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COLREG

Rule 25: Sailing Vessels Underway and Vessels Under Oars

Rule 25 requires a sailing vessel underway to show the prescribed lights and provides options for small vessels and sailing vessels also propelled by machinery.

Rule 25 of 41

Decide

An interactive 3D illustration is shown here. The same content is described in the rule text and key takeaways below.

Decision 1 of 3

It is night and you see only these lights. What vessel is it?

The rule in plain words

  1. Which lights does a sailing vessel underway show?She shows the sidelights and sternlight.
  2. What may she do if she is less than 20 metres in length?She may carry those lights in one combined lantern at or near the top of the mast, where it can best be seen.
  3. Is there an additional option?She may show two vertical all-round lights, red above and green below, but not in conjunction with the combined lantern.
  4. What about small sailing vessels and vessels under oars?Under 7 m: prescribed lights if practicable; otherwise torch or lantern showing white light ready. Under oars: sailing lights or white light ready. Exhibit that light in time to prevent collision. Sail with machinery: cone forward, apex down.
A vessel under sail and also propelled by machinery is power-driven: it shows the corresponding lights and, by day, a cone with its apex downward.
1 of 3

Before you read on: what would you do?

Question 1/1

Which lights does a sailing vessel underway exhibit?

See it in 3D

Drag the vessel to turn it: 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.

Frequent mistakes

  • Wrong: "Below 20 metres, the lights must be separate."

    Right: They may be combined in one lantern carried at or near the masthead.

  • Wrong: "The red and green lights always replace the combined lantern."

    Right: They are additional, and must not be shown with the combined lantern.

  • Wrong: "A vessel under oars needs no light ready."

    Right: If she does not show the prescribed lights, she keeps an electric torch or lighted white lantern ready.

Exam keys & interpretation

  1. "Less than 20 metres" excludes exactly 20 metres: the number decides whether the combined lantern is allowed.
  2. Red above green is additional, not the combined lantern: look for "shall not be exhibited in conjunction".
  3. "Apex downwards" identifies the conical shape for a sailing vessel also propelled by machinery.

How to read it

Read "may" as an option: the two all-round lights are not mandatory.

"If practicable" and "in sufficient time" allow practical judgment, but require the white light to be ready when prescribed lights are not shown.

Key points

  • Sailing underway → sidelights + sternlight.
  • Less than 20 m → combined lantern at or near the masthead.
  • Two all-round lights: red above, green below · never with the combined lantern.
  • Under 7 m/oars → prescribed or white light ready; sailing with machinery → cone down.

Test your knowledge

3 questions to check it. Pick an answer to see why.

Question 1/3

A sailing vessel is less than 20 metres in length. What does Rule 25 allow?

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Official text

IMO COLREG 1972 U.S. Coast Guard — International Navigation Rules (2024)

Full wording of the rule, reproduced from the official source below.

(a) A sailing vessel underway shall exhibit: (i) sidelights; (ii) a sternlight. (b) In a sailing vessel of less than 20 meters in length the lights prescribed in paragraph (a) of this Rule may be combined in one lantern carried at or near the top of the mast where it can best be seen. (c) A sailing vessel underway may, in addition to the lights prescribed in paragraph (a) of this Rule, exhibit at or near the top of the mast, where they can best be seen, two all-round lights in a vertical line, the upper being red and the lower green, but these lights shall not be exhibited in conjunction with the combined lantern permitted by paragraph (b) of this Rule. (d) (i) A sailing vessel of less than 7 meters in length shall, if practicable, exhibit the lights prescribed in paragraph (a) or (b) of this Rule, but if she does not, she shall have ready at hand an electric torch or lighted lantern showing a white light which shall be exhibited in sufficient time to prevent collision. (ii) A vessel under oars may exhibit the lights prescribed in this Rule for sailing vessels, but if she does not, she shall have ready at hand an electric torch or lighted lantern showing a white light which shall be exhibited in sufficient time to prevent collision. (e) A vessel proceeding under sail when also being propelled by machinery shall exhibit forward where it can best be seen a conical shape, apex downwards.
Source:

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Reviewed by Ramon Curto, Nautical teacher at IECNPC · Last reviewed:

Frequently asked questions about COLREG Rule 25

Which lights does a sailing vessel underway show?

Sidelights and a sternlight (Rule 25(a)). A sailing vessel shows no masthead light, and in a vessel of less than 20 metres the sidelights and sternlight may be combined in one lantern at or near the top of the mast.

What do red over green all-round lights at the masthead mean?

They mark a sailing vessel underway. Rule 25(c) lets her exhibit, in addition to sidelights and sternlight, two all-round lights in a vertical line at or near the top of the mast, the upper red and the lower green. They are optional and must not be used together with the combined mast-top lantern permitted by Rule 25(b).

What light must a vessel under oars show?

She may exhibit the lights prescribed for sailing vessels, or else have ready at hand an electric torch or lighted lantern showing a white light to prevent collision (Rule 25(d)).