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21

Definitions (Lights)

Defines masthead light, sidelights, sternlight, towing light, all-round light and flashing light.

Rule 21 defines the types of navigation lights:

a
Masthead light: a white light over an arc of 225 degrees, visible from dead ahead to 22.5 degrees abaft the beam on each side
b
Sidelights: green on starboard and red on port, each showing over an arc of 112.5 degrees from dead ahead to 22.5 degrees abaft the beam
c
Sternlight: a white light over an arc of 135 degrees, shown from dead astern to 67.5 degrees on each side
d
Towing light: a yellow light with the same characteristics as a sternlight
e
All-round light: a light showing over an arc of 360 degrees
f
Flashing light: a light flashing at regular intervals at a frequency of 120 flashes or more per minute

Recognition Sequence

1

Classify the vessel state first: underway, making way, stopped, at anchor, aground, towing, fishing, pilotage or special condition.

2

Read special lights vertically from top to bottom before using sidelights and sternlight to confirm aspect.

3

Then confirm the answer with the day shape, vessel length and any extra signal such as towing lights, deck illumination or a cylinder.

Exam Focus

1

Avoid identifying a vessel from one colour alone. Many mistakes come from spotting a red light and guessing before checking the full pattern.

2

If the question mentions 'making way', 'underway but stopped', 'at anchor' or 'aground', that wording usually determines which extra lights or shapes appear.

Key Takeaways

1

Masthead light covers 225 degrees (ahead to 22.5 abaft beam)

2

Sidelights cover 112.5 degrees each

3

Sternlight covers 135 degrees

4

All-round light covers full 360 degrees

Common Mistakes

Confusing the arcs of visibility for different light types

Forgetting towing light is yellow, same arc as sternlight

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