What does a vessel under sail but also being propelled by machinery exhibit?

A vessel under sail that is also being propelled by machinery exhibits a conical shape, apex down. According to the COLREGs, this specific shape indicates to other vessels that the sailing vessel is not only using its sails but also has its engine running, which is an important distinction for other mariners to be aware of.

The conical shape, when displayed apex down, signifies that the vessel is operating under both sail and power. This helps to communicate the vessel's status clearly to nearby vessels, allowing them to take appropriate action or navigate safely around the powered sailing vessel.

The other options do not represent the correct visual signals for a vessel under these circumstances. A conical shape with the apex up, a single all-round white light, and two vertical red lights pertain to different maritime situations and do not accurately describe the status of a vessel being propelled by machinery while under sail.