The three fold purpose of navigation lights is to alert other vessels to your presence, direction of travel and vessel type.
There is no confusion in our discussion regarding what lights a sailing vessel under sail only is supposed to display. There does seem to be some confusion regarding what lights that same vessel when under power can display.
For the purposes of this note, I will refer to the combination red and green bow light as "side lights". I will refer to a white light on the mast which faces forward only as a "mast head" light, even though that light may be fixed to the forward side of the mast below the very top of the mast. I will refer to the light at the top of the mast which shines 360 degrees around the horizon as the "360" light. The white, stern facing light is the "stern" light
Vessels under power of less than 39 feet (that's us) must display sidelights and either the white forward facing mast head light and white stern light, OR vessels may display a single white 360 light. Both arrangements are perfectly acceptable. The rules specify, the white forward facing light or the 360 light must be a minimum of at least 1meter above the colored sidelights. There is no maximum height limitation for the distance of the white mast head light or 360 white light above the side lights. See Rule 23 (c-d)(i) and Rules Annex Section 84.02 (d)
Thus, the 360 white light does double duty. It serves as an Anchor light and it can also serve as the white Navigation light when under power.
Which brings us to the stern light. It is used under power ONLY IF the forward facing masthead white light is being used. It cannot be used if one is under power displaying the 360 white light because, when seen from another vessel approaching from astern, that white over white light combination will confuse that navigator regarding both the size and direction of the mis-lighted vessel.
This is a long way of saying if one's boat does not have a forward facing white mast head light but does have a 360 degree white light at the top of the mast, that vessel is within the rules to power at night with the sidelights and 360 degree light illuminated, provided the white stern light is turned off. That's why an on/off switch in the stern light circuit is useful and to my mind measurably easier to install than is a forward facing mast head light.
Kind regards
Hugh Straub