I have seen two different configurations for running lights on a sailboat. Which one is correct for sailing and for steaming?
Thanks for your question. The situation can seem a little convoluted, but the way this breaks down is that all vessels underway or at anchor must display at least one navigation light at night or in times of limited visibility.
The confusion might arise from the requirement that different vessels carry different sets of navigation lights (or running lights) so that they can be more readily identified. Sailboats less than 23 feet that are underway should display side lights (red to port and green to starboard) and a white stern light. A boat of this description can also get by with a flashlight or a lantern. Sailboats that are larger than 23 feet should display red and green side lights and a white stern light when under sail or being rowed. When under power at night or in limited visibility, that vessel is subject to the same rules that apply to powerboats. What that means is that the sailboat should display illuminated side lights, a stern light, and a masthead steaming light.
What might get a little tricky is that the masthead steaming light actually isn't at the masthead, but somewhere above the side lights, usually a third of the way up the mast. Vessels longer than 164 feet are required to carry a second steaming light abaft of and higher than the first.
If you'd like to see further reference on this, have a look at SailNet's graphic tutorial on Lights and Shapes. You can also have a look at some printed resources that explain the various uses of lights for navigation on board. These include Chapman Piloting and John Rousmaniere's The Annapolis Book of Seamanship.