What lights does a power-driven vessel show underway?

Under Rule 23: a masthead light forward, a second masthead light abaft of and higher than the forward one, sidelights, and a sternlight. Below 50 metres the second masthead light is optional, and below 12 metres she may substitute an all-round white light plus sidelights.

When are two masthead lights required?

At 50 metres and over. A vessel of less than 50 metres may exhibit the second masthead light but is not obliged to, so seeing a single white masthead light does not by itself tell you the vessel is small — it tells you she is either under 50 metres or has chosen not to fit the second light.

Why is the after masthead light higher than the forward one?

Because the two lights together form a range. When you see them vertically separated you are looking at the vessel roughly abeam; as she turns towards or away from you the lights close together, and when they line up you are looking near her bow or stern. It is the cheapest aspect indicator on a ship.

What does an all-round flashing yellow light mean?

An air-cushion vessel — a hovercraft — operating in the non-displacement mode. She still shows the ordinary Rule 23 lights; the flashing yellow is added, warning that she may be moving fast and cannot be expected to manoeuvre like a displacement hull.

Does Inland Rule 23 differ from COLREGs?

In one respect that matters on the Great Lakes: 33 CFR 83.23 lets a power-driven vessel on the Great Lakes carry an all-round white light in place of the second masthead light and the sternlight, positioned where the second masthead light would be. COLREGs has no equivalent.

Inland Rule 23 — Power-driven Vessel Lights (33 CFR 83.23)

Rule text#

Source

Verbatim 33 CFR §83.23 as published in the Code of Federal Regulations — public domain, retrieved 2026-07-26.

(a)A power-driven vessel underway shall exhibit:
(i)A masthead light forward;
(ii)A second masthead light abaft of and higher than the forward one; except that a vessel of less than 50 meters in length shall not be obliged to exhibit such light but may do so;
(iii)Sidelights; and
(iv)A sternlight.
(b)An air-cushion vessel when operating in the non-displacement mode shall, in addition to the lights prescribed in paragraph (a) of this Rule, exhibit an all-round flashing yellow light where it can best be seen.
(c)A WIG craft only when taking off, landing and in flight near the surface shall, in addition to the lights prescribed in paragraph (a) of this Rule, exhibit a high intensity all-round flashing red light.
(d)A power-driven vessel of less than 12 meters in length may, in lieu of the lights prescribed in paragraph (a) of this Rule, exhibit an all-round white light and sidelights.
(e)A power-driven vessel when operating on the Great Lakes may carry an all-round white light in lieu of the second masthead light and sternlight prescribed in paragraph (a) of this Rule. The light shall be carried in the position of the second masthead light and be visible at the same minimum range.

Why it shows up on the exam#

The distinguishing detail is the second masthead light: required at 50 metres and over, optional below it, and always abaft of and HIGHER than the forward one — which is how you read a large vessel's heading at night from the vertical separation of two white lights. The flashing yellow of an air-cushion vessel in non-displacement mode is the other frequently-tested item.

Exam checkpoints#

The standard set
2 masthead · sidelights · stern
A masthead light forward, a second masthead light abaft of and higher than the forward one, sidelights, and a sternlight. The vertical separation between the two mastheads is what reveals a big vessel's aspect at night.
Under 50 metres
Second masthead optional
A vessel of less than 50 metres in length is not obliged to exhibit the second masthead light, though she may do so. One white light forward therefore tells you nothing about her length beyond 'under 50 metres or choosing not to'.
Under 12 metres
All-round white + sidelights
A power-driven vessel of less than 12 metres in length may, in lieu of the masthead and stern lights, exhibit an all-round white light and sidelights.
Air-cushion & WIG
Flashing yellow / flashing red
An air-cushion vessel operating in the non-displacement mode exhibits, in addition to her Rule 23 lights, an all-round flashing yellow light. A WIG craft only when taking off, landing and in flight near the surface exhibits a high-intensity all-round flashing red light.

Inland-specific notes#

Inland adds a Great Lakes option COLREGs does not have: a power-driven vessel operating on the Great Lakes may carry an all-round white light in lieu of the second masthead light and sternlight, mounted where the second masthead light would go. Candidates sitting a Great Lakes route exam should expect this one.

Rule 23 — common exam questions#

What lights does a power-driven vessel show underway?: Under Rule 23: a masthead light forward, a second masthead light abaft of and higher than the forward one, sidelights, and a sternlight. Below 50 metres the second masthead light is optional, and below 12 metres she may substitute an all-round white light plus sidelights.
When are two masthead lights required?: At 50 metres and over. A vessel of less than 50 metres may exhibit the second masthead light but is not obliged to, so seeing a single white masthead light does not by itself tell you the vessel is small — it tells you she is either under 50 metres or has chosen not to fit the second light.
Why is the after masthead light higher than the forward one?: Because the two lights together form a range. When you see them vertically separated you are looking at the vessel roughly abeam; as she turns towards or away from you the lights close together, and when they line up you are looking near her bow or stern. It is the cheapest aspect indicator on a ship.
What does an all-round flashing yellow light mean?: An air-cushion vessel — a hovercraft — operating in the non-displacement mode. She still shows the ordinary Rule 23 lights; the flashing yellow is added, warning that she may be moving fast and cannot be expected to manoeuvre like a displacement hull.
Does Inland Rule 23 differ from COLREGs?: In one respect that matters on the Great Lakes: 33 CFR 83.23 lets a power-driven vessel on the Great Lakes carry an all-round white light in place of the second masthead light and the sternlight, positioned where the second masthead light would be. COLREGs has no equivalent.

Authoritative source#

Source links checked against the current public rule corpus on 2026-07-26.

33 CFR §83.23 — the section this text comes from, with its neighbours.
The Inland Navigation Rules are codified at 33 CFR Part 83 and are public domain. The international COLREGs text is published separately by the IMO.

Practice this rule#

One real exam question on this rule, cited to source. No sign-up.

A power-driven vessel of 25 meters in length is operating on the Great Lakes at night. Which substitution is she permitted to make regarding her lights?

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Power-driven Vessel Underway