Rule 23 — Power-Driven Vessel Underway: Required Lights

TL;DR — A power-driven vessel underway must show a forward masthead light, sidelights, and a sternlight; vessels 50 meters or more in length must also show a second masthead light abaft of and higher than the forward one. Size-based alternatives exist for vessels under 12 meters and for Great Lakes operations.

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What the Rule Says

The Standard Light Configuration (≥ 50 m)

A power-driven vessel underway is required to exhibit four elements 33 CFR §83.23:

1. A masthead light forward — a white light showing over an arc of 225° from dead ahead to 22.5° abaft the beam on each side. 2. A second masthead light — placed abaft of and higher than the forward one. For vessels 50 meters or more in length, this second masthead light is mandatory. 33 CFR §83.23 3. Sidelights — red to port, green to starboard, each showing 112.5°. 4. A sternlight — white, showing 135° centered on dead astern.

For vessels less than 50 meters in length, the second masthead light is not required but may be carried. 33 CFR §83.23

Vertical Placement Requirements

The forward masthead light on a vessel 20 meters or more in length must be placed at a height above the hull of not less than 5 meters. If the vessel's breadth exceeds 5 meters, the light must be at least as high as the breadth, but need not exceed 8 meters above the hull. When two masthead lights are carried, the after one must be at least 2 meters vertically higher than the forward one. 33 CFR §84.02

The vertical separation must be sufficient so that, in all normal conditions of trim, the after light is seen over and separate from the forward light at a distance of 1,000 meters from the stem when viewed from water level. 33 CFR §84.02

For a vessel 12 meters but less than 20 meters in length, the masthead light must be placed at a height above the gunwale of not less than 2.5 meters. 33 CFR §84.02

For a vessel less than 12 meters in length, the masthead light (or the all-round white light used in lieu of the standard configuration) must be carried at least one meter higher than the sidelights. 33 CFR §84.02

Sidelights on a power-driven vessel must be placed at least one meter lower than the forward masthead light and must not be so low as to be interfered with by deck lights. 33 CFR §84.02

Horizontal Placement Requirements

When two masthead lights are carried, the horizontal distance between them must not be less than one quarter of the vessel's length, but need not exceed 50 meters. The forward masthead light must be placed not more than one half of the vessel's length from the stem. 33 CFR §84.03

On a vessel of 20 meters or more in length, sidelights must not be placed forward of the forward masthead light and must be placed at or near the side of the vessel. 33 CFR §84.03

When only one masthead light is carried, it must be exhibited forward of amidships. On a vessel less than 20 meters in length, it shall be placed as far forward as is practicable. 33 CFR §84.03

The Small Vessel Alternative (< 12 m)

A power-driven vessel of less than 12 meters in length may, in lieu of the standard four-element configuration, exhibit:

• An all-round white light, and
• Sidelights

33 CFR §83.23

This is a complete substitution — the vessel does not also carry a forward masthead light and sternlight when using this option. The all-round white light must be carried at least one meter higher than the sidelights. 33 CFR §84.02

Great Lakes Special Rule

A power-driven vessel operating on the Great Lakes may carry an all-round white light in lieu of the second masthead light and the sternlight. This substitution light must be carried in the position of the second masthead light and must be visible at the same minimum range as those lights. 33 CFR §83.23

Note carefully what this substitution does and does not replace: the forward masthead light and sidelights are still required. Only the after masthead light and sternlight are replaced by the single all-round white light. 33 CFR §83.23

Air-Cushion Vessels (Non-Displacement Mode)

An air-cushion vessel operating in the non-displacement mode must exhibit all lights required under the standard Rule 23(a) configuration plus an all-round flashing yellow light placed where it can best be seen. 33 CFR §83.23

WIG Craft

A wing-in-ground (WIG) craft, only when taking off, landing, and in flight near the surface, must exhibit all lights required under Rule 23(a) plus a high-intensity all-round flashing red light. 33 CFR §83.23

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Why It Matters on the Exam

The OUPV and Master 100 GT written examinations test Rule 23 in several recurring ways:

1. Identifying the correct light configuration by vessel length. The exam will present a scenario — a power-driven vessel of a stated length — and ask which lights are required. The critical length thresholds are:

• < 12 m: May substitute all-round white + sidelights for the full Rule 23(a) suite. 33 CFR §83.23
• < 50 m: Second masthead light is optional, not required. 33 CFR §83.23
• ≥ 50 m: Second masthead light is mandatory. 33 CFR §83.23

2. Placement and spacing of masthead lights. Questions frequently ask about minimum heights and the required vertical separation between forward and after masthead lights. The 2-meter minimum vertical difference between the two masthead lights and the 1,000-meter separation test are both exam-tested specifics. 33 CFR §84.02

3. Horizontal spacing. The rule that the horizontal distance between two masthead lights must be at least one quarter of the vessel's length — and that the forward light must be within one half the vessel's length of the stem — appears in exam questions about light placement compliance. 33 CFR §84.03

4. Great Lakes substitution. Candidates frequently confuse which lights the Great Lakes all-round white light replaces. It replaces the second masthead light and the sternlight only — not the forward masthead light or the sidelights. 33 CFR §83.23

5. Special vessel types. Air-cushion vessels and WIG craft carry additional lights on top of the Rule 23(a) suite. The exam distinguishes between the flashing yellow (air-cushion, non-displacement mode) and the high-intensity flashing red (WIG craft, during takeoff/landing/flight near surface). 33 CFR §83.23

6. Relationship to Rule 24. When a power-driven vessel is towing or pushing, Rule 24 modifies the masthead light configuration — replacing the single forward masthead light with two (or three) masthead lights in a vertical line. The sidelights and sternlight requirements from Rule 23 are retained. 33 CFR §83.24 Candidates must not confuse the Rule 23 standard configuration with the Rule 24 towing configuration.

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Common Pitfalls

Pitfall 1: Assuming the second masthead light is always required. It is only mandatory for vessels 50 meters or more in length. A 45-meter power-driven vessel is not obliged to carry it, though it may. 33 CFR §83.23

Pitfall 2: Misapplying the < 12 m alternative. The all-round white light plus sidelights option is available only to vessels less than 12 meters in length. A 12-meter vessel does not qualify — it must use the full Rule 23(a) configuration. 33 CFR §83.23

Pitfall 3: Confusing the Great Lakes substitution scope. The Great Lakes all-round white light replaces the after masthead light and sternlight. The forward masthead light and sidelights remain required. Candidates who think the substitution covers the forward masthead light will select the wrong answer. 33 CFR §83.23

Pitfall 4: Forgetting the 1,000-meter separation test. The vertical separation between masthead lights is not just a fixed measurement — it must be sufficient so the after light is seen over and separate from the forward light at 1,000 meters from the stem at water level. This is a functional requirement, not merely a dimensional one. 33 CFR §84.02

Pitfall 5: Placing sidelights forward of the forward masthead light. On vessels 20 meters or more in length, sidelights must not be placed forward of the forward masthead light. This is a placement violation that exam questions test directly. 33 CFR §84.03

Pitfall 6: Confusing air-cushion and WIG craft additional lights. Air-cushion vessel (non-displacement mode): all-round flashing yellow. WIG craft (takeoff/landing/near-surface flight): high-intensity all-round flashing red. These are not interchangeable. 33 CFR §83.23

Pitfall 7: Applying Rule 23 to a vessel under sail with machinery. A vessel proceeding under sail while also being propelled by machinery is treated as a power-driven vessel and must exhibit a conical shape, apex downward, forward where it can best be seen (vessels less than 12 meters are not required to exhibit this shape). 33 CFR §83.25 The lights, however, follow the power-driven vessel rules, not the sailing vessel rules.

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Quick Check

Q1: A power-driven vessel is 38 meters in length and underway at night. Is a second masthead light required?

No. A vessel of less than 50 meters in length is not obliged to exhibit a second masthead light, though it may do so. The required lights are: one forward masthead light, sidelights, and a sternlight. 33 CFR §83.23

Q2: A power-driven vessel is 9 meters in length. What is the simplified light option available to it at night?

A power-driven vessel of less than 12 meters in length may, in lieu of the standard Rule 23(a) lights, exhibit an all-round white light and sidelights. The all-round white light must be carried at least one meter higher than the sidelights. 33 CFR §83.23 33 CFR §84.02

Q3: A power-driven vessel operating on the Great Lakes substitutes an all-round white light for certain lights. Which lights does it replace?

The all-round white light replaces the second masthead light and the sternlight. The forward masthead light and sidelights are still required. The substitution light must be carried in the position of the second masthead light and be visible at the same minimum range. 33 CFR §83.23

Q4: Two masthead lights are carried on a power-driven vessel. What is the minimum required vertical difference in height between them?

The after masthead light must be at least 2 meters vertically higher than the forward masthead light. Additionally, the separation must be sufficient so the after light is seen over and separate from the forward light at 1,000 meters from the stem when viewed from water level. 33 CFR §84.02

Q5: An air-cushion vessel is operating in non-displacement mode. What additional light must it show beyond the standard Rule 23(a) lights?

An all-round flashing yellow light, placed where it can best be seen, in addition to all lights prescribed in Rule 23(a). 33 CFR §83.23

Q6: A power-driven vessel is 55 meters in length. What is the minimum horizontal distance required between its two masthead lights?

Not less than one quarter of the vessel's length. For a 55-meter vessel, that is not less than 13.75 meters. The distance need not exceed 50 meters. The forward masthead light must be placed not more than 27.5 meters (one half the vessel's length) from the stem. 33 CFR §84.03

Q7: What additional light does a WIG craft exhibit, and under what conditions?

A WIG craft exhibits a high-intensity all-round flashing red light, but only when taking off, landing, and in flight near the surface. This is in addition to all lights prescribed in Rule 23(a). 33 CFR §83.23