1.12 Lights & Shapes — General Rules, Visibility & Definitions (Rules 20–22)

Rules 20-22: Lights and Shapes — the Foundation

Part C of the Navigation Rules (Rules 20-31) tells you which lights and day shapes every kind of vessel must display. Before you memorize the individual patterns in the next four sections, you must own three building blocks from Rules 20, 21, and 22: when lights are shown, the arc each light covers, and how far each must be visible. Get these cold and every later light pattern becomes readable.

Rule 20 — when lights and shapes are shown

• Lights are exhibited from sunset to sunrise, and also from sunrise to sunset in restricted visibility (fog, heavy rain) and any other time it is deemed necessary. During those hours no other lights may be shown that could be mistaken for the prescribed lights or impair their visibility.
• Day shapes (balls, cones, cylinders, diamonds — all black) are exhibited by day.
• The rules apply in all weathers. You do not get to skip your lights because it is a clear, calm night with no traffic.

Rule 21 — the six light definitions and their arcs

Every prescribed light is defined by an arc of the horizon it covers. Learn the arcs as a set:

The numbers interlock beautifully. Two sidelights (112.5° + 112.5° = 225°) cover exactly the same forward arc as the masthead light. Add the sternlight's 135° and you get 225° + 135° = 360° — the masthead-plus-sidelights forward arc and the sternlight together ring the whole horizon with no gap and no overlap. The seam between "forward lights" and the sternlight falls at 22.5° abaft the beam on each side. That 22.5°-abaft-the-beam line is the same boundary that defines the overtaking sector in Rule 13 — if you can see only a vessel's sternlight and neither sidelight, you are in her overtaking sector.

Reading a vessel by her lights at night

Because the arcs are fixed, the lights you can see tell you your aspect — how you are looking at the other vessel:

• Both sidelights + masthead light(s) → you are seeing her nearly head-on or fine on the bow.
• One sidelight + masthead light → a crossing aspect; a green sidelight means you see her starboard side (she is crossing right-to-left), a red means her port side.
• Only a white sternlight (no sidelights) → you are astern, in her overtaking sector.

Rule 22 — minimum visibility ranges by vessel length

Rule 22 sets the minimum range (in nautical miles) at which each light must be visible, scaled to the vessel's length. Six-pack candidates are regularly asked for the sub-50-metre and sub-12-metre rows:

The pattern to remember: the masthead light always reaches the farthest (it is the "here I am, and here is my heading" light), and every range shrinks as the vessel gets shorter. Your typical six-pack is under 12 metres or in the 12–20 m band, so its masthead light need only carry 2 or 3 miles and its sidelights 1 mile. That short range is precisely why you cannot assume a small boat will be seen early — and why a proper lookout (Rule 5) and safe speed (Rule 6) matter so much at night.

Day shapes — the vocabulary

All day shapes are black. You only need four basic solids, and every later rule combines them:

• Ball — anchored vessel (one ball forward); also part of NUC (two balls) and RAM/aground signals.
• Cone — apex down means a sailing vessel proceeding under sail and power (motorsailing); apex up marks the direction of a fishing vessel's outlying gear.
• Cylinder — a vessel constrained by her draught (International only).
• Diamond — displayed by a vessel being towed, and the middle shape of the RAM ball-diamond-ball.

Traps

• All lights, all weather, all night. There is no "clear night, no traffic" exemption.
• Do not confuse the arc with the range. A sidelight's arc is 112.5°; its range on a small boat is 1 mile. Two different rules.
• The towing light is yellow and sits above the sternlight — it is not a second white light.
• Shapes are black and shown by day; lights are shown at night and in reduced visibility.