COLREG Rule 24 Explained — Towing and Pushing Lights

A plain-language guide to COLREG Rule 24: how a vessel towing astern, pushing ahead or towing alongside is lit at night, what the yellow towing light means, and why the 200-metre tow length changes everything.

What does Rule 24 actually say?

COLREG Rule 24 — "Towing and pushing" — describes how vessels engaged in towing or pushing operations are lit and marked. It builds directly on Rule 23 (the lights of an ordinary power-driven vessel) by adding or substituting lights so that other vessels can see, at a glance, that there is a tow — and crucially, how long it is.

The danger Rule 24 exists to prevent is simple and deadly: a towing vessel and her tow may be separated by hundreds of metres of barely-visible towline. A vessel passing between them — "cutting the tow" — can be capsized or sunk. Rule 24's extra lights and the day diamond are the warning that you must pass around the whole assembly, never between the towing vessel and what she is pulling.

Towing astern — the full light set

Rule 24(a): a power-driven vessel when towing astern shall exhibit —

The signature of a towing vessel at night is therefore two white lights stacked vertically at the foremast, and a yellow light above a white light at the stern. The yellow-over-white at the stern is unique to towing — no other vessel shows it — so it is a high-value recognition cue.

The 200-metre rule

The single most-tested point in Rule 24 is what happens when the tow gets long. The "length of the tow" is measured from the stern of the towing vessel to the after end of the tow — i.e. the whole assembly, not just the line.

So if you count three white masthead lights in a vertical line, you are looking at a towing vessel with more than 200 metres of tow behind her — give her, and a very large area astern of her, a wide berth.

Pushing ahead and towing alongside

Rule 24(c): a power-driven vessel pushing ahead or towing alongside (except a composite unit, below) shows —

• Two masthead lights in a vertical line (replacing the single forward masthead light);
• Sidelights;
• A sternlight.

Composite units

Rule 24(b): when a pushing vessel and the vessel being pushed ahead are rigidly connected in a composite unit — effectively one vessel — they are lit as a single power-driven vessel under Rule 23. No two-masthead towing signal, because manoeuvrability-wise it behaves as one hull. This is common with integrated tug-barge units.

Lights of the vessel being towed

Rule 24(e): a vessel or object being towed shall show —

• Sidelights;
• A sternlight;
• A diamond shape where it can best be seen, if the tow exceeds 200 m.

Note what the tow does not show: no masthead lights, no towing light. So the assembly reads as: leading vessel with two (or three) vertical masthead lights and a yellow towing light, then — possibly far astern — a set of sidelights and a sternlight with no masthead light. Recognising that the trailing lights "belong" to the leading vessel's tow is the whole skill.

Inconspicuous and partly submerged tows

Rule 24(g) covers inconspicuous, partly submerged vessels or objects being towed — semi-submersible barges, large floating structures. Because they are hard to see, the rule prescribes all-round white lights at the forward and after ends (with spacing rules for wide or long objects) and a diamond shape for the most difficult cases. Rule 24(h) adds that where it is impracticable for a towed vessel or object to show the normal lights, all possible measures shall be taken to light it or at least indicate its presence.

Why this matters — the long dark line

Plenty of serious incidents come from a vessel passing between a tug and its tow at night, snagging or being thrown by an unseen towline. Rule 24 is your defence: if you see two or three stacked white masthead lights, or a yellow-over-white at a vessel's stern, assume a tow extends behind her — possibly a long way — and plan to pass clear of the entire assembly. By day, the diamond shapes carry the same message.

Exam tips and common confusions

• Two vs three masthead lights. Two = tow up to 200 m. Three = tow over 200 m. This is the classic exam discriminator.
• Yellow towing light is astern-only. Towing astern shows it; pushing ahead and towing alongside do not.
• Measuring the tow. From the stern of the towing vessel to the after end of the tow — the whole length, including the towed vessel, not just the line.
• The towed vessel has no masthead light. Sidelights and sternlight only (plus diamond if over 200 m). Don't mistake it for a separate sailing vessel.
• Composite units are lit as one vessel under Rule 23 — no towing signal.
• Day shapes. The diamond only appears when the tow exceeds 200 m — on both the towing vessel and the tow.

Practise towing lights on the simulator

Towing arrangements are among the hardest light pictures to read because the lights are split across two (or more) hulls. SkipperCheck's AIS / radar / COLREG bridge simulator lets you practise exactly this:

• Towing vessel lights — the Rule 24 picture
• Power-driven vessel lights — the Rule 23 baseline it builds on
• Power-driven vs sailing — tell vessel types apart by their lights
• Anchor lights — Rule 30, for contrast

The simulator is part of the Skipper Refresher Course, with free demo scenarios to try first.

Frequently asked questions

What is COLREG Rule 24?

Rule 24 — "Towing and pushing" — sets out the lights and shapes for vessels towing or pushing. A vessel towing astern shows two masthead lights in a vertical line (three if the tow exceeds 200 m), sidelights, a sternlight and a yellow towing light, plus a diamond shape by day if the tow is over 200 m.

How many masthead lights does a towing vessel show?

Two in a vertical line for a tow of 200 m or less; three in a vertical line for a tow over 200 m. A towing/pushing vessel of 50 m or more also carries the second, higher after masthead light of Rule 23(a)(ii).

What is the yellow towing light?

A yellow light with the same characteristics as the sternlight (135° astern), shown directly above the sternlight on a vessel towing astern. Yellow-over-white at the stern is unique to towing astern. Vessels pushing ahead or towing alongside do not show it.

What is the 200 metre rule?

When the tow — measured from the stern of the towing vessel to the after end of the tow — exceeds 200 metres, the towing vessel shows three masthead lights instead of two, and both the towing vessel and the towed vessel show a diamond shape by day.

What lights does the vessel being towed show?

Sidelights and a sternlight, plus a diamond shape if the tow exceeds 200 m. It shows no masthead lights and no towing light. Inconspicuous, partly submerged towed objects show special all-round white lights and shapes under Rule 24(g).

Why must I never pass between a tug and its tow?

The towline between them may be hundreds of metres long and almost invisible at night. Passing between the two risks snagging the line or being struck or capsized. Rule 24's stacked masthead lights, yellow towing light and day diamonds warn you to pass clear of the whole assembly.

Related reading

• COLREG Rule 23 Explained — Lights for Power-Driven Vessels — the baseline Rule 24 builds on
• COLREG Rule 18 Explained — Responsibilities Between Vessels — who gives way to whom
• Short Range Certificate (SRC) — Complete Guide — the marine VHF qualification
• Maritime glossary — sailing, COLREG, VHF and navigation terms