A sailing vessel is not permitted to show all-round red over green lights if what condition is met?

The correct response highlights a specific requirement for the lighting configuration of sailing vessels under navigational rules. A sailing vessel is not allowed to display all-round red over green lights when her sidelights and stern light are combined into a single lantern and shown on the mast. This is essential because the all-round red over green lights specifically indicate a sailing vessel that is under sail and not using its motor power.

When sidelights and a stern light are combined into one lantern, it does not allow for the proper indication of the vessel's sailing status according to the regulations, as the individual light requirements for sidelights and the stern light need to be distinctly shown. The combined lantern configuration can lead to ambiguity regarding the vessel’s activities, particularly whether it is under sail or under power, which could pose risks during navigation at night or in reduced visibility conditions. Thus, the adherence to specific lighting configurations helps maintain clarity and safety on the waterways.