Power-driven Vessels:

Powerboats less than 164 feet (50 m) but longer than 39.4 feet (12 m)

While underway – these vessels must display a green sidelight at the starboard side and a red sidelight at the port side. These sidelights should show an unbroken light from dead ahead to 112.5 degrees on either side and should be visible at a distance of 2 nautical miles (NM). Power vessels less than 65.6 ft (20 m) in length may combine the sidelights in a single (“bi-color”) fixture carried on the fore-aft centerline of the boat.

You must also carry an aft facing white stern light, as nearly as practicable at the stern, so its beam is an unbroken arc of 135 degrees showing 67.5 degrees on each side of the vessel (typically mounted on the centerline). This light must also be visible at a distance of 2 NM.

A forward facing white “masthead” light must be carried on the centerline of the boat. Often mounted on a pole or the front of the bridge, the masthead light must show an unbroken beam of 225 degrees (112.5 degrees on either side of the boat) with a visibility of 3 NM, and should not be obscured by any equipment while underway. Not mounted literally at the top of the mast, the masthead light has specific height requirements which depend on the length and beam of the vessel, and place this light higher than the side lights. Refer to your copy of the USCG Navigation Rules and Regulations Handbook (legally required on all boats of this size, and often referred to simply as the “Rules”), Annex I, Section 84 for specific height requirements for your vessel.

Powerboats less than 39.4 feet (12 meters)

While underway, these vessels must display the same side and stern lights as above, but these lights are only required to have a visibility of 1 NM along with a masthead light with a 2 mile visibility.

On power driven vessels (only) less than 39.4 feet, the masthead and stern functions may be combined into a single all-around white light (225° plus 135°). If used, make sure the all-around white light is at least 3.3 feet (1 meter) above your sidelights. When not underway, this all-round combination light can also be used as an anchor light, see below.

For power boats less than 23 feet (7 meters) that do not exceed 7 knots boat speed, you may display just one all-around white light. However, sidelights are recommended.

When is a Sailboat considered a Power Boat?

• If sailboat is powered by machine propulsion of any type, then the sailboat is classified as a power-driven vessel and must comply with the rules in this section.

Sailboats:

Sailboats less than 65.6 feet (20 meters)

While under sail, you have several options for displaying navigation lights:

• Display a green light at the starboard side, a red light at the port side (or a bi-color light that combines both), and a white light off your stern – all with the same beam angle and distance visibility requirements as for powerboats, above; or
• Display a tri-color light off the top of your mast. This light includes a red light to port, a green light to starboard and a white light aft – all in a single light creating a full circle. This light can only be used when under sail. If under power, or motor-sailing with your sails set, regular navigation lights must be displayed – including a steaming (masthead) light, see below. A tri-color light increases the boat’s visibility to others at sea, and can also conserve electrical power when all three lights are illuminated by a single bulb; or
• Display a green light off your starboard bow, a red light off your port bow, a white light off your stern with the option to display addition red and green all-round lights at the top of your mast, 1 meter above the uppermost sail. The upper light should be red and the lower light should be green. This option is rarely used today, but is still accepted. Again, the red and green all-round lights at the top of the mast can only be used while under sail.

Visibility of navigation lights for sailboats this size should be as follows:

• Masthead light – 3 NM
• Sidelights/Bi-color light – 2 NM
• Stern or all-round light – 2 NM

NOTE: A sailboat operating under power at night or at times of restricted visibility - whether the sails are set, or not – is required to follow the same rules as a power boat, as outlined above, since they are no longer entitled to the sailboat’s usual right of way privileges due to restricted maneuverability. For this reason, most sailboats will often comply with option 1 above, and will mount a masthead (or “steaming”) light on the front of the mast. Although called a “masthead” light, this light is not literally carried at the top of the sailboat’s mast. Instead there are specific minimum height requirements for various length sailboats, which will be found in Annex I, Section 84 of the Navigation Rules and Regulations Handbook.

It's also important to note that the function of an all-round white light located at the top of a sailboat mast is an anchor light, not a masthead light. This light should not be used when sailing or powering at night – it should only be used at anchor.