A vessel constrained by her draught 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 by day a cylinder.
A training aid, not the official text. The COLREGs are an IMO convention, and your national authority publishes the version that applies in its waters. Check the rules themselves before you rely on them at sea. Download the rules.