Lights for a Power-driven Vessel Underway (Rule 23)

What the rule requires

Paragraph (a) is a single sentence with four items in it, and the exam works that sentence harder than the whole rest of the rule.

A masthead light forward.
A second masthead light, abaft of and higher than the forward one. A vessel of less than 50 metres in length is not obliged to exhibit it but may do so.
Sidelights.
A sternlight.

That is the wording of 33 CFR §83.23, and it is word for word the wording of COLREGs Rule 23. When a question on paragraph (a) is labelled "(Both International & Inland)", there is no trap in the label — the light arrangement for a power-driven vessel underway is identical under both sets.

So 50 metres is the line, and the second masthead light is mandatory at or above it, permissive below it. The rule does not forbid the smaller vessel from carrying and showing one.

Rule 23 also carries no speed condition. The trigger is a power-driven vessel underway; paragraph (a) makes no distinction between a vessel driving ahead at fourteen knots and one lying stopped with her engines idling off a berth. Same four lights.

A 46-metre offshore supply vessel is on a night transit showing two masthead lights, sidelights and a sternlight. Is she in compliance?

Yes. At less than 50 metres she is not obliged to exhibit the second masthead light, but the rule expressly permits her to. An answer choice claiming she is over-lit is wrong. Had she been 50 metres or more, the second masthead light would have been required rather than optional.

Under 12 metres: the all-round white light

A power-driven vessel of less than 12 metres in length may, in lieu of the lights prescribed in paragraph (a), exhibit an all-round white light and sidelights . The same option exists under . Two things about it decide questions: the substitution is optional, and sidelights are named inside the substitution itself, so they are not among the lights dispensed with.

Most six-pack candidates are answering this one about their own launch.

Under International there is a further construction allowance for the same size band: on a vessel of less than 12 metres the masthead light or all-round white light may be displaced from the fore and aft centreline where centreline fitting is not practicable, provided the sidelights are combined in one lantern carried on the centreline, or as nearly as practicable in the same fore and aft line as that white light COLREGs Rule 23(d). The text of 33 CFR §83.23 carries no equivalent paragraph. It is rarely asked; know that it exists and that it is an International provision.

The Great Lakes option — Inland only

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, carried in the position of the second masthead light and visible at the same minimum range .

That substitution takes out two lights, not one, and it leaves the forward masthead light and the sidelights exactly as paragraph (a) prescribes them. COLREGs Rule 23 has no counterpart, so any question turning on it is an "(Inland Only)" question.

Under 7 metres and not more than 7 knots — International only

A power-driven vessel of less than 7 metres in length whose maximum speed does not exceed 7 knots may, in lieu of the paragraph (a) lights, exhibit an all-round white light, and shall if practicable also exhibit sidelights .

Both halves of the trigger have to be satisfied: the length and the speed. A 6-metre skiff that will do 25 knots does not qualify. This is the only place in Rule 23 where the sidelight requirement is softened to "if practicable", and it is the only paragraph of Rule 23 flagged International only.

Your 6-metre skiff, maximum speed 6 knots, is underway after dark on inland waters. May you exhibit an all-round white light alone?

No. The 7-metre / 7-knot provision is in COLREGs Rule 23(d) and has no counterpart in 33 CFR §83.23. On inland waters you fall back on the under-12-metre option — an all-round white light and sidelights — or the full paragraph (a) arrangement.

Air-cushion vessels, WIG craft, composite units

An air-cushion vessel operating in the non-displacement mode exhibits, in addition to the paragraph (a) lights, an all-round flashing yellow light, which the Inland text places where it can best be seen . The requirement is the same under International .
A WIG craft only when taking off, landing and in flight near the surface exhibits, in addition to the paragraph (a) lights, a high intensity all-round flashing red light. Both regimes.
A pushing vessel and a vessel being pushed ahead rigidly connected in a composite unit are regarded as a power-driven vessel and exhibit the lights prescribed in Rule 23 COLREGs Rule 24(b), and the Inland rule is the same.

In all three the trigger is a condition rather than a vessel type: the mode she is operating in, the phase of flight, the way the units are connected.

Telling it apart

Four different single white lights appear in this material, and the criterion that separates them is what each one stands in for.

All-round white light, vessel under 12 metres — stands in for the whole of paragraph (a); sidelights unaffected. Both regimes.
All-round white light, Great Lakes — stands in for the second masthead light and the sternlight only, carried in the position of the second masthead light at the same minimum range. Forward masthead light and sidelights unchanged. Inland only.
All-round white light, under 7 metres and 7 knots — stands in for the whole of paragraph (a), sidelights required only if practicable. International only.
Combined lantern, sailing vessel under 20 metres — the lights prescribed in Rule 25(a) may be combined in one lantern carried at or near the top of the mast COLREGs Rule 25(b). This is a Rule 25 permission for a sailing vessel; it is not a Rule 23 light and it is not a masthead light, notwithstanding where it is fitted.

Separate the two masthead lights the same way, by position: the required one is forward, and the conditional one is abaft of and higher than it.

Working a question

A 30-metre twin-screw crew boat is underway at night on the Houston Ship Channel, holding station at idle while she waits for a berth. What must she exhibit?

Fix the regime. Inland waters, so 33 CFR §83.23 governs. Do this first on every lights question, because three of the substitutions above live in only one of the two rule sets.
Confirm Rule 23 applies. She is power-driven and underway. Engines at idle changes nothing — paragraph (a) attaches to "underway" with no speed condition attached to it.
Take the length threshold. 30 metres is less than 50, so the second masthead light is optional. The forward masthead light, sidelights and sternlight are required.
Test each substitution. Under 12 metres? No, she is 30. Operating on the Great Lakes? No. Neither substitution is available to her, so paragraph (a) stands in full.
Test construction and mode. Not an air-cushion vessel, not a WIG craft, so no flashing yellow and no flashing red.
Answer. Masthead light forward, sidelights, sternlight — plus a second masthead light if she is fitted with one and elects to show it.

Now change one fact. Move the same vessel to a night passage on Lake Erie: 33 CFR §83.23(e) opens up, and she may carry an all-round white light in the position of the second masthead light, at the same minimum range, in place of both the second masthead light and the sternlight. Her forward masthead light and sidelights are unchanged. Change the fact back to a run from Galveston out to a rig offshore and that option closes again.

Where candidates lose the point

Reading the under-50-metre exemption as a prohibition. The distractor is the answer saying a 40-metre vessel showing two masthead lights is not complying. The rule says she is not obliged to exhibit the second one but may do so.
Taking the under-12-metre option as licence to show one white light and nothing else. The attractive answer is "an all-round white light only", because that is what a great many small craft actually show. The clause reads all-round white light and sidelights.
Using the 7-metre / 7-knot option on inland waters. It is memorable because of the doubled sevens, so it gets applied everywhere. It is in COLREGs Rule 23(d) and nowhere in 33 CFR §83.23.
Halving the Great Lakes substitution. Candidates recall that the all-round white light replaces the second masthead light and stop there, then pick an answer that keeps the sternlight burning. It replaces the sternlight as well.
Turning the Great Lakes provision into an Inland provision generally. It names the Great Lakes. It does not reach the Western Rivers or any other inland waters.
Flashing yellow on an air-cushion vessel whenever she is under way. The trigger is the non-displacement mode, not the hull form; the same vessel proceeding in displacement mode is on her paragraph (a) lights.
Flashing red on a WIG craft that is merely proceeding on the surface. The high intensity all-round flashing red light is for taking off, landing and flight near the surface only.
Reaching for towing lights on a rigidly connected composite unit. Rule 24(b) treats the unit as a power-driven vessel and sends you straight back to Rule 23.

Check yourself

You are on watch aboard a 52-metre inspected passenger vessel making a night run on inland waters. Which lights are you required to exhibit, and which are optional?

At 52 metres she is at or above the 50-metre threshold, so the second masthead light abaft of and higher than the forward one is required, not optional. Required: forward masthead light, second masthead light, sidelights, sternlight. Nothing in paragraph (a) is discretionary for her.

Your 10-metre power-driven launch is underway at night. You elect to use the option Rule 23 gives a vessel of her length. What do you show?

An all-round white light and sidelights. The all-round white light substitutes for the paragraph (a) masthead and sternlight arrangement; the sidelights are inside the substitution and are still exhibited. The option is identical under International and Inland.

Northbound on Lake Michigan at night, your 35-metre vessel is showing a forward masthead light, an all-round white light in the position of the second masthead light, and sidelights. No sternlight is burning. Are you compliant?

Yes, on the Great Lakes. 33 CFR §83.23(e) permits the all-round white light in lieu of both the second masthead light and the sternlight, carried in the position of the second masthead light and visible at the same minimum range. Take the same arrangement to sea under COLREGs and it has no authority.

An air-cushion vessel is proceeding in the displacement mode on a night passage. What does she exhibit?

The paragraph (a) lights for her length, and no flashing yellow light. The all-round flashing yellow light is required only when she is operating in the non-displacement mode.

Your towing vessel is made up rigidly to a loaded barge ahead of her as a composite unit, proceeding at night. Which rule governs your lights?

Rule 24(b): rigidly connected, the pushing vessel and the vessel pushed ahead are regarded as a power-driven vessel and exhibit the lights prescribed in Rule 23. Same under Inland. Break the rigid connection and you are no longer in this provision.

You sight a single white lantern near the top of a mast, with no other lights showing high above it. Does that tell you the vessel is power-driven?

No. A sailing vessel of less than 20 metres may combine the lights prescribed in Rule 25(a) in one lantern carried at or near the top of the mast, and that is a Rule 25 permission with nothing to do with the masthead light of Rule 23. Position high on a mast does not by itself identify a Rule 23 masthead light.

A 6-metre power-driven skiff whose maximum speed is 20 knots is underway at night on international waters. May she exhibit an all-round white light alone?

No. She meets the length half of the trigger but not the speed half — the provision requires a maximum speed not exceeding 7 knots. She falls back to the under-12-metre option, all-round white light and sidelights, or to the full paragraph (a) arrangement.