A sailing vessel using propelling machinery by day must show which of the following?

A sailing vessel using propelling machinery by day is required to show a black cone. This visual signal serves as an indication of the vessel’s status while it is under mechanical propulsion. The presence of the black cone signifies to other vessels that the sailing vessel is utilizing its engine, which is crucial for navigation safety as it alerts other mariners to its operational condition.

The design of the black cone helps distinguish these vessels quickly, as it is a standard visual aid recognized in maritime navigation rules. While other shapes like the black ball or black cylinder may convey different meanings, they do not indicate that the vessel is under power from its machinery, which is the fundamental reason why the black cone is the appropriate choice in this scenario.