Thanks to ilearntoboat for this:
Navigation lights are a very important safety system on a boat. These lights help all boats navigating the waters between sunset and sunrise and in events with reduced visibility such as rain and fog. Using these lights appropriately helps boats navigate safely and identify the give-way vessel to avoid collisions.
Sidelights (or combination lights) are red and green lights that are only visible to approaching vessels. The red light designates the vessel’s left, or port, side, while the green light designates the right, or starboard, side.
A sternlight is a white light that is located at the stern of the boat and is only visible from behind the vessel.
A masthead light is required on all power-driven vehicles. This white light shines forward and to both sides and must be displayed by all vessels 39.4 feet in length or longer when under engine power.
An all-round white light may be used to replace the masthead light and sternlight in vessels less than 39.4 feet in length. This light can be seen by other vessels in any direction.
The rules to determine which navigation lights should be displayed depends on:
- The length of your vessel.
- How your vessel is powered—engine or sail.
- The location of your vessel—inland or international waters.
- Whether your vessel is at anchor.
Powered Boat Navigation Lights
Powered recreational vessels, including sailboats while under power, are required to use the following navigation lights while operating between sunset and sunrise or in periods of restricted visibility.
Powered Boats Less Than 39.4 Feet, or 12 Meters
- One all-round white light that illuminates 360 degrees and for two miles, and…
- One pair of red and green sidelights that illuminate 112.5 degrees and for one mile.
The all-round white light needs to be positioned at least 39 inches above the sidelights.
Powered Boats Greater Than 39.4 Feet but Less Than 65.6 Feet, or 20 Meters
- One masthead light that illuminates 225 degrees and for two miles
- One stern light that illuminates 135 degrees for two miles
- One set of red and green sidelights that illuminate 112.5 degrees for one mile
Note that the radius of the masthead light and the stern light combined is 360 degrees. The masthead light must be positioned at least 8 feet above the gunnel.
- Masthead light (forward)—225 degrees visible from two miles
- Sternlight (aft)—135 degrees visible from two miles
- Sidelights—112.5 degrees visible from one mile
Boat Navigation Lights at Anchor
When boat is at anchor and not in a designated anchoring area, such as a marina, all-round white lights should be used to ensure the vessel can be seen by all boats in the area.
Unpowered Boat Navigation Lights
All sailboats over 23 feet in length, or 7 meters, operating between sunset and sunrise and in periods with restricted visibility, are required to have the following lights.
- A white stern light that illuminates 135 degrees for two miles
- Red and green sidelights that illuminate 112.5 degrees for one mile
These vessels can use independent sidelights and white lights or use a tricolor light combination in their place. The tricolor light combines red, green, and white lights into one unit. The tricolor light can never be used while a vessel is under power, regardless of whether or not its sails are hoisted. Tricolor lights also should not be displayed at the same time as regular sidelights.
Vessels under Oars or Paddles and Sailboats under 23 Feet
Sailboats under 23 feet, or 7 meters, are only required to display a white light, such as a lantern or a flashlight, when operating between sunset and sunrise or during periods of restricted visibility. Sidelights are also a good idea, but not required.
Canoes, kayaks, and rowboats are also required to display a white light between sunset and sunrise and during periods of restricted visibility.
More info from Boatus