Browse the COLREGS Manual

(a) A power-driven vessel when towing astern shall exhibit: (i) Instead of the light prescribed in Rule 23(a)(i) or Rule 23(a)(ii), two masthead lights in a vertical line. When the length of the tow, measuring from the stern of the towing vessel to the after end of the tow, exceeds 200 meters, three such lights in a vertical line; (ii) sidelights; (iii) a sternlight; (iv) a towing light in a vertical line above the sternlight; and (v) when the length of the tow exceeds 200 meters, a diamond shape where it can best be seen.

(b) When a pushing vessel and a vessel being pushed ahead are rigidly connected in a composite unit they shall be regarded as a power-driven vessel and exhibit the lights prescribed in Rule 23(a).

(c) A power-driven vessel when pushing ahead or towing alongside, except in the case of a composite unit as required by Rule 24(b) and (g)(i), shall exhibit: (i) instead of the light prescribed in Rule 23(a)(i) or Rule 23(a)(ii), two masthead lights in a vertical line; (ii) sidelights; and (iii) a sternlight two towing lights in a vertical line.

(d) A power-driven vessel to which Rule 24(a) or (c) applies shall also comply with Rule 23(a) (a)(i) and (a)(ii).

(e) A vessel or object being towed, other than those referred to in Rule 24(g), shall exhibit: (i) sidelights; (ii) a sternlight; and (iii) when the length of the tow exceeds 200 meters, a diamond shape where it can best be seen.

(f) Provided that any number of vessels being towed alongside or pushed in a group shall be lighted as one vessel except as provided in Rule 24(f)(iii): (i) a vessel being pushed ahead, not being part of a composite unit, shall exhibit at the forward end, sidelights, and a special flashing light; (ii) a vessel being towed alongside shall exhibit a sternlight and at the forward end, sidelights, and a special flashing light.

(g) An inconspicuous, partly submerged vessel or object, or combination of such vessels or objects being towed, shall exhibit:

(iii) if it exceeds 100 meters in length, additional all-round white lights between the lights prescribed in Rule 24(g)(i) and (ii) and so that the distance between the lights shall not exceed 100 meters, provided that any vessels or objects being towed alongside each other shall be lighted as one vessel or object; (iv) a diamond shape at or near the aftermost extremity of the last vessel or object being towed; and if the length of the tow exceeds 200 meters an additional diamond shape where it can best be seen and located as far forward as is practicable.

(h) Where from any sufficient cause it is impracticable for a vessel or object being towed to exhibit the lights or shapes prescribed in Rule 24(e) or (g), all possible measures shall be taken to light such the unlighted vessel or object.

(i) Where from any sufficient cause it is impracticable for a vessel not normally engaged in towing operations to display the lights prescribed in Rule 24(a) or Rule 24(c), such vessel shall not be required to exhibit those lights when engaged in towing another vessel in distress or otherwise in need of assistance. All possible measures shall be taken to indicate the nature of the relationship between the towing vessel and the vessel being towed as authorized by Rule 36, in particular by illuminating the towline and the vessel being assisted. The searchlight authorized by Rule 36 may be used to illuminate the tow.

Color note: black text applies generally; blue is International Waters; green is Inland Waters.

MyCOLREGS explanation

A power-driven vessel when towing astern shall exhibit: (i) Instead of the light prescribed in Rule 23(a)(i) or Rule 23(a)(ii), two masthead lights in a vertical line.

A power-driven vessel when towing astern at a distance exceeding 200 meters from the stern of the towing vessel to the after end of the tow shall exhibit: three masthead lights in a vertical line.

Summary

A towing vessel's lights announce that there is more to her than meets the eye. Towing astern, she shows two (or three, if the tow exceeds 200 meters) masthead lights in a vertical line instead of the usual one or two, plus sidelights, a sternlight, and a towing light above the sternlight - and a diamond shape by day if the tow is long. A rigidly connected pushed unit lights up as a single power-driven vessel, but pushing ahead or towing alongside without a rigid connection calls for two masthead lights plus sidelights and stern-facing lights, with some wording differences between international and inland waters on exactly which stern-facing lights. The tow itself - the vessel or object actually being towed - carries its own sidelights, sternlight, and (if long enough) a diamond shape, and an inconspicuous, partly submerged tow needs extra all-round white lights sized to its breadth and length. If lighting the tow properly is not practicable, every possible measure must still be taken to mark it or the tow's presence somehow, including when the towing vessel is herself engaged in a distress rescue.

Real World Example

The real hazard isn't the tug or the tow themselves but the towline between them, which can run for hundreds of feet of nearly invisible cable under real tension - enough to capsize or dismast a small boat that snags it. A boat cutting between a tug's lights and her tow's lights at night, thinking she's found open water, may actually be crossing straight over gear that no light on either vessel is required to mark.