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.
We have a friend who was well into a voyage across the Atlantic when the oil pressure alarm went off on her engine.
She and her crew made all the usual checks but to no avail.
When they finally got to the Azores under sail and with no engine for charging, the mechanic found parts of the seal from a bottle of oil blocking the lube oil pump pickup tube.
My guess is that someone had let these fragments drop into the container of oil, or maybe a whole seal, a surprisingly easy thing to do, and then dumped the oil into the engine.
It truly is the little things that can get you. And when they do on a voyage it sucks ten times more.
This kind of thing is one of the many reasons I have always done all the routine maintenance on critical systems like the engine myself, and would have done so even if we could have afforded to pay someone else.
Sure we all hope that the guy in the boatyard tasked with doing oil changes would be careful not to let this happen, and strain the oil if it did…