What is the defining feature of an Inboard engine?

The defining feature of an inboard engine is that it is typically a propulsion system where the engine is mounted within the hull of the vessel, driving a propeller that is usually located under the boat. This design allows for a more efficient transfer of power from the engine to the propeller, which in turn provides better control and stability while navigating.

The option stating that it powers through a water jet does not accurately define an inboard engine, as water jet propulsion systems represent a different type of propulsion entirely, often associated with jet boats. Similarly, powering through a propeller mounted externally describes outboard engines rather than inboard ones, as inboard engines utilize props that are housed within the hull. The mention of a steerable propulsion unit is more characteristic of certain modern inboard/outboard or jet systems rather than the conventional fixed propeller arrangement seen in most inboard setups. Finally, an engine mounted outside would pertain to outboard engines, thus distinguishing them clearly from inboard engines.

Overall, the essence of an inboard engine lies in its internal positioning and direct attachment to the drive system within the vessel's hull, primarily utilizing a propeller for propulsion beneath the water's surface.