COLREGs day shapes explained

Day shapes are visual signals that vessels display during daylight to show a particular status, activity or circumstance.

This StandOn guide explains the day-shape concepts used in our learning material, with a short description of what each signal indicates and the relevant COLREG reference.

Recognising a shape is only one part of applying the COLREGs. You still need to consider the circumstances of the encounter and the rules that apply.

What do day shapes tell you?

A day shape tells you something about another vessel during daylight: for example, that she is at anchor, aground, fishing, or limited in how she can manoeuvre. That information helps you understand her situation early, often before you could tell it from her movement alone.

Recognition on its own does not decide what you should do. The action you take in an encounter still depends on the rules that apply, the circumstances, and a proper lookout by all available means.

The basic shapes

Day signals are built from four basic black shapes:

Ball: a sphere.
Cone: shown point upwards or point downwards, or as two cones with their points together.
Diamond: two cones joined at their bases.
Cylinder: a cylinder taller than it is wide.

Many day signals use a single shape or a vertical combination of shapes. Some signals require shapes in different positions to indicate a vessel’s operation or the side on which an obstruction exists. Minimum sizes and the spacing between shapes are set out in Annex I of the COLREGs. Vessels under 20 metres may use shapes of smaller dimensions commensurate with the size of the vessel, with the spacing reduced accordingly.

Common day signals at a glance

Common COLREGs day signals and their rule references
Signal / vessel status	Shape description	COLREG reference
Vessel at anchor	One ball, shown in the fore part of the vessel where it can best be seen.	Rule 30(a)(i)
Vessel aground	Three balls in a vertical line.	Rule 30(d)(ii)
Vessel not under command (NUC)	Two balls in a vertical line.	Rule 27(a)(ii); definition in Rule 3(f)
Vessel restricted in her ability to manoeuvre	A ball, a diamond and a ball in a vertical line.	Rule 27(b)(ii); definition in Rule 3(g)
Restricted in her ability to manoeuvre: obstruction on the port side, clear to pass on the starboard side	The ball, diamond, ball signal, plus two balls in a vertical line on the port side and two diamonds in a vertical line on the starboard side.	Rule 27(b)(ii) and Rule 27(d)(ii)
Diving operations: International Code flag A	Where the vessel’s size makes it impracticable to display all the shapes required by Rule 27(d), a rigid replica of International Code flag A, at least 1 metre high, is displayed and arranged for all-round visibility.	Rule 27(e)(ii)
Mine-clearance operations	Three balls: one near the foremast head and one at each end of the fore yard.	Rule 27(f)
Vessel constrained by her draught	May exhibit a cylinder where it can best be seen.	Rule 28; definition in Rule 3(h)
Tow over 200 m	For an ordinary tow astern exceeding 200 metres, the towing vessel displays a diamond where it can best be seen; the vessel or object being towed also displays a diamond.	Rule 24(a)(v) and Rule 24(e)(iii)
Fishing vessel	Two cones with their points together in a vertical line.	Rule 26(b)(i) and Rule 26(c)(i)
Fishing vessel with outlying gear extending more than 150 m horizontally	In addition to the fishing signal, one cone point upwards, in the direction of the gear.	Rule 26(c)(ii)
Sailing vessel also being propelled by machinery	One cone, point downwards, shown forward where it can best be seen.	Rule 25(e)

Day shapes in detail

Vessel at anchor

Shape: One ball, shown in the fore part of the vessel where it can best be seen.
Indicates: The vessel is at anchor.
COLREG reference: Rule 30(a)(i)

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Vessel aground

Shape: Three balls in a vertical line.
Indicates: The vessel is aground.
COLREG reference: Rule 30(d)(ii)

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Vessel not under command (NUC)

Shape: Two balls in a vertical line.
Indicates: Through some exceptional circumstance, the vessel is unable to manoeuvre as required by the rules and so cannot keep out of the way of another vessel.
COLREG reference: Rule 27(a)(ii); definition in Rule 3(f)

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Vessel restricted in her ability to manoeuvre

Shape: A ball, a diamond and a ball in a vertical line.
Indicates: The nature of the vessel's work restricts her ability to manoeuvre as required by the rules, so she cannot keep out of the way of another vessel.
COLREG reference: Rule 27(b)(ii); definition in Rule 3(g)

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Restricted in her ability to manoeuvre: obstruction on the port side, clear to pass on the starboard side

Shape: The ball, diamond, ball signal, plus two balls in a vertical line on the port side and two diamonds in a vertical line on the starboard side.
Indicates: The vessel is engaged in work such as dredging or underwater operations. An obstruction exists on the side showing the two balls; other vessels may pass on the side showing the two diamonds.
COLREG reference: Rule 27(b)(ii) and Rule 27(d)(ii)

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Diving operations: International Code flag A

Shape: Where the vessel’s size makes it impracticable to display all the shapes required by Rule 27(d), a rigid replica of International Code flag A, at least 1 metre high, is displayed and arranged for all-round visibility.
Indicates: Diving operations are being carried out from the vessel.
COLREG reference: Rule 27(e)(ii)

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Mine-clearance operations

Shape: Three balls: one near the foremast head and one at each end of the fore yard.
Indicates: The vessel is engaged in mine clearance and it is dangerous for another vessel to approach within 1,000 metres.
COLREG reference: Rule 27(f)

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Vessel constrained by her draught

Shape: May exhibit a cylinder where it can best be seen.
Indicates: Because of her draught in relation to the available depth and width of navigable water, the vessel is severely restricted in her ability to deviate from the course she is following.
COLREG reference: Rule 28; definition in Rule 3(h)

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Tow over 200 m

Shape: For an ordinary tow astern exceeding 200 metres, the towing vessel displays a diamond where it can best be seen; the vessel or object being towed also displays a diamond.
Indicates: The length of the tow, measured from the stern of the towing vessel to the after end of the tow, exceeds 200 metres.
COLREG reference: Rule 24(a)(v) and Rule 24(e)(iii)

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Fishing vessel

Shape: Two cones with their points together in a vertical line.
Indicates: The vessel is engaged in fishing, whether trawling or fishing by other methods.
COLREG reference: Rule 26(b)(i) and Rule 26(c)(i)

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Fishing vessel with outlying gear extending more than 150 m horizontally

Shape: In addition to the fishing signal, one cone point upwards, in the direction of the gear.
Indicates: A vessel engaged in fishing other than trawling has gear extending more than 150 metres horizontally from the vessel in that direction.
COLREG reference: Rule 26(c)(ii)

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Sailing vessel also being propelled by machinery

Shape: One cone, point downwards, shown forward where it can best be seen.
Indicates: The vessel is under sail and also being propelled by machinery.
COLREG reference: Rule 25(e)

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Day shapes and navigation lights

Many of the vessel statuses represented by day shapes also have prescribed light configurations. Lights are displayed from sunset to sunrise and, if carried, are also displayed during daylight in restricted visibility.

Practise recalling day shapes

Reading a reference helps you understand what each shape means. Being able to recall it when you need it takes practice. StandOn Practice asks you to recall day-shape meanings from memory, again and again over time.

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A revision reference, not a substitute for training

This page is a revision reference. It is not a complete guide to the COLREGs and does not replace recognised training, course material, instructor guidance or practical experience.

StandOn is an independent sailing revision platform and is not affiliated with or endorsed by the Royal Yachting Association.