Reading a light at night: what you see and what to do
1The lesson in plain words
Use light patterns to read aspect and vessel type. Repeated compass bearings help you judge collision risk.
Read the colours
A sidelight helps you read her aspect, the angle from which you see her. Red means you see her port side; green, her starboard side. Both together suggest a bow-on or nearly bow-on aspect. With separate lights, her sternlight alone places you more than 22.5° abaft her beam. If you are catching her up from that sector, you are overtaking and must keep clear (Rule 13).
Read the whites
Confirm the whole pattern. An identified masthead light with sidelights indicates power; white above red alone is inconclusive. Small motorboats may use an all-round white instead. Two masthead lights, the after one higher, help show aspect. Nearly aligned from ahead, they suggest a nearly bow-on view. Their spacing can change as either vessel moves; it does not prove she has turned. Sidelights without a visible white light may indicate sail; look again before deciding.
Check the bearing
Take repeated compass bearings: directions read on a hand-bearing compass. A fixed point on deck gives only a relative bearing, which changes when your boat turns. If a vessel is closing and her compass bearing hardly changes, assume risk of collision. A changing bearing does not always remove risk, especially at close range or with a large vessel. Recheck as often as the situation requires. If in doubt, assume risk (Rule 7).
Decide what to do
Check overtaking first (Rule 13). For two ordinary power-driven vessels crossing with collision risk, the one with the other to starboard gives way (Rule 15). The stand-on vessel initially keeps course and speed. She may act when the other fails to act appropriately; she must act when the other alone cannot avoid collision. If acting early, avoid turning to port for a vessel on your port side if circumstances allow (Rule 17). Two power-driven vessels meeting head-on or nearly head-on with collision risk both turn to starboard (Rule 14).
Special lights
Extra all-round lights can identify a vessel’s work or limits on manoeuvring. Identify the complete pattern before deciding which rule applies. Learn these special-status lights at /colreg/rule-24 to /colreg/rule-31 and practise at /lights-shapes.
Under sail
Your windward side is the side opposite your mainsail (Rule 12(b)). If two sailing vessels risk collision, the one with wind from port keeps clear of the one with wind from starboard. With wind from the same side, the windward boat (upwind of the other) keeps clear. With wind from port, keep clear if unsure which side a windward boat has the wind on (Rule 12). Power normally gives way to sail (Rule 18); overtaking, narrow-channel and traffic-lane duties still apply.
2Step by step
Reading a light at night
- 1
Note every colour and how the lights are arranged.
One colour alone can mislead you; identify the complete pattern.
- 2
Take a compass bearing with a hand-bearing compass and note the time.
A steady bearing on a closing vessel means risk of collision (Rule 7).
- 3
Repeat compass bearings as often as the situation requires and watch the light pattern.
Alignment helps judge aspect; it does not prove a turn. Use compass bearings to judge collision risk.
- 4
Decide which rule applies and whether you give way or stand on.
Rules 12 to 18 depend on what both vessels are and on the aspect you see.
- 5
If you give way, act early and make a clear, large change.
Rule 16 and Rule 8: the other vessel must be able to see your action.
- 6
Keep watching until she is past and clear.
Rule 8(d): check that your action works; she may alter too.
3Key points
- 1
Red = her port side; green = her starboard side; both suggest a bow-on or nearly bow-on aspect.
- 2
A masthead light with sidelights indicates power; small motorboats may use an all-round white with sidelights.
- 3
Aligned fore-and-aft masthead lights suggest a nearly bow-on aspect; changing alignment alone does not prove a turn.
- 4
A steady compass bearing on a closing light means risk of collision.
4Frequent mistakes
Wrong: Judge the danger by how close the light looks.
Right: Distance is hard to judge at night. Take compass bearings: a steady bearing on an approaching vessel means risk of collision.
Wrong: Red and green with no white light above is a small motorboat.
Right: Power-driven vessels show a white masthead or all-round light. Sidelights alone usually mean a sailing vessel.
Wrong: Seeing only a white light means she is at anchor.
Right: It may be a sternlight, or a small motorboat’s all-round white seen from astern. Check the bearing and look again.
5Practice case & exam keys
You are motoring on 000° in open water at night. On your starboard bow you identify a power-driven vessel’s masthead light and red sidelight, with no special-status lights. Her compass bearing stays at 045° as she closes.
Show the answer
This is a power-driven vessel crossing from your starboard. The steady compass bearing and decreasing range mean collision risk. You must give way early (Rules 15 and 16). If safe, alter clearly to starboard to pass astern, or reduce speed as appropriate. Keep checking until she is past and clear.
Exam keys
- Red = her port side, green = her starboard side, both suggest a bow-on or nearly bow-on aspect.
- With separate lights, sternlight only = more than 22.5° abaft her beam; coming up from there is overtaking: keep clear.
- Steady bearing and closing = risk of collision (Rule 7); act early and clearly (Rules 8 and 16).
- Power-driven crossing with collision risk: the vessel with the other to starboard gives way (Rule 15).
Finish this unit
When you've read it, match the terms, then answer three questions. Get them all right to finish the unit; opening the page alone doesn't count.
My notes for this unit
No notes