Part C — Lights and Shapes
COLREGS Rule 23 — Power-driven Vessels Underway
Rule 23 specifies the lights a power-driven vessel underway must show. The basic scheme is a forward masthead light, a second masthead light farther aft and higher (for vessels 50m or more), port and starboard sidelights, and a white sternlight. Small vessels have reduced requirements.
Rule Text
A power-driven vessel underway shall exhibit: a masthead light forward; a second masthead light abaft and higher than the forward light (vessels 50m or more must; vessels under 50m may); sidelights; and a sternlight. A power-driven vessel of less than 7 meters in length whose maximum speed does not exceed 7 knots may, in lieu of the prescribed lights, exhibit an all-around white light and, if practicable, sidelights. A power-driven vessel underway of less than 12 meters may show the forward masthead light and sidelights and sternlight combined in a single lantern on the vessel's fore and aft centerline.
What it means on the water
- →Standard power-driven vessel: masthead light forward + sternlight + sidelights.
- →Vessels 50m or more: second masthead light required (abaft and higher than the forward).
- →Vessels under 50m: second masthead light is optional.
- →Vessel under 7m AND max speed not over 7 knots: may show all-around white light only (sidelights if practicable).
- →Vessels under 12m: may use a single combined lantern instead of separate lights.
Common exam mistakes
- ✗Stating a second masthead light is required for vessels under 50m — it is optional, not required.
- ✗Forgetting that the all-around white substitute is only for vessels under 7m AND under 7 knots (both conditions required).
- ✗Mixing up forward/aft placement: the second masthead light is ABAFT and HIGHER than the forward light.
USCG exam questions — Rule 23
These questions are drawn from the same pool used in real USCG licensing exams. Correct answers and explanations are shown.
1. A power-driven vessel underway and less than 50 meters in length is required to show which lights at night?
- A.One masthead light, sidelights, and a sternlight✓
- B.Two masthead lights (forward lower, aft higher), sidelights, and a sternlight
- C.One masthead light and an all-round white light aft
- D.Sidelights and a sternlight only
Why: Rule 23(b) allows a power-driven vessel less than 50 meters in length to show a single masthead light instead of the two required for vessels 50 meters or more. Sidelights and a sternlight are still required.
2. A power-driven vessel 50 meters or more in length underway at night must show:
- A.One masthead light forward, sidelights, sternlight
- B.Two masthead lights (forward higher than aft), sidelights, sternlight
- C.Two masthead lights at the same height, sidelights, sternlight
- D.Two masthead lights (forward lower than aft), sidelights, sternlight✓
Why: Rule 23(a) requires a forward masthead light and a second masthead light abaft and higher than the forward one. This gives a distinctive profile — the farther light is always the higher one — helping observers judge the vessel's heading.
3. A power-driven vessel less than 7 meters in length whose maximum speed does not exceed 7 knots may, in lieu of the prescribed lights, show:
- A.Sidelights and a sternlight only
- B.A single all-round white light only✓
- C.An all-round white light and sidelights
- D.A flashing yellow light
Why: Rule 23(d)(i) permits the smallest, slowest power-driven vessels — under 7 meters and under 7 knots — to show only an all-round white light as a practical minimum. If practicable, sidelights shall also be shown.
4. You observe a vessel at night showing two white masthead lights (forward lower), green and red sidelights, and a white sternlight. What type of vessel is it?
- A.A sailing vessel with auxiliary engine
- B.A vessel towing with a tow more than 200 meters
- C.A power-driven vessel 50 meters or more underway✓
- D.A pilot vessel on station
Why: Two white masthead lights with the forward one lower, combined with sidelights and sternlight, is the standard light configuration for a power-driven vessel 50 meters or more in length underway. The staggered masthead lights are the key identifier.
5. What is the minimum visibility range required for the masthead light of a power-driven vessel less than 12 meters in length?
- A.1 mile
- B.2 miles✓
- C.3 miles
- D.5 miles
Why: Rule 22(c) prescribes a 2-mile visibility range for masthead lights on vessels less than 12 meters in length. Sidelights on such vessels need only 1 mile, and the all-round white light also requires 2 miles.
6. A power-driven vessel underway of 50 meters or more in length shall exhibit which masthead lights?
- A.A forward masthead light and a second masthead light abaft and higher than the forward one✓
- B.A single masthead light visible from ahead only
- C.Two masthead lights of equal height separated horizontally
- D.A forward masthead light and an optional second masthead light
Why: Rule 23(a) requires power-driven vessels of 50 meters or more to show a forward masthead light and a second masthead light abaft and higher than the forward light. This creates a distinctive profile that indicates the vessel's heading.
7. A power-driven vessel underway that is less than 7 meters in length and whose maximum speed does not exceed 7 knots may, in lieu of the lights prescribed in Rule 23, show what?
- A.An all-round white light and, if practicable, sidelights✓
- B.A single white stern light only
- C.Sidelights and a stern light only
- D.A masthead light and sidelights only
Why: Rule 23(d) provides that power-driven vessels under 7 meters with a maximum speed of 7 knots or less may show an all-round white light and, if practicable, sidelights, as a simplified alternative to the standard light configuration.
8. What is the minimum arc of visibility for masthead lights on power-driven vessels?
- A.225 degrees✓
- B.112.5 degrees
- C.360 degrees
- D.135 degrees
Why: Under COLREGS/Inland Rule 21(a), a masthead light shows an unbroken white light over an arc of 225 degrees — from right ahead to 22.5 degrees abaft the beam on each side — which distinguishes it from sidelights (112.5 degrees, Rule 21(b)) and the sternlight (135 degrees, Rule 21(c)). (Rule 23 prescribes that a power-driven vessel underway exhibit this masthead light; Annex I covers only the lights' positioning and technical details, not the arc itself.)
Frequently asked questions
- What lights does a power-driven vessel show when underway?
- A power-driven vessel underway shows a white masthead light forward, port sidelight (red), starboard sidelight (green), and a white sternlight. Vessels 50 meters or more must also show a second white masthead light abaft and higher than the forward light. Vessels under 50 meters may add the second light but are not required to.
- Can a small power-driven vessel show just one white light?
- Yes, but only under strict conditions: the vessel must be under 7 meters in length AND its maximum speed must not exceed 7 knots. In that case it may substitute an all-around white light for the full light configuration, with sidelights shown if practicable.
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