Interactive 3D Viewer
3DDrag to rotate
Active lights
Masthead lightAft mastheadPort sidelightStarboard sidelightSternlight
Part CCritical
Rule 23: Power-driven vessels underway
Power-driven vessels underway show a standard light set, with clear size and craft-type variations.
Detailed Explanation
(a) Power-driven vessel underway shows: (i) forward masthead; (ii) second masthead abaft/higher (not mandatory if <50 m); (iii) sidelights; (iv) sternlight.
(b) Air-cushion vessel in non-displacement mode adds all-round flashing yellow.
(c) WIG craft taking off/landing/near-surface flight adds high-intensity all-round flashing red.
(d) Small vessel options: (i) <12 m may show all-round white + sidelights; (ii) <7 m and max speed ≤7 kn may show all-round white and, if practicable, sidelights; (iii) centerline exceptions permitted under stated conditions.
Key Points
- Power-driven underway: masthead + sidelights + sternlight
- ≥50m: second (higher aft) masthead light required
- <12m: may show all-round white + sidelights instead
- <7m and <7 knots: just all-round white light if practicable
Examples
- You see two white masthead lights vertically separated, a green sidelight and a sternlight: a power-driven vessel 50 m or over showing her starboard side, as prescribed by Rule 23(a).
- A small boat shows only an all-round white light and sidelights — this is a power-driven vessel under 12 m using the Rule 23(d)(i) simplified arrangement.
- A high-speed catamaran ferry displays a single masthead light and the standard sidelights/sternlight. At 40 m length, she needs only one masthead light per Rule 23(a).
Common Mistakes
- Expecting a vessel under 50 metres to show two masthead lights when one is sufficient.
- Confusing the all-round white light option for vessels under 12 metres with a towing light arrangement.
- Forgetting to show sidelights when a power-driven vessel under 7 metres exceeds 7 knots.