COLREGs: Lights, Shapes and Sound Signals for Large Ships

Prerequisites

This lesson covers Part C (Rules 20 to 31), Part D (Rules 32 to 37) and Annexes I to IV of the International Regulations for Preventing Collisions at Sea, 1972 (COLREGs) at the depth the MCA expects in the Officer of the Watch (Unlimited) oral examination. Before you start you should:

Know the steering and sailing rules (Rules 4 to 19) from the earlier lesson, including the definitions in Rule 3 (power-driven vessel, vessel not under command, vessel restricted in her ability to manoeuvre, vessel constrained by her draught, underway, making way).
Already recognise the basic lights of a power-driven vessel and a sailing vessel from yacht or cadet training.
Know that in UK law the COLREGs apply through the Merchant Shipping (Distress Signals and Prevention of Collisions) Regulations 1996, published with the full text as MSN 1781 Amendment 3.

The examiner will not let you look anything up. You must be able to identify any combination of lights at a glance, say what the vessel is doing, which way she is heading, roughly how big she is, and what you are going to do about it. You must also be able to recite the sound signals and the distress signals of Annex IV word for word.

Learning Objectives

By the end of this lesson you will be able to:

State the arcs, colours and minimum ranges of every navigation light defined in Rules 21 and 22, by vessel length.
Describe the lights and shapes for power-driven vessels, vessels towing and pushing, sailing vessels, fishing vessels, vessels not under command, restricted in their ability to manoeuvre, constrained by their draught, pilot vessels, and vessels at anchor and aground (Rules 23 to 30).
Use the light arcs to work out another vessel's heading, aspect and rough length at night.
Explain the key positioning rules of Annex I that let you judge aspect from masthead lights, and the extra fishing signals of Annex II.
State the manoeuvring and warning signals of Rule 34, the restricted visibility signals of Rule 35, and the sound signal equipment and ranges of Rule 33 and Annex III.
List the distress signals of Rule 37 and Annex IV, and explain how they are recognised in the UK (MIN 542).

Why Lights and Shapes Matter to a Ship's Officer

On a yacht you look at a set of lights and ask, "Is that a ship?" On the bridge of a 300 m container ship you need far more: is she a ship over 50 m, which way is she heading, is she free to manoeuvre, is she fishing with gear out on one side, is she towing something 600 m long that will not show up on radar? Each answer changes your action under Rules 13 to 18.

Lights do three jobs:

Identify the type and status of the vessel (fishing, NUC, RAM, CBD, pilot, anchored, aground).
Show aspect: which side of her you are looking at, so you can judge her heading.
Show size: a second masthead light means 50 m or more (or a smaller vessel choosing to show one).

Radar and AIS help, but Rule 7 forbids assumptions on scanty information, and AIS status is often wrong. A properly read set of lights, confirmed by compass bearings, is still the primary evidence for the OOW.

Rule 20: When Lights and Shapes Apply

Lights are shown from sunset to sunrise, and in that period no other lights may be shown that could be mistaken for them, impair their visibility or distinctive character, or interfere with a proper lookout. Deck working lights that dazzle the lookout breach this rule.
Lights shall also be shown from sunrise to sunset in restricted visibility, and may be shown in any other circumstance when it is deemed necessary.
Shapes are shown by day.
The technical detail sits in Annex I (positioning and technical details), Annex II (additional signals for fishing vessels fishing in close proximity), Annex III (sound signal appliances) and Annex IV (distress signals).

Rule 21: Definitions of Lights

Light	Colour	Arc	Where it shines
Masthead light	White	225°	Right ahead to 22.5° abaft the beam on each side
Sidelights	Green (starboard), red (port)	112.5° each	Right ahead to 22.5° abaft the beam on that side
Sternlight	White	135°	67.5° from right aft on each side
Towing light	Yellow	135°	Same arc as the sternlight
All-round light	Any	360°	Unbroken horizon
Flashing light	Any	As fitted	120 flashes or more per minute

The diagram below shows how these arcs fit together on a power-driven vessel. Notice that the masthead light and sidelights share the same after limit, 22.5° abaft the beam, and the sternlight covers exactly the gap behind. From any direction you see either the sternlight or a sidelight (plus masthead lights), never both, apart from a narrow overlap allowed at the sector edges by Annex I.

Arcs of masthead lights, sidelights and sternlight on a power-driven vessel, with the minimum ranges for vessels of 50 m or more

The 22.5° abaft the beam line is not an accident. It is the same line Rule 13 uses to define an overtaking vessel. If at night you can see only her sternlight and none of her sidelights, you are overtaking her, and you must keep out of her way until finally past and clear.

Rule 22: Visibility of Lights

The minimum ranges are given for a dark night with clear atmosphere. Learn them as a table.

Light	50 m or more	12 m to under 50 m	Under 12 m
Masthead	6 nm	5 nm (3 nm if under 20 m)	2 nm
Sidelights	3 nm	2 nm	1 nm
Sternlight	3 nm	2 nm	2 nm
Towing light	3 nm	2 nm	2 nm
All-round (white, red, green, yellow)	3 nm	2 nm	2 nm

An inconspicuous, partly submerged vessel or object being towed shows an all-round white light visible at 3 nm.

Two practical points follow:

First sighting: a big ship's masthead lights appear about 6 nm off, but her sidelights only at about 3 nm. At a closing speed of 30 knots that is six minutes. Take bearings on the masthead lights; do not wait for the sidelights to decide what she is doing.
Small craft: a fishing boat under 12 m may have sidelights visible at only 1 nm. If a single white light is close and the bearing is steady, treat it as a small vessel until proved otherwise.

Rule 23: Power-Driven Vessels Underway

A power-driven vessel underway shows:

A masthead light forward.
A second masthead light abaft of and higher than the forward one. This is mandatory for vessels of 50 m or more; vessels under 50 m may show it.
Sidelights.
A sternlight.

Additional cases:

An air-cushion vessel operating in the non-displacement mode also shows an all-round flashing yellow light.
A WIG craft (wing-in-ground) taking off, landing or in flight near the surface also shows a high-intensity all-round flashing red light.
A power-driven vessel under 12 m may show an all-round white light and sidelights instead. Under 7 m with a maximum speed not over 7 knots, an all-round white light alone, with sidelights if practicable.

Reading aspect from the masthead lights

Annex I requires the two masthead lights of a vessel of 20 m or more to be separated horizontally by at least half her length (need not exceed 100 m), with the after light at least 4.5 m higher than the forward one. That geometry is what lets you read her heading:

Lights in line, one above the other: she is heading straight at you or straight away (sidelights tell you which).
Lights opening out with the after light to the right: you are seeing her port side and she is heading to your left.
Lights closing up: she is turning towards you. Opening up: turning away.

If the horizontal gap between her masthead lights is closing while the compass bearing stays steady, she is altering towards you and a close-quarters situation is developing fast.

Rule 24: Towing and Pushing

Towing lights are the most examined area of Part C because the tow can be long, low and invisible on radar.

Power-driven vessel towing astern

Two masthead lights in a vertical line instead of the single forward (or after) masthead light. If the length of tow exceeds 200 m, three masthead lights in a vertical line. Length of tow is measured from the stern of the towing vessel to the after end of the tow.
Sidelights and a sternlight.
A towing light (yellow, same arc as the sternlight) in a vertical line above the sternlight.
By day, where the tow exceeds 200 m, a diamond shape where it can best be seen.
A tug of 50 m or more must still show her after masthead light as well.

Pushing ahead or towing alongside

Two masthead lights in a vertical line, sidelights and a sternlight. No towing light.
A pushing vessel and the pushed vessel rigidly connected as a composite unit are lit as a single power-driven vessel.

The vessel or object being towed

Sidelights and a sternlight, and where the tow exceeds 200 m, a diamond shape.
Several vessels towed alongside in a group are lit as one vessel.
An inconspicuous, partly submerged vessel or object (for example a barge awash or a dracone) shows all-round white lights at each end, or along its length if over 100 m, and a diamond at the after end. If over 200 m long, the diamond is shown at both ends.
Where it is impracticable to light the tow properly, all possible measures must be taken to light it or indicate its presence; the tug may illuminate the tow line with a searchlight (Rule 24(i)).

A tug towing who is severely restricted in her ability to deviate from her course also shows the RAM lights or shapes of Rule 27(b) in addition. Note that a tow in itself does not make a vessel RAM; only a towing operation that severely restricts her ability to deviate does.

Rule 25: Sailing Vessels and Vessels Under Oars

Sidelights and sternlight. Under 20 m these may be combined in a tricolour lantern at or near the masthead.
Optional: two all-round lights in a vertical line at or near the masthead, red over green (not with a tricolour).
A vessel under sail which is also being propelled by machinery shows by day a cone, apex downwards, forward. At night she is simply a power-driven vessel.

From a big ship, sailing yachts are often visible only at the last moment, and a tricolour on a pitching masthead can be mistaken for a sidelight far closer than it is.

Rule 26: Fishing Vessels

A vessel engaged in fishing (with gear that restricts her manoeuvrability, not trolling lines) shows:

Trawling

Two all-round lights in a vertical line, green over white ("green over white, trawling at night").
A masthead light abaft and higher than the green light. Vessels under 50 m are not obliged to show it.
When making way through the water, sidelights and a sternlight.

Other fishing (drift nets, long lines, purse seines)

Two all-round lights, red over white ("red over white, fishing at night").
Where outlying gear extends more than 150 m horizontally, an all-round white light (or by day a cone apex upwards) in the direction of the gear. Pass on the other side.
When making way, sidelights and sternlight.

Day shapes

Two cones with their apexes together in a vertical line. A vessel under 20 m may show a basket instead.

Annex II: fishing in close proximity

Annex II gives extra signals, visible at 1 nm or more but less than the navigation lights:

Vessel	Signal	Meaning
Trawler, 20 m or more	Two white lights in a vertical line	Shooting nets
Trawler, 20 m or more	White over red	Hauling nets
Trawler, 20 m or more	Two red lights in a vertical line	Net fast on an obstruction
Pair trawlers	Searchlight directed forward and towards the other vessel	Pair trawling
Purse seiner	Two yellow lights in a vertical line, flashing alternately every second	Hampered by gear

Large ships transiting fishing grounds at night, such as the southern North Sea or the Western Approaches, meet these signals constantly. A trawler with two red lights is stopped and fast: treat her as an obstruction.

Rule 27: Not Under Command and Restricted in Ability to Manoeuvre

Not under command (NUC)

Two all-round red lights in a vertical line where best seen.
Two balls in a vertical line by day.
When making way, sidelights and sternlight as well. No masthead lights.

A vessel NUC with no way on shows only the two reds. "Red over red, the captain is dead" is the old rhyme: the vessel cannot keep out of your way, and under Rule 18 a power-driven vessel, sailing vessel and fishing vessel must keep out of her way.

Restricted in ability to manoeuvre (RAM)

Three all-round lights in a vertical line: red, white, red.
Three shapes: ball, diamond, ball.
When making way: masthead light(s), sidelights and sternlight as well.
At anchor: the RAM lights plus the anchor lights or ball.

Dredging and underwater operations

A RAM vessel dredging or working underwater with an obstruction on one side also shows:

Two all-round red lights or two balls in a vertical line on the side where the obstruction exists.
Two all-round green lights or two diamonds on the side where another vessel may pass.

Diving and mine clearance

Where a small vessel engaged in diving cannot show the full RAM signals, a rigid replica of the International Code flag "A" at least 1 m high is shown.
A vessel engaged in mine clearance shows three all-round green lights or three balls (one at or near the foremast head, one at each end of the fore yard). It is dangerous to approach within 1,000 m.

Rule 28: Constrained by Her Draught

A power-driven vessel constrained by her draught may show, in addition to her power-driven lights:

Three all-round red lights in a vertical line.
A cylinder by day.

Note "may": it is optional. Under Rule 18(d), any vessel other than NUC or RAM shall, if circumstances admit, avoid impeding her safe passage. But the CBD vessel herself must still navigate with particular caution; she is not a stand-on vessel by virtue of the signal. CBD status exists only for power-driven vessels, and only because of her draught in relation to the available depth and width of water, as defined in Rule 3(h).

The panel below summarises the day shapes you must be able to name instantly.

Day shapes: anchored, NUC, RAM, CBD, aground, fishing, towing with a tow over 200 m, and a sailing vessel under power

Rule 29: Pilot Vessels

A pilot vessel engaged on pilotage duty shows:

Two all-round lights in a vertical line at or near the masthead, white over red ("white over red, pilot ahead").
When underway, sidelights and sternlight as well. No masthead lights.
At anchor, the anchor lights or ball as well.

When not on pilotage duty, she shows the lights of a vessel of her length. By day a pilot vessel shows the International Code flag "H".

Rule 30: Anchored Vessels and Vessels Aground

At anchor

In the fore part, an all-round white light or one ball.
At or near the stern and lower than the forward light, an all-round white light.
A vessel under 50 m may show a single all-round white light where it can best be seen instead.
A vessel at anchor may, and a vessel of 100 m or more shall, also use available working or equivalent lights to illuminate her decks.

The forward light is the higher one. That lets you work out which way she is lying at anchor, and so the direction of the tidal stream at her position.

Aground

The anchor lights (and deck lights where appropriate), plus two all-round red lights in a vertical line.
By day, three balls in a vertical line.

A vessel under 7 m at anchor (not in or near a narrow channel, fairway or anchorage, or where other vessels normally navigate) need not show anchor lights or the ball, and a vessel under 12 m aground need not show the aground signals.

Seaplanes and WIG craft (Rule 31)

Where it is impracticable for a seaplane or WIG craft to comply exactly, she shows lights and shapes as closely similar as possible.

Reading Lights at Night

The diagram below shows six sets of lights an OOW must decode instantly. Work through each one aloud as you would in the oral: what she is, which way she is heading, and what it means for you.

Six sets of lights at night: a power-driven vessel of 50 m or more head-on, NUC making way, a trawler head-on, a pilot vessel, and anchored and aground vessels of 50 m or more

Two white masthead lights in line, green on your left, red on your right: a power-driven vessel of 50 m or more heading straight at you. Rule 14 head-on situation: both alter to starboard.
Two reds in a vertical line plus a red sidelight: NUC, making way, showing you her port side. She is heading to your left. Keep out of her way.
Green over white, both sidelights: a trawler under 50 m heading towards you. A trawler of 50 m or more would also show a masthead light abaft and above the green.
White over red, both sidelights: a pilot vessel underway on duty, heading towards you. Expect her to manoeuvre to put a pilot aboard.
Two whites, the forward one higher: a vessel of 50 m or more at anchor. The higher light marks her bow, so you are looking at her starboard side here with the bow to your right.
Two whites plus two reds in a vertical line: a vessel aground. Her position marks shoal water. Check your own position and the chart at once.

A method for unknown lights

When the combination is confusing, follow a fixed routine:

Take a compass bearing and start a radar plot or ARPA track immediately.
Identify the sidelight or sternlight you can see. That gives you her aspect.
Count the white masthead lights and note whether they are in line or spread. That gives heading and size, or towing.
Look for all-round lights in a vertical line at the masthead. Those give status: red-red NUC, red-white-red RAM, red-red-red CBD, green-white trawling, red-white fishing, white-red pilot.
Look for a yellow towing light above the sternlight. If you see a tug's lights, look for the tow before crossing astern of her.
If still in doubt, treat her as the vessel that would give you the most difficulty, call the master if required by standing orders, and do not cross ahead.

Part D: Sound and Light Signals

Rule 32: Definitions

Whistle: any sound-signalling appliance that complies with Annex III.
Short blast: about one second.
Prolonged blast: four to six seconds.

Rule 33: Equipment

Vessel length	Required
12 m or more	Whistle
20 m or more	Whistle and bell
100 m or more	Whistle, bell, and a gong (whose tone cannot be confused with the bell)
Under 12 m	Not obliged to carry these, but must have some other means of making an efficient sound signal

Annex III: whistle frequencies and audibility

Vessel length	Fundamental frequency	Audibility range
200 m or more	70 to 200 Hz	2 nm
75 m to under 200 m	130 to 350 Hz	1.5 nm
20 m to under 75 m	250 to 525 Hz	1 nm
Under 20 m	250 to 700 Hz	0.5 nm

The bigger the ship, the deeper the note. A low, booming whistle in fog means a large vessel. Bells for vessels of 20 m or more have a mouth diameter of at least 300 mm. These ranges are guidance only: sound is easily distorted in fog, and Rule 35 signals give no reliable direction or distance.

Rule 34: Manoeuvring and warning signals (vessels in sight of one another)

Signal	Meaning
One short blast	I am altering my course to starboard
Two short blasts	I am altering my course to port
Three short blasts	I am operating astern propulsion
Two prolonged, one short	(Narrow channel) I intend to overtake you on your starboard side
Two prolonged, two short	(Narrow channel) I intend to overtake you on your port side
Prolonged, short, prolonged, short	(Narrow channel) Agreement by the vessel about to be overtaken
At least five short and rapid blasts	I fail to understand your intentions, or doubt you are taking sufficient action to avoid collision
One prolonged blast	Approaching a bend or area where other vessels may be obscured; answered by one prolonged blast

Key points:

The signals in Rule 34(a) are mandatory for a power-driven vessel manoeuvring as authorised or required by the Rules when vessels are in sight of one another. They are not used in restricted visibility when the other vessel is detected by radar alone.
They may be supplemented by light signals: one flash to starboard, two to port, three astern. Each flash lasts about one second, with about one second between flashes, and at least ten seconds between successive signals. The light is an all-round white light visible at a minimum of 5 nm (Annex I).
Ships of 12 m or more fitted with more than one whistle more than 100 m apart use only one whistle for manoeuvring signals.
Five short blasts may be supplemented by five short and rapid flashes. Use the doubt signal early and then act; do not sound it and wait.

Rule 35: Signals in restricted visibility

In or near an area of restricted visibility, whether by day or night:

Vessel	Signal	Interval
Power-driven, making way	One prolonged	Not more than 2 min
Power-driven, underway but stopped, no way on	Two prolonged, about 2 s apart	Not more than 2 min
NUC, RAM, CBD, sailing, fishing, towing or pushing; and fishing or RAM at anchor	One prolonged, two short	Not more than 2 min
Vessel towed (last of the tow, if manned)	One prolonged, three short, immediately after the tug's signal	Not more than 2 min
At anchor	Rapid ringing of the bell for about 5 s	Not more than 1 min
At anchor, 100 m or more	Bell in the fore part, then gong in the after part, each for about 5 s	Not more than 1 min
At anchor (additional, optional)	One short, one prolonged, one short	To warn an approaching vessel
Aground	Bell signal (and gong if required) preceded and followed by three separate and distinct strokes on the bell	Not more than 1 min
Pilot vessel on duty (optional)	Four short (identity signal)	In addition to her other signals
Under 12 m	Not obliged to give the above, but must give some other efficient sound signal	Not more than 2 min

A pushing vessel and pushed vessel rigidly connected as a composite unit are regarded as a power-driven vessel and give the power-driven signals.

On a large ship the fog signal is usually automatic, but the OOW must check it is set to the right signal. Switching from making way (one prolonged) to stopped (two prolonged) when you take the way off is a common oversight; so is leaving the automatic signal running at anchor instead of the bell.

Rule 36: Signals to attract attention

Any vessel may make light or sound signals that cannot be mistaken for any signal authorised by the Rules, or direct a searchlight towards the danger, provided it does not embarrass another vessel. Strobe lights and high-intensity intermittent or revolving lights are to be avoided for this purpose.

Rule 37 and Annex IV: Distress signals

When a vessel is in distress and requires assistance she uses or exhibits the signals in Annex IV. Use of any of them except for indicating distress and need of assistance is prohibited, as is the use of other signals which may be confused with them.

The Annex IV signals, used together or separately, are:

A gun or other explosive signal fired at intervals of about a minute.
A continuous sounding with any fog-signalling apparatus.
Rockets or shells throwing red stars fired one at a time at short intervals.
A signal made by any signalling method consisting of the group SOS in Morse code.
A signal sent by radiotelephony consisting of the spoken word MAYDAY.
The International Code signal of distress indicated by NC.
A signal consisting of a square flag having above or below it a ball or anything resembling a ball.
Flames on the vessel (as from a burning tar barrel or oil barrel).
A rocket parachute flare or a hand flare showing a red light.
A smoke signal giving off orange-coloured smoke.
Slowly and repeatedly raising and lowering arms outstretched to each side.
A distress alert by means of digital selective calling (DSC) on VHF channel 70, or MF/HF on 2187.5 kHz, 8414.5 kHz, 4207.5 kHz, 6312 kHz, 12577 kHz or 16804.5 kHz.
A ship-to-shore distress alert transmitted by the ship's Inmarsat or other mobile satellite service provider ship earth station.
Signals transmitted by emergency position-indicating radio beacons.
Approved signals transmitted by radiocommunication systems, including survival craft radar transponders.

Annex IV also lists signals for attention from the air: a piece of orange-coloured canvas with a black square and circle or other appropriate symbol, and a dye marker.

In the UK, MIN 542 Amendment 3 sets out the recognised distress signals and the advertised alternatives to pyrotechnic flares, such as electronic visual distress signals (EVDS). For the OOW the key point is recognition: an orange strobe-like light sequence, an EVDS or an AIS-SART target on the radar or ECDIS must be treated as a distress signal and reported, and the master called at once.

Your duty when you see a distress signal

SOLAS Chapter V Regulation 33 requires the master of a ship at sea in a position to provide assistance, on receiving information from any source that persons are in distress at sea, to proceed with all speed to their assistance. As OOW you:

Note the time, bearing and, if possible, range of the signal, and your own position.
Call the master immediately.
Inform the coastguard (MRCC) by the fastest means, for example a VHF call on channel 16, and listen for any DSC alert or MAYDAY.
Keep the signal in sight and continue a proper lookout and watch.
Log everything.

Annex I: Positioning That Matters at Sea

You will not be asked to recite all of Annex I, but these points explain what you see:

Forward masthead light on a vessel of 20 m or more: at least 6 m above the hull, and if her breadth exceeds 6 m, at a height not less than the breadth, but need not be more than 12 m.
After masthead light: at least 4.5 m vertically higher than the forward one, so that at 1,000 m ahead at sea level you see them separated.
Horizontal separation of the two masthead lights: not less than half the vessel's length, but need not exceed 100 m. The forward light is not more than one quarter of the length from the stem.
Sidelights on vessels of 20 m or more are not placed in front of the forward masthead lights, and have matt black inboard screens to cut off the arc accurately.
Lights in a vertical line on vessels of 20 m or more are spaced at least 2 m apart, and the lowest (other than a towing light) at least 4 m above the hull.
Shapes are black. A ball is at least 0.6 m in diameter; a cone has a base of 0.6 m and a height equal to its diameter; a cylinder is 0.6 m in diameter and twice as high; a diamond is two cones base to base. Shapes are at least 1.5 m apart.
Sector cut-off: sidelights must fall to practical cut-off between 1° and 3° outside their prescribed sector, and must show at least the minimum intensity up to 22.5° abaft the beam. This small overlap is why, very close to the 22.5° line, you may briefly see a sternlight and a sidelight together.

Worked Example

Situation: You are OOW on a 180 m bulk carrier, course 045°, speed 13 knots, in clear weather at 0200 in the Dover Strait south-west traffic lane. Visibility is good. At 0203 you sight, fine on the starboard bow, bearing 052°:

Three white lights in a vertical line.
A red light below them.
On radar, a target at 5.2 nm, and about 0.5 nm astern of it a much weaker echo.

Step 1: identify. Three white masthead lights in a vertical line mean a power-driven vessel towing with a length of tow over 200 m. The red sidelight shows you her port side.

Step 2: aspect. You see red: she is crossing from your starboard side to your port side, or heading roughly towards you on a reciprocal with a small red aspect. The ARPA, once it has a stable track at about 0206, shows her course 250° at 7 knots, aspect red 20°, CPA 0.4 nm ahead, TCPA 19 minutes.

Step 3: the tow. A tow 0.5 nm long is effectively a second vessel. Rule 24 tells you a tow over 200 m shows sidelights and a sternlight and, by day, a diamond. The weak radar echo is the tow; acquire it on ARPA as a separate target.

Step 4: rules. She is a power-driven vessel. Is she RAM? Only if you also see red-white-red, which you do not. So this is a crossing situation under Rule 15; she is on your starboard side, so you are the give-way vessel. You must avoid crossing ahead of her, and, critically, avoid passing between the tug and her tow.

Step 5: action. At 0207, with a CPA of 0.4 nm, you alter early and substantially to starboard to pass astern of the whole tow. A 40° alteration to 085° should put both the tug and the tow on your port side; confirm it on the ARPA trial manoeuvre before you turn, and check the starboard side for traffic in the lane. Sound one short blast as you alter, because you are in sight of one another. Watch the bearing of the tow open, not just the tug.

Step 6: record and report. Log the time, alteration, and reason. Call the master if the standing orders require it for a CPA under 1 nm.

A daytime variation

The same vessel by day would show a diamond (tow over 200 m) on the tug and another diamond on the tow. If the tug also showed ball-diamond-ball, she would be RAM because towing severely restricts her ability to deviate. You would still give way, but under Rule 18 rather than Rule 15.

Common Mistakes

Mixing up the masthead light arc (225°) with the sternlight arc (135°). Both are white. Use the 22.5° abaft the beam line to remember which is which: masthead and sidelights share it.
Forgetting the tow. Seeing a tug's masthead lights and crossing just astern of her is how vessels end up hitting tow wires. Count the masthead lights, look for the yellow towing light, and track the tow on radar.
Assuming CBD means stand-on. A CBD vessel is not stand-on over a vessel crossing from her starboard side; other vessels are required by Rule 18(d) only to avoid impeding her if circumstances admit.
Reading NUC as RAM. NUC: red-red, two balls. RAM: red-white-red, ball-diamond-ball. Get the middle one right.
Using Rule 34 signals in fog. Manoeuvring signals are for vessels in sight of one another. In restricted visibility you use Rule 35 signals and act under Rule 19.
Leaving the fog signal set to "making way" when stopped. Two prolonged blasts mean underway but stopped. The difference matters to the other ship.
Waiting for the sidelights. Big-ship masthead lights show twice as far as sidelights. Start bearings and plotting on first sighting.
Misjudging an anchored ship as underway. An anchored ship of 100 m or more is lit up with deck lights that can drown the anchor lights. Look for the lack of a bow wave on radar, zero speed on ARPA, and the two white lights at different heights.
Confusing the trawler's green over white with a power-driven vessel's starboard sidelight and masthead light. All-round lights are close together in a vertical line at the masthead; a sidelight is low and to one side.

Summary

Masthead 225°, sidelights 112.5° each, sternlight and towing light 135°; all after limits meet at 22.5° abaft the beam.
Vessels of 50 m or more: masthead 6 nm, everything else 3 nm. A second masthead light abaft and higher is mandatory at 50 m.
Status lights at the masthead: red-red NUC; red-white-red RAM; red-red-red CBD; green-white trawling; red-white other fishing; white-red pilot.
Shapes: ball anchored; two balls NUC; ball-diamond-ball RAM; cylinder CBD; three balls aground; two cones apex together fishing; diamond tow over 200 m; cone apex down motor-sailing.
Towing: two masthead lights in a vertical line, three if the tow exceeds 200 m, plus a yellow towing light above the sternlight.
Short blast about 1 s, prolonged 4 to 6 s. One short starboard, two short port, three short astern; five short means doubt.
In restricted visibility: one prolonged making way, two prolonged stopped, one prolonged two short for the "special" vessels, bell (and gong at 100 m) at anchor every minute.
Know the Annex IV distress signals word for word and act on them under SOLAS V/33.

Check Your Understanding

At night you see a red light low down and two white lights above it, the higher white light being to the right of the lower one and the gap between them slowly widening. Describe the vessel and what she is doing.

Answer: A power-driven vessel, most likely 50 m or more, showing you her port side. The higher light is her after masthead light; because it is to the right, her bow is to your left and she is crossing from your right to your left. The widening gap means she is turning away from you (or her aspect is broadening). Take compass bearings at once: a vessel crossing from your starboard side on a steady bearing is the stand-on vessel under Rule 15, and you must give way.

What are the minimum ranges of a masthead light and a sidelight on a vessel of 60 m?

Answer: Masthead light 6 nm, sidelights 3 nm (Rule 22(a)). The sternlight, towing light and all-round lights are also 3 nm.

A tug is showing three white masthead lights in a vertical line, sidelights, a sternlight and a yellow light above the sternlight. What does this tell you, and what shape would she show by day?

Answer: She is a power-driven vessel towing astern with a length of tow exceeding 200 m. By day she shows a diamond shape where it can best be seen, and so does the tow. Do not pass between the tug and the tow.

Distinguish between the lights of a vessel not under command making way and a vessel restricted in her ability to manoeuvre making way.

Answer: NUC making way: two all-round red lights in a vertical line, sidelights and sternlight, and no masthead lights. RAM making way: all-round red, white, red in a vertical line, plus masthead light(s), sidelights and sternlight.

In fog you hear one prolonged blast followed by two short blasts, repeated about every two minutes. What could the vessel be?

Answer: A vessel not under command, restricted in her ability to manoeuvre, constrained by her draught, sailing, engaged in fishing, or towing or pushing; or a vessel engaged in fishing or RAM at anchor. You cannot tell which from the signal alone, nor her bearing reliably, so you must act under Rule 19.

What is the signal for a vessel of 120 m at anchor in restricted visibility?

Answer: At intervals of not more than one minute, the bell rung rapidly for about 5 seconds in the fore part of the vessel, immediately followed by the gong sounded rapidly for about 5 seconds in the after part. She may additionally sound one short, one prolonged and one short blast to warn an approaching vessel.

You are overtaking another vessel in a narrow channel and intend to pass on her port side. What signal do you make, and what reply do you expect if she agrees?

Answer: Two prolonged blasts followed by two short blasts. If she agrees, she replies with one prolonged, one short, one prolonged, one short blast. If in doubt she sounds at least five short and rapid blasts.

Name the day shapes for a vessel aground and for a vessel constrained by her draught.

Answer: Aground: three balls in a vertical line. Constrained by her draught: a cylinder.

Under Annex III, what frequency range and audibility would you expect from the whistle of a 250 m tanker?

Answer: A fundamental frequency between 70 and 200 Hz, audible at a range of 2 nm. It is a low, deep note.

List six distress signals from Annex IV.

Answer: Any six of: a gun fired at about one-minute intervals; continuous sounding of a fog-signalling apparatus; red star rockets or shells fired one at a time at short intervals; SOS by any signalling method; the spoken word MAYDAY by radiotelephony; the code flags NC; a square flag with a ball above or below it; flames on the vessel; a red parachute rocket flare or red hand flare; orange smoke; slowly and repeatedly raising and lowering outstretched arms; a DSC distress alert; an Inmarsat or other satellite distress alert; EPIRB signals; approved radiocommunication signals including survival craft radar transponders.