Under sail, a red sidelight to port, a green sidelight to starboard and a white stern light (COLREGs Rule 25). Under engine you add a white masthead light shining forward, and at anchor you show an all-round white light.
Navigation lights must be shown from sunset to sunrise and in restricted visibility. They tell other vessels what you are and which way you are heading.
Under sail (Rule 25), a sailing vessel under way shows:
- a red sidelight on the port side and a green one on the starboard side, each shining from dead ahead to 22.5 degrees abaft the beam;
- a white stern light, shining aft over the remaining arc.
A sailing vessel under 20 metres may combine all three in a single tricolour lantern at or near the masthead, where it is higher and easier to see over waves. She may also show an all-round red light over an all-round green light at the masthead, but not together with a tricolour. A sailing vessel under 7 metres that cannot carry fixed lights must at least have a white torch or lantern ready to show in time to prevent a collision.
Under engine (Rule 23), the boat is a power-driven vessel and shows sidelights, a stern light and a white masthead light shining forward over 225 degrees. The tricolour must not be used under power.
At anchor (Rule 30), a vessel shows an all-round white light where it can best be seen. By day, an anchored vessel shows a black ball.
Reading other vessels' lights works the same way. Red and green together mean she is heading straight at you; red alone means you are looking at her port side; green alone, her starboard side; a white light alone is usually her stern, or a vessel at anchor. A white light above the sidelights means she is under power.
Check your lights before dark, carry spare bulbs or a backup torch, and remember that a boat's deck lights or cabin lights can make the navigation lights hard for others to read.
More in the book: Chapter 13, The rules of the road