What shape does a vessel proceeding under sail and propelled by machinery display?

A vessel that is proceeding under sail while also being propelled by machinery displays a cone with the apex pointing downwards. This shape, which is typically a cone apex down, is a specific signal that indicates the state of the vessel to other nearby vessels. The reason for this distinctive signaling is rooted in navigation safety and is defined by the COLREGs (International Regulations for Preventing Collisions at Sea) to ensure that all mariners can clearly identify the status and capabilities of other vessels.

The reason this signal is vital is that it informs other vessels that the sailboat can maneuver effectively under both the power of its sails and by the machinery, which may influence how it interacts with other vessels in terms of right of way and navigational decisions. This is particularly important in busy waterways where multiple vessels are present.

Other shapes mentioned in the options do not correspond to a vessel under sail and also using machinery. For instance, a cone apex up might indicate different operational conditions or vessel types, while a cylinder and a ball represent other situations entirely and would not provide the necessary information about the combined sail and power propulsion. This clear signaling helps to prevent collisions and enhances overall maritime safety.