Which system typically uses a tachometer to indicate propulsion shaft speed?

A tachometer is an instrument utilized to measure the rotational speed of an object, often displayed in revolutions per minute (RPM). In the context of propulsion systems, it plays a crucial role in monitoring the performance and efficiency of engines and motors.

The tachometer system specifically pertains to devices that provide real-time feedback on the rotational speed of the propulsion shaft, facilitating better control and diagnostics of machinery. By indicating shaft speed, it allows operators to optimize engine performance and ensure that it operates within safe and efficient parameters.

In contrast, options related to the torque converter system, pneumatic system, and gearbox system may involve the use of tachometers but are not specifically designed to function as a tachometer system itself. They serve different purposes within mechanical and hydraulic systems, where monitoring speed may be relevant but not their primary function. Thus, the correct answer underscores the specific role and definition of a tachometer system in indicating propulsion shaft speed directly.