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Towing: tow length and light functions

The measurement starts at the towing vessel’s stern; a towing light is not all-round.

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In this guide
Unscaled fictional example: towing stern to tow bow190m, towed hull20m, total210m to final stern. Simple tow above200m functions: three white mastheads vertically and yellow above white sternlight, plus sidelights and23(a)(ii). Measurement line is not cable or track.
Towing · extent and functions

Original interpretation schematic with declared functions and references. No scale, approved installation or operational track.

insideSailOriginal insideSail artwork — all rights reserved
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Unscaled fictional example: towing stern to tow bow190m, towed hull20m, total210m to final stern. Simple tow above200m functions: three white mastheads vertically and yellow above white sternlight, plus sidelights and23(a)(ii). Measurement line is not cable or track.

A tow occupies more water than the hull first seen. Rule 24 measures from the towing vessel’s stern to the after end of the last towed vessel or object.

The cable is only part of that length. More than two hundred metres changes lights and shapes; exactly two hundred is not more than two hundred.

Rule 21(d) defines the yellow towing light with the same 135-degree aft sector as a sternlight, not all-round.

Towing astern

Under 24(a), machinery towing shows two masthead lights vertically instead of the specified masthead, increasing to three when tow length exceeds two hundred metres. It also shows sidelights, sternlight and yellow towing light vertically above the white sternlight.

A diamond is required above that tow-length threshold. Paragraph (d) retains 23(a)(ii), including the second masthead compulsory from fifty metres.

Two or three vertical whites therefore need not be every masthead light on the towing vessel.

An ordinary tow under 24(e) shows sidelights and sternlight, plus a diamond above the threshold. A masthead is not assigned merely because it has an installed engine that is not propelling it.

Towing-vessel and tow lights may overlap visually or be widely separated. Identifying the nearer hull does not identify the whole tow’s end.

A later bearing change does not cancel overtaking obligations or appraisal of the entire tow’s collision risk.

Pushing and towing alongside differ

Pushing ahead or towing alongside outside a composite unit, 24(c) uses two vertical mastheads, sidelights and sternlight, without the yellow towing light prescribed in (a). Pushed groups and alongside tows have display conditions in (f).

A rigid connection forming a composite unit takes, under 24(b), the Rule 23 lights of a power-driven unit. Proximity or connection by ropes does not alone establish the required rigidity.

Inconspicuous or partly submerged objects have all-round whites and diamonds under 24(g), according to breadth, length and position. The rule does not promise easy detection.

Justified inability under (h), and exceptional assistance towing under (i), retain measures to indicate presence and connection; they are not a general exemption for missing equipment. Towing that severely restricts ability to deviate can add 27(c) signals; not every tow has that status.

The figure measures a declared example and organises light functions for the simple case. It does not draw a real cable, installation dimension or passage between components.

No measurement line is an authorised route. Annex I and all Rule 24 conditions remain necessary for design and compliance.

For recognition, retain three questions: where the whole tow ends, which towing category is declared and which signals the observed aspect might hide.

Sources and references

  1. RIEAM / COLREGs — Rule 13: Overtaking ↗

    Instituto Hidrográfico / International Maritime Organization · Rule 13, normative Portuguese printed p.19 (PDF p.25); International English cross-check in USCG handbook.

    Rule 13; IH normative Portuguese printed p.19, PDF p.25; English International column cross-check. Individual provision commencement not established: valid_from remains null.

    Checked on
  2. RIEAM / COLREGs — Rule 36: Signals to attract attention ↗

    Instituto Hidrográfico / International Maritime Organization · Rule 36, normative Portuguese printed p.59 (PDF p.65); International English cross-check in USCG handbook.

    Rule 36; IH normative Portuguese printed p.59, PDF p.65; English International column cross-check. Individual provision commencement not established: valid_from remains null.

    Checked on
  3. RIEAM / COLREGs — Rule 20: Application of lights and shapes ↗

    Instituto Hidrográfico / International Maritime Organization · Rule 21, normative Portuguese printed p.29 (PDF p.35); International English cross-check in USCG handbook.

    Rule 21; IH normative Portuguese printed p.29, PDF p.35; English International column cross-check. Individual provision commencement not established: valid_from remains null.

    Checked on
  4. USCG amalgamated rules — International current web cross-check ↗

    United States Coast Guard Navigation Center · International/shared Rules 3,20–30,32–35; International Annexes I–III. International-only wording identified by angle-bracket legend; all Inland insertions excluded. Rule35(j)/(k) compared with IH normative Portuguese printed55/PDF61 and English appendix89–90/PDF95–96.

    Directly read 11 October2026 alongside freshly fetched corrected-August2024 official handbook. Current page confirms under12 paragraph(j) and pilot(k); Portuguese IH agrees; its English appendix differs. Access is not qualified legal approval. No unsupported exact consolidation date assigned.

    Checked on
  5. RIEAM / COLREGs — Rule 23: Power-driven vessels underway ↗

    Instituto Hidrográfico / International Maritime Organization · Rule 23, normative Portuguese printed p.31 (PDF p.37); International English cross-check in USCG handbook.

    Rule 23; IH normative Portuguese printed p.31, PDF p.37; English International column cross-check. Individual provision commencement not established: valid_from remains null.

    Checked on
  6. RIEAM / COLREGs — Rule 2: Responsibility ↗

    Instituto Hidrográfico / International Maritime Organization · Rule 3, normative Portuguese printed p.3 (PDF p.9); International English cross-check in USCG handbook.

    Rule 3; IH normative Portuguese printed p.3, PDF p.9; English International column cross-check. Individual provision commencement not established: valid_from remains null.

    Checked on
  7. RIEAM / COLREGs — Rule 6: Safe speed ↗

    Instituto Hidrográfico / International Maritime Organization · Rule 7, normative Portuguese printed p.8 (PDF p.14); International English cross-check in USCG handbook.

    Rule 7; IH normative Portuguese printed p.8, PDF p.14; English International column cross-check. Individual provision commencement not established: valid_from remains null.

    Checked on
  8. COLREGs Annex I — Positioning and technical details of lights and shapes ↗

    Instituto Hidrográfico / International Maritime Organization · Annex I; printed p.65, PDF p.71; International cross-check USCG.

    Annex I, IH printed p.65 (PDF p.71); cross-check International annex in USCG handbook. Installation/approval is not taught. International/Inland distinction checked in the USCG PDF: International §3(a) half-length/100m agrees with IH; the quarter-length/50m value belongs to U.S. Inland and is not imported. No dimensional installation prescription is reproduced.

    Checked on
  9. RIEAM / COLREGs — Rule 27: Vessels not under command or restricted in their ability to manoeuvre ↗

    Instituto Hidrográfico / International Maritime Organization · Rule 27, normative Portuguese printed p.43 (PDF p.49); International English cross-check in USCG handbook.

    Rule 27; IH normative Portuguese printed p.43, PDF p.49; English International column cross-check. Individual provision commencement not established: valid_from remains null.

    Checked on
  10. RIEAM / COLREGs — Rule 24: Towing and pushing ↗

    Instituto Hidrográfico / International Maritime Organization · Rule 24, normative Portuguese printed p.34 (PDF p.40); International English cross-check in USCG handbook.

    Rule 24; IH normative Portuguese printed p.34, PDF p.40; English International column cross-check. Individual provision commencement not established: valid_from remains null.

    Checked on

Regulatory references

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Rule 13 overtaking Risk of collision

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