What is a common instrument used to measure engine speed in a marine engine?

A tachometer is specifically designed to measure the rotational speed of an engine's crankshaft, often displayed in revolutions per minute (RPM). This measurement is crucial for marine engine operation because it helps the operator understand how hard the engine is working and to maintain optimal performance and efficiency. Tachometers provide vital information that aids in preventing engine damage, as running an engine at excessively high RPM can lead to overheating or mechanical failure.

Other instruments in the options, while important for different aspects of marine operation, do not serve the same function as a tachometer. A speedometer measures the speed of the vessel through the water, while a voltmeter monitors electrical voltage, and a pressure gauge tracks the pressure of fluids within the system. None of these instruments give a direct reading of engine speed, which underscores the tachometer's unique role in marine engine monitoring.