Rule 23 – Explained

Part C · Lights and shapes · Rule 23

Power-driven vessels underway

23(a) A power-driven vessel underway shall exhibit:

(i) a masthead light forward;

(ii) a second masthead light abaft of and higher than the forward one; except that a vessel of less than 50 metres in length shall not be obliged to exhibit such light but may do so;

(iii) sidelights;

(iv) a sternlight.

“A power-driven vessel underway shall exhibit”Underway means not at anchor, made fast to the shore or aground (Rule 3(i)), so these lights are shown whether she is making way through the water or stopped.
“(i) a masthead light forward; (ii) a second masthead light abaft of and higher than the forward one”One white masthead light forward and a second further aft and higher. Seen from ahead, the two lights in line mean she is heading straight at you; as she turns away from that line, they open out, and the lower forward light points the way she is heading.
“except that a vessel of less than 50 metres in length shall not be obliged to exhibit such light but may do so”Under 50 metres the second masthead light is optional. So two masthead lights tell you she is probably 50 metres or more, but not for certain; one masthead light means she is probably under 50 metres.
“(iii) sidelights; (iv) a sternlight”Red to port, green to starboard and a white sternlight, with the arcs in Rule 21.

Power-driven vessel underway: masthead light(s), sidelights and sternlight. Two masthead lights means probably 50 m or more.

Power-driven vessel less than 50 metres in length

Power-driven vessel less than 50m, Bow Aspect, Underway, possibly making way – day — Power-driven vessel less than 50mBow AspectUnderway, possibly making way

Power-driven vessel less than 50m, Bow Aspect, Underway, possibly making way – night — Power-driven vessel less than 50mBow AspectUnderway, possibly making way

Power-driven vessel less than 50m, Port Aspect, Underway, possibly making way – day — Power-driven vessel less than 50mPort AspectUnderway, possibly making way

Power-driven vessel less than 50m, Port Aspect, Underway, possibly making way – night — Power-driven vessel less than 50mPort AspectUnderway, possibly making way

Power-driven vessel less than 50m, Starboard Aspect, Underway, possibly making way – day — Power-driven vessel less than 50mStarboard AspectUnderway, possibly making way

Power-driven vessel less than 50m, Starboard Aspect, Underway, possibly making way – night — Power-driven vessel less than 50mStarboard AspectUnderway, possibly making way

Power-driven vessel less than 50m, Stern Aspect, Underway, possibly making way – day — Power-driven vessel less than 50mStern AspectUnderway, possibly making way
This could also be:
Sailing vessel, stern aspect
Power-driven vessel pushing, stern aspect
Power-driven vessel, probably over 50m, stern aspect
Power-driven vessel less than 7m and max speed does not exceed 7 knots
Vessel less than 50m at anchor
Sailing vessel less than 7m
Vessel under oars

Power-driven vessel less than 50m, Stern Aspect, Underway, possibly making way – night — Power-driven vessel less than 50mStern AspectUnderway, possibly making way
This could also be:
Sailing vessel, stern aspect
Power-driven vessel pushing, stern aspect
Power-driven vessel, probably over 50m, stern aspect
Power-driven vessel less than 7m and max speed does not exceed 7 knots
Vessel less than 50m at anchor
Sailing vessel less than 7m
Vessel under oars

Power-driven vessel less than 20 metres in length

Power-driven vessel less than 20m, Bow Aspect, Underway, possibly making way – day — Power-driven vessel less than 20mBow AspectUnderway, possibly making way

Power-driven vessel less than 20m, Bow Aspect, Underway, possibly making way – night — Power-driven vessel less than 20mBow AspectUnderway, possibly making way

Power-driven vessel probably over 50 metres in length

Power-driven vessel probably over 50m, Bow Aspect, Underway, possibly making way – day — Power-driven vessel probably over 50mBow AspectUnderway, possibly making way

Power-driven vessel probably over 50m, Bow Aspect, Underway, possibly making way – night — Power-driven vessel probably over 50mBow AspectUnderway, possibly making way

Power-driven vessel probably over 50m, Port Aspect, Underway, possibly making way – day — Power-driven vessel probably over 50mPort AspectUnderway, possibly making way

Power-driven vessel probably over 50m, Port Aspect, Underway, possibly making way – night — Power-driven vessel probably over 50mPort AspectUnderway, possibly making way

Power-driven vessel probably over 50m, Starboard Aspect, Underway, possibly making way – day — Power-driven vessel probably over 50mStarboard AspectUnderway, possibly making way

Power-driven vessel probably over 50m, Starboard Aspect, Underway, possibly making way – night — Power-driven vessel probably over 50mStarboard AspectUnderway, possibly making way

Power-driven vessel probably over 50m, Stern Aspect, Underway, possibly making way – day — Power-driven vessel probably over 50mStern AspectUnderway, possibly making way
This could also be:
Sailing vessel, stern aspect
Power-driven vessel pushing, stern aspect
Power-driven vessel less than 50m, stern aspect
Power-driven vessel less than 7m and max speed does not exceed 7 knots
Vessel less than 50m at anchor
Sailing vessel less than 7m
Vessel under oars

Power-driven vessel probably over 50m, Stern Aspect, Underway, possibly making way – night — Power-driven vessel probably over 50mStern AspectUnderway, possibly making way
This could also be:
Sailing vessel, stern aspect
Power-driven vessel pushing, stern aspect
Power-driven vessel less than 50m, stern aspect
Power-driven vessel less than 7m and max speed does not exceed 7 knots
Vessel less than 50m at anchor
Sailing vessel less than 7m
Vessel under oars

23(b) An air-cushion vessel when operating in the non-displacement mode shall, in addition to the lights prescribed in paragraph (a) of this Rule, exhibit an all-round flashing yellow light.

“An air-cushion vessel when operating in the non-displacement mode”A hovercraft riding on her air cushion, clear of the water.
“shall, in addition to the lights prescribed in paragraph (a) of this Rule, exhibit an all-round flashing yellow light”She shows all the normal power-driven lights plus an all-round flashing yellow light (120 or more flashes a minute, Rule 21(f)).

Normal power-driven lights plus a flashing yellow light: an air-cushion vessel (hovercraft) on her cushion.

Air cushioned vessel operating in non-displacement mode

Air cushioned vessel operating in non-displacement mode less than 50m, Port Aspect, Underway, possibly making way – day — Air cushioned vessel operating in non-displacement mode less than 50mPort AspectUnderway, possibly making way

Air cushioned vessel operating in non-displacement mode less than 50m, Port Aspect, Underway, possibly making way – night — Air cushioned vessel operating in non-displacement mode less than 50mPort AspectUnderway, possibly making way

Air cushioned vessel operating in non-displacement mode less than 50m, Starboard Aspect, Underway, possibly making way – day — Air cushioned vessel operating in non-displacement mode less than 50mStarboard AspectUnderway, possibly making way

Air cushioned vessel operating in non-displacement mode less than 50m, Starboard Aspect, Underway, possibly making way – night — Air cushioned vessel operating in non-displacement mode less than 50mStarboard AspectUnderway, possibly making way

Air cushioned vessel operating in non-displacement mode less than 50m, Stern Aspect, Underway, possibly making way – day — Air cushioned vessel operating in non-displacement mode less than 50mStern AspectUnderway, possibly making way

Air cushioned vessel operating in non-displacement mode less than 50m, Stern Aspect, Underway, possibly making way – night — Air cushioned vessel operating in non-displacement mode less than 50mStern AspectUnderway, possibly making way

23(c) A WIG craft only when taking off, landing and in flight near the surface shall, in addition to the lights prescribed in paragraph (a) of this Rule, exhibit a high intensity all-round flashing red light.

“A WIG craft only when taking off, landing and in flight near the surface shall, in addition to the lights prescribed in paragraph (a) of this Rule, exhibit a high intensity all-round flashing red light”A WIG craft taking off, landing or flying near the surface shows the normal power-driven lights plus a high-intensity all-round flashing red light. In flight she must keep well clear of all vessels (Rule 18(f)(i)). On the water, she is simply a power-driven vessel (Rule 18(f)(ii)) and doesn’t show the red flashing light.

Normal power-driven lights plus a high-intensity flashing red light: a WIG craft taking off, landing or in flight near the surface.

WIG craft, when taking off, landing and in flight near the surface.

Wing-in-ground aircraft, Bow Aspect, Underway, possibly making way – day — Wing-in-ground aircraftBow AspectUnderway, possibly making way

Wing-in-ground aircraft, Bow Aspect, Underway, possibly making way – night — Wing-in-ground aircraftBow AspectUnderway, possibly making way

Wing-in-ground aircraft, Port Aspect, Underway, possibly making way – day — Wing-in-ground aircraftPort AspectUnderway, possibly making way

Wing-in-ground aircraft, Port Aspect, Underway, possibly making way – night — Wing-in-ground aircraftPort AspectUnderway, possibly making way

Wing-in-ground aircraft, Starboard Aspect, Underway, possibly making way – day — Wing-in-ground aircraftStarboard AspectUnderway, possibly making way

Wing-in-ground aircraft, Starboard Aspect, Underway, possibly making way – night — Wing-in-ground aircraftStarboard AspectUnderway, possibly making way

Wing-in-ground aircraft, Stern Aspect, Underway, possibly making way – day — Wing-in-ground aircraftStern AspectUnderway, possibly making way

Wing-in-ground aircraft, Stern Aspect, Underway, possibly making way – night — Wing-in-ground aircraftStern AspectUnderway, possibly making way

23(d)

(i) A power-driven vessel of less than 12 metres in length may in lieu of the lights prescribed in paragraph (a) of this Rule exhibit an all-round white light and sidelights;

“(i) A power-driven vessel of less than 12 metres in length may in lieu of the lights prescribed in paragraph (a) of this Rule exhibit an all-round white light and sidelights”Instead of a masthead light and sternlight, a power-driven vessel under 12 metres may show one all-round white light, plus sidelights (separate or combined in one lantern, Rule 21(b)).

Under 12 m: an all-round white light and sidelights may replace the masthead light and sternlight. From ahead she looks the same, but from astern you see only a white light.

Lights of a power-driven vessel under 12 metres using the Rule 23(d)(i) option: all-round white light and sidelights

23(d)(ii) a power-driven vessel of less than 7 metres in length whose maximum speed does not exceed 7 knots may in lieu of the lights prescribed in paragraph (a) of this Rule exhibit an all-round white light and shall, if practicable, also exhibit sidelights;

“a power-driven vessel of less than 7 metres in length whose maximum speed does not exceed 7 knots may in lieu of the lights prescribed in paragraph (a) of this Rule exhibit an all-round white light”A small, slow power-driven vessel (under 7 metres and no faster than 7 knots, both conditions) may show just an all-round white light.
“and shall, if practicable, also exhibit sidelights”She must also show sidelights if she can fit them. If she shows only the white light, she can look like a vessel at anchor (Rule 30) or a small sailing vessel or rowing boat (Rule 25(d)).

Under 7 m and 7 knots or less: an all-round white light, plus sidelights if practicable.

Lights of a power-driven vessel under 7 metres, maximum speed 7 knots or less: all-round white light

23(d)(iii) the masthead light or all-round white light on a power-driven vessel of less than 12 metres in length may be displaced from the fore and aft centre line of the vessel if centreline fitting is not practicable, provided that the sidelights are combined in one lantern which shall be carried on the fore and aft centre line of the vessel or located as nearly as practicable in the same fore and aft line as the masthead light or the all-round white light.

“the masthead light or all-round white light on a power-driven vessel of less than 12 metres in length may be displaced from the fore and aft centre line of the vessel if centreline fitting is not practicable”On a small power-driven vessel under 12 metres, for example a fast RIB with a centre console, the white light can be fitted off the centreline if it can’t go on it.
“provided that the sidelights are combined in one lantern which shall be carried on the fore and aft centre line of the vessel or located as nearly as practicable in the same fore and aft line as the masthead light or the all-round white light”If so, her sidelights must be in one combined lantern, either on the centreline or as nearly as practicable in line, fore and aft, with the white light. This keeps the lights looking right to another vessel.

Under 12 m, the white light may be off-centre only if the sidelights are one combined lantern lined up with it.