Blue Sea 8239 AC Digital Frequency Meter

The Blue Sea Systems 8239 AC Digital Frequency Meter reads 40 to 90 hertz behind a splashproof front.

Frequency is the number that tells you how a generator is actually running, because engine speed sets it directly. A set drooping to 57 hertz under load is working too hard, and one drifting high is running fast. Motors, compressors and anything with a transformer care about that, and voltage alone will not reveal it.

Shore power rarely moves off 60 hertz, so this meter belongs on a generator boat and has little to say anywhere else. For a boat that only ever takes dock power, the money is better spent on a voltmeter.

It is a single purpose instrument, which keeps the reading large and unambiguous rather than sharing a display. The front is splashproof and display brightness has three levels. Explore more marine electrical meters.

• Frequency shows how hard the generator is working
• 57 hertz under load means the set is struggling
• Voltage alone will not reveal it
• Little use on a boat that only takes shore power