Originally Posted by junkrig
It is difficult to tell for sure from the text of the ColRegs whether it would be permissible to have green / red lights at the masthead on a power boat, above your steaming lights, but in 100% of the illustrations the white steaming (forward visible) is above the red / green sidelights. That would imply that you would have to turn off your masthead light and use deck or cabin sidelights to run under power.
Length is not a factor in these illustrations.
However, he said, correcting himself:
the point at which the 12 meter breakpoint does come into play is that, for boats under 12 meters, it is acceptable under power to use sidelights plus one 360* white light, whereas over 12 meters the requirement is for a 225* white masthead light plus a white 67.5* sternlight. Boats over 12 meters seem not uncommon on SN.