Part C — Lights and Shapes
COLREGS Rule 25 — Sailing Vessels Underway and Vessels Under Oars
Rule 25 governs the lights for sailing vessels underway and vessels under oars. A sailing vessel shows sidelights and a sternlight. Additional options include a combined lantern and tri-color masthead light for vessels under 20m. A sailing vessel using its engine simultaneously is a power-driven vessel and must show a cone by day.
Rule Text
A sailing vessel underway shall exhibit sidelights and a sternlight. In a vessel of less than 20 meters in length the lights prescribed above may be combined in one lantern carried at or near the top of the mast. A sailing vessel underway may, in addition to the lights prescribed above, exhibit at or near the top of the mast two all-around lights in a vertical line (red over green) — but not in conjunction with the combined lantern. When underway, a sailing vessel shall exhibit forward, where it can best be seen, a conical shape, apex downwards, when proceeding under sail when also under power.
What it means on the water
- →Sailing vessel: sidelights (red/green) + white sternlight.
- →Under 20m option: combined lantern (sidelights + sternlight in one unit).
- →Optional: red over green all-around at top of mast (not used with combined lantern).
- →Sailing + engine = power-driven vessel: show cone apex downward by day.
- →Vessel under oars: may exhibit sailing vessel lights OR just a flashlight/lantern to prevent collision.
Common exam mistakes
- ✗Thinking a sailing vessel can show red over green AND the combined lantern at the same time — they cannot be used together.
- ✗Forgetting that a sailing vessel motoring must show the cone by day.
- ✗Missing that the red over green all-around lights are optional, not required.
USCG exam questions — Rule 25
These questions are drawn from the same pool used in real USCG licensing exams. Correct answers and explanations are shown.
1. A sailing vessel underway at night shall show:
- A.A tricolor light at the masthead only
- B.Sidelights, a sternlight, and a white masthead light
- C.Sidelights and a sternlight only✓
- D.Sidelights, a sternlight, and two all-round lights (red over green) at the masthead
Why: Rule 25(a) requires a sailing vessel underway to show sidelights and a sternlight only, so C is the mandatory minimum. The optional two all-round lights (red over green) at the masthead are authorized by Rule 25(c) — not 25(b), which is the combined tricolor lantern for vessels under 20m — and a white masthead light would falsely indicate a power-driven vessel.
2. A sailing vessel less than 20 meters in length may combine the sidelights and sternlight into:
- A.A single all-round white light
- B.A bicolor lantern at the bow
- C.An all-round red and green light at the stern
- D.A tricolor lantern at the masthead✓
Why: Rule 25(b) permits sailing vessels less than 20 meters to combine sidelights and sternlight into a tricolor lantern at or near the top of the mast. When this tricolor is used, the separate all-round red-over-green lights at the masthead must NOT also be shown.
3. A sailing vessel proceeding under sail AND using her engine shall display by day:
- A.A black ball shape forward
- B.A black cone with apex downward forward✓
- C.A black diamond shape
- D.No special day shape is required
Why: Rule 25(e) requires a sailing vessel proceeding under sail when also being propelled by machinery to show a conical shape, apex downwards, at the forward part of the vessel. This alerts other mariners that despite the sails, she is operating as a power-driven vessel for rules purposes.
4. A vessel under oars less than 7 meters in length shall, in lieu of prescribed lights, have ready at hand:
- A.A red flare
- B.A white flare only
- C.Sidelights and a sternlight are always required
- D.A white all-round electric torch or lighted lantern✓
Why: Rule 25(d)(ii) permits a vessel under oars less than 7 meters to show only the lights in Rule 25(a) or (b), but if this is not practicable, she shall have ready at hand an electric torch or lighted lantern showing a white light. This is a practical concession for small rowboats.
5. You observe a vessel showing only a tricolor lantern at the masthead. What type of vessel is this most likely to be?
- A.A power-driven vessel less than 12 meters in length
- B.A sailing vessel less than 20 meters in length underway✓
- C.A vessel at anchor with a single white light
- D.A pilot vessel on duty
Why: Rule 25(b) permits sailing vessels less than 20 meters to combine the sidelights and sternlight into a tricolor lantern at or near the masthead. The tricolor replaces the separate sidelights and stern light — it is never used with a white masthead light.
6. A sailing vessel underway may exhibit at or near the top of the mast where they can best be seen which combination lights?
- A.A combined lantern showing a red light over a green light✓
- B.A tri-color lantern showing red, green, and white sectors
- C.Separate red and green lights only
- D.A single all-round white light
Why: Under COLREGS Rule 25(c), a sailing vessel underway may — in addition to the sidelights and sternlight required by Rule 25(a) — 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 red and the lower green. There is no length restriction on these optional lights (the under-20-meter limit applies instead to the separate Rule 25(b) combined tri-color masthead lantern, which may not be shown together with the 25(c) red-over-green lights).
7. A sailing vessel under 20 meters underway may use which simplified light configuration in lieu of separate sidelights and stern light?
- A.A tri-color lantern at the masthead showing red, green, and white sectors✓
- B.A combined red over green lantern at the masthead
- C.A single all-round white light and separate sidelights
- D.A single masthead light visible from all directions
Why: Rule 25(b) permits sailing vessels under 20 meters to use a single tri-color lantern at or near the masthead showing red to port, green to starboard, and white aft — replacing the need for separate sidelights and stern light.
8. A sailing vessel of less than 7 meters in length, if unable to exhibit the lights prescribed by Rule 25, shall exhibit what?
- A.An electric torch or lighted lantern showing a white light in sufficient time to prevent collision✓
- B.An all-round red light only
- C.Sidelights from a flashlight when another vessel approaches
- D.No light is required for vessels this small
Why: Rule 25(d) provides that sailing vessels under 7 meters and vessels under oars that cannot exhibit standard lights shall have ready an electric torch or lighted lantern showing a white light to display in sufficient time to prevent collision.
Frequently asked questions
- What lights must a sailing vessel show when underway at night?
- A sailing vessel underway must show port sidelight (red), starboard sidelight (green), and a white sternlight. Vessels under 20 meters may combine these into a single combined lantern. Any sailing vessel may optionally show red over green all-around lights at the masthead, but not at the same time as the combined lantern.
- What must a sailing vessel show when using its engine?
- A sailing vessel underway using its engine — whether or not it also has sails set — is considered a power-driven vessel and must comply with Rule 23. During daylight hours, it must show a conical shape, apex downward, visible forward. At night it must show the lights of a power-driven vessel.
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