There are all kinds of good reasons to check our engine RPM, including making sure:
- We are not overloading before the engine warms up.
- We are not under-loading after warm up.
- To figure out fuel burn.
- To check that the prop is not under- or over-sized.
- To check that the engine has not lost RPM at wide-open throttle, an early sign of all kinds of things that should be fixed before damage is done.
But the problem is that tachometers can drift into inaccuracy.
And an even bigger problem is that if we change our alternator to a larger and more robust one, as many of us cruisers do, and most of us should, it’s likely that the tachometer will be wildly inaccurate afterward because the new alternator sends a different number of pulses per revolution.
Wait, there’s more. If we change to a serpentine belt, again as we should, that likely changes the ratio between the crank shaft and alternator RPM, making the inaccuracy even worse.
The good news is that handheld RPM counters are both accurate and relatively cheap.
And then once we know the actual RPM, it’s reasonably easy…OK, a pain in the ass…to recalibrate the tachometer.
Phyllis spent an hour crunched up in the cockpit locker poking at the little calibration button while I read the rapidly changing numbers and yelled “press”, “let go”, “press”.
Why an hour? Because I adjusted the wrong way…twice…but let’s not go there.
Worth it to have an accurate tachometer, though…at least as far as I’m concerned…not asking Phyllis.
You can usually find the calibration instructions for your tachometer online, but here’s a typical set of instructions.