Hey all,
For our size vessels, when under power, we must show a 360* visible white light, as well as side lights (red/green bow lights). That 360* can be made in multiple ways; all that matters is that from a distance other vessels can only distinguish a single white light from 360*.
On a sailboat, this is generally accomplished with a single forward-facing 225*-arc "masthead" light (which is NOT mounted at the top of the mast, but generally mid-mast somewhere), and an aft-facing 135*-arc "stern" light.
When sailing you may only display side lights and an aft-facing 135*-arc stern light.
~A