Rule 24 — Lights for Towing Astern, Pushing Ahead and Towing Alongside

What the rule requires

Rule 24 sorts by one question: where is the tow, and how is it connected? Get the paragraph right and the light list follows from it. Get the paragraph wrong and every distractor on the page looks plausible.

Towing astern

A power-driven vessel towing astern exhibits two masthead lights in a vertical line in place of the Rule 23(a)(i) or (a)(ii) masthead light, sidelights, a sternlight, a towing light in a vertical line above the sternlight, and, when the length of the tow exceeds 200 meters, a diamond shape where it can best be seen COLREGs Rule 24(a). Past 200 meters the vertical masthead lights become three. Measure the tow from the stern of the towing vessel to the after end of the tow, not the length of the towline and not the length of the barge.

The towing light is the identifying feature. A towing light is a yellow light having the same characteristics as the sternlight COLREGs Rule 21(d). One towing light sitting above the sternlight means towing astern, and that vertical pair reads the same under International and Inland 33 CFR §83.24.

Paragraph (d) is masthead-light housekeeping. A vessel to which (a) or (c) applies must also comply with Rule 23(a), and the after masthead light remains optional for a vessel of less than 50 meters in length under both regimes COLREGs Rule 24(d).

Pushing ahead or towing alongside — the divergence

International — two masthead lights in a vertical line, sidelights, and a sternlight, except in the case of a composite unit COLREGs Rule 24(c).
Inland — two masthead lights in a vertical line, sidelights, and two towing lights in a vertical line, except where paragraph (b) or (i) applies .

Two towing lights in a vertical line answers Inland pushing ahead or towing alongside. One towing light above a sternlight answers towing astern in either regime. Candidates who hold those two sentences take most of the light-identification questions on this rule.

(Inland Only) You sight a vessel showing two masthead lights in a vertical line, a green sidelight, and two yellow lights in a vertical line at her stern. What are you looking at?

A power-driven vessel pushing ahead or towing alongside. Two towing lights in a vertical line is the Inland signature for that configuration under 33 CFR §83.24(c). Had you seen a single towing light above a sternlight, she would be towing astern.

The composite unit

When a pushing vessel and a vessel being pushed ahead are rigidly connected in a composite unit, they are regarded as a power-driven vessel and exhibit the lights prescribed in Rule 23 COLREGs Rule 24(b). The unit is lit as one power-driven vessel: no towing lights, no special flashing light, no vertical pair of masthead lights. Both regimes agree.

The definition decides the question. A composite unit is a pushing vessel rigidly connected by mechanical means to the vessel being pushed so that they react to sea and swell as one vessel, and "mechanical means" expressly excludes lines, hawsers, wires and chains 33 CFR §82.3. A barge winched up hard on face wires until nothing moves still falls outside the definition, and the tug shows the pushing-ahead lights.

A tug is pushing a loaded barge ahead, made up with face wires set up so tight the two work as one in a following sea. Is she a composite unit?

No. 33 CFR §82.3 excludes wires from "mechanical means," along with lines, hawsers and chains, so the arrangement cannot be a composite unit regardless of how rigidly it behaves. She exhibits pushing-ahead lights. Inland: two masthead lights in a vertical line, sidelights, two towing lights. International: two masthead lights in a vertical line, sidelights, a sternlight.

What the tow itself shows

A vessel or object being towed, other than the partly submerged tows of paragraph (g), exhibits sidelights, a sternlight, and a diamond shape where it can best be seen when the length of the tow exceeds 200 meters COLREGs Rule 24(e). That is identical under both regimes. On a tow astern over 200 meters, two diamonds are in the water: one on the towing vessel under (a)(v), one on the tow under (e)(iii).

Any number of vessels towed alongside or pushed in a group is lighted as one vessel. Beyond that the regimes part company:

Pushed ahead, not part of a composite unit — International: sidelights at the forward end COLREGs Rule 24(f). Inland: sidelights and a special flashing light at the forward end .
Towed alongside — International: a sternlight, and sidelights at the forward end. Inland: a sternlight, and sidelights and a special flashing light at the forward end.
Towed alongside on both sides of the towing vessel (Inland only) — a sternlight on the stern of the outboard vessel on each side, a single set of sidelights as far forward and as far outboard as practicable, and a single special flashing light.

The special flashing light exists only in the Inland rule. On a question labelled International, any option containing one is a distractor.

Western Rivers

Pushing ahead is the standard towing arrangement on the Western Rivers, and the light arrangements differ significantly from open-water towing astern USCG Boatswain Manual Ch. 3 §3.3.

The governing text there is Inland paragraph (j). Notwithstanding (c), on the Western Rivers except below the Huey P. Long Bridge at mile 106.1 Above Head of Passes on the Mississippi River, and on waters specified by the Secretary, a power-driven vessel pushing ahead or towing alongside, composite units excepted, exhibits sidelights and two towing lights in a vertical line . No masthead lights at all.

The boundary is arbitrary and has to be memorised. Mississippi River mileage runs upward as you proceed upriver from the Head of Passes, so waters "below" the bridge are the waters downriver of mile 106.1 AHP, carrying lower AHP numbers, toward the Gulf. In that stretch paragraph (c) governs and the masthead pair goes back on.

Inconspicuous, partly submerged tows

Low-frequency material, but the breadth thresholds get asked. A tow of less than 25 meters in breadth carries one all-round white light at or near each end under both regimes, though International excuses dracones from the forward light COLREGs Rule 24(g). At 25 meters or more in breadth the two texts reach the same display of four all-round white lights. Inland states the result directly as four all-round white lights marking length and breadth ; International arrives at four by adding two lights at the extremities of the breadth to the two end lights already required. Over 100 meters in length, both require additional all-round white lights between them so that no gap exceeds 100 meters.

Shapes. Both regimes place a diamond at or near the aftermost extremity of the last vessel or object towed. International requires a second diamond, as far forward as practicable, when the length of the tow exceeds 200 meters; the Inland rule prescribes no forward diamond at any tow length. Inland adds that the towing vessel may direct a searchlight in the direction of the tow to indicate its presence to an approaching vessel.

Where it is impracticable for the tow to display its prescribed lights or shapes, all possible measures must be taken to light it or at least indicate its presence COLREGs Rule 24(h). Where a vessel not normally engaged in towing operations is assisting a vessel in distress and cannot display the towing lights, she is not required to exhibit them, but all possible measures must be taken to indicate the relationship: International speaks of illuminating the towline as authorized by Rule 36 COLREGs Rule 24(i), Inland of the searchlight authorized by Rule 36 to illuminate the tow.

Telling it apart

The criterion is the connection: where the tow sits relative to the towing vessel, and whether the coupling is a rigid mechanical connection. Neither the tug's size nor the barge's cargo enters into it.

Composite unit. Rigid mechanical connection, reacting to sea and swell as one vessel. Rule 23 lights, both regimes. The trap is a hard push on face wires, which §82.3 excludes by name.
Pushing ahead, not composite. Tug shows the (c) or (j) lights. The barge shows sidelights forward, with a special flashing light added under Inland. Candidates who read a solid make-up as rigid connection answer with Rule 23 lights and lose the question.
Towing alongside. Tow on the hip, tug lights as for pushing ahead in each regime, and the tow adds a sternlight. Watch for a special flashing light offered on an International answer.
Towing astern. Tow on a hawser, one towing light above the sternlight in both regimes. This is the only configuration in which the 200-meter masthead and diamond triggers operate. Two towing lights answered here is the Inland pushing-ahead display misapplied.

Working a question

(Inland Only) You are on a 25-meter towing vessel pushing a single loaded barge ahead, made up on face wires, on the Mississippi River at mile 60 Above Head of Passes. What lights are required on your vessel and on the barge?

Fix the configuration. Pushed ahead on wires. Wires are not mechanical means under 33 CFR §82.3, so paragraph (b) is out and this is not a composite unit. The candidate paragraphs are (c) and (j).
Fix the regime. The question is labelled Inland, so 33 CFR §83.24 governs and a sternlight is not the answer for the towing vessel.
Test the Western Rivers exception. Paragraph (j) does not reach the Mississippi below the Huey P. Long Bridge at mile 106.1 AHP. Mississippi mileage is counted upward from the Head of Passes, so a lower AHP number lies farther downriver, toward the Gulf. Mile 60 AHP is therefore below mile 106.1 AHP, inside the excluded water, and paragraph (c) applies. The masthead lights stay.
Build the towing vessel's lights. Two masthead lights in a vertical line, sidelights, and two towing lights in a vertical line. Nothing further: the 25-meter length keeps the after masthead light optional, and the diamond is a towing-astern item triggered by tow length, not by pushing.
Build the barge's lights. Paragraph (f)(i): sidelights and a special flashing light at the forward end. No sternlight, because she is pushed ahead rather than towed alongside.
Check the counterfactual the exam will offer. Move the same vessel to mile 150 AHP, upriver of the bridge, and paragraph (j) applies: sidelights and two towing lights only, masthead lights off.

Where candidates lose the point

Vessel length and tow length control different things. The third vertical masthead light answers to the length of the tow exceeding 200 meters, measured from the stern of the towing vessel to the after end of the tow. The length of the towing vessel answers only to whether the after masthead light is optional, which it is below 50 meters.

The sternlight and the special flashing light are not interchangeable. Under Inland, a barge pushed ahead carries sidelights and a special flashing light forward and no sternlight; the sternlight belongs to a tow made up alongside.

The Western Rivers display does not run the length of the Mississippi. Downriver of mile 106.1 AHP the exception drops out and the two-masthead-light display of paragraph (c) is correct.

Diamonds come in pairs on a long tow astern. The towing vessel shows one under (a)(v) and the vessel or object towed shows its own under (e)(iii).

Check yourself

(Both) You are towing a barge astern on a hawser; the distance from your stern to the after end of the barge is 250 meters. What must you exhibit?

Three masthead lights in a vertical line, sidelights, a sternlight, a towing light in a vertical line above the sternlight, and a diamond shape where it can best be seen. The tow exceeds 200 meters, which triggers both the third masthead light and the diamond under Rule 24(a). The barge separately shows sidelights, a sternlight and her own diamond under (e).

(International Only) At night you see two masthead lights in a vertical line, a red sidelight, and a sternlight. Two towing lights are not present. What is she doing?

Pushing ahead or towing alongside. COLREGs Rule 24(c) prescribes two masthead lights in a vertical line, sidelights and a sternlight for that configuration; the absence of towing lights is expected under International, where they are not part of the display.

(Inland Only) A tug on the Western Rivers above the Huey P. Long Bridge is pushing ahead. What does she show?

Sidelights and two towing lights in a vertical line. 33 CFR §83.24(j) displaces paragraph (c) on the Western Rivers except below mile 106.1 AHP, and masthead lights are not required. A composite unit under (b) is excepted and still shows Rule 23 lights.

(Both) Your 40-meter fishing vessel, not normally engaged in towing, takes a disabled vessel in tow. You have no towing light fitted. Are you in violation?

No. Where it is impracticable for a vessel not normally engaged in towing operations to display the paragraph (a) or (c) lights, she is not required to exhibit them when towing a vessel in distress or otherwise in need of assistance. All possible measures must still be taken to indicate the relationship. Under Inland, the searchlight authorized by Rule 36 may illuminate the tow; under International, the towline is illuminated in particular.

(Inland Only) You have barges made up alongside on both sides of your towing vessel. How are the tow's lights arranged?

A sternlight on the stern of the outboard vessel on each side, a single set of sidelights as far forward and as far outboard as practicable, and a single special flashing light. 33 CFR §83.24(f)(iii) is the only exception to lighting a group as one vessel.

(Both) You are towing an inconspicuous, partly submerged object 120 meters long and 30 meters in breadth. What lights and shape does it carry?

Four all-round white lights, marking length and breadth, since the breadth is 25 meters or more. Both regimes reach the same four: Inland states the total outright, while International adds two lights at the extremities of the breadth to the two end lights. Length exceeding 100 meters requires additional all-round white lights so that no gap between lights exceeds 100 meters. A diamond shape goes at or near the aftermost extremity. The International second diamond, located as far forward as practicable, is triggered only by a tow exceeding 200 meters and so does not arise here; the Inland rule prescribes no forward diamond in any case.