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Rule 23: Power-driven vessels underway
Power-driven vessels underway show a standard light set, with clear size and craft-type variations.
Detailed Explanation
Power-driven vessels underway display a standard masthead–sidelight–sternlight set, with variations by size and craft type.
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.