Difference between port and starboard engine

Either I'm not understanding what you mean, or I'm missing something.
Let's take a typical gearbox like the one below.
It's directly coupled to the engine, and its only external rotating part is the flange at the bottom, to which the shaft is connected.
If you put the tape on that flange, or on the shaft connected to it, you certainly can measure the rpm, but with the two caveats of my post #13:
1) you can't check idle speed with gear disengaged
2) the rpm that you measure is affected by the g/box reduction ratio



Mind, as I said I normally attach the reflective tape on the pulley, so the gearbox and its layout is totally irrelevant.
In just one occasion, when the very tight space on the front side of the engine did not allow to keep the photo tach in a decent readable position, I attached the tape to the g/box output flange.
I did have to consider the reduction ratio for calculating the actual engine rpm, though.

If there is a third alternative (vs. either the front pulley or the g/box output), I'd be curious to understand what it is.

I have to type slower or your going to have to read the post to the bottom;
From #19 above;
Regardless of where the foil tape is placed, just the same place on both engines if just comparing rpm.
If your checking against your helm tach, Best on the balancer or crank pulley base.

There are setups like that already out there. They display the amount of slip in clutches with trolling valves.

Well there ya go. You probably have the much better instrument anyway.
cr

Good point, but did you experience slipping only depending on some conditions, like rpm or load?
I'm asking because for some occasional checks it would have been sufficient to put the tape on both the front pulley and the gearbox output, and check the consistency.
Nowhere near as convenient as having a gauge on the dashboard of course, but I would guess (and hope!) that a gearbox doesn't start slipping badly in the middle of a passage with no previous warning signs.

Consider swapping the coolers to see if the problem follows the cooler?

Some folks don't like sleeping dawgs.

Or, just put on a newer and larger cooler. This would also keep your hands off of the other engine at this time.

Ok, fine. Don't touch the coolers. Use your heat gun on the incoming and discharge ports of the heat exchanger. Your temperature delta on each engine should be the same if the heat exchangers are performing as they should. But the very fact that the temps aren't climbing suggest that there is no blockage.

+1 on the principle if it ain't broke, etc.

BUT, the consistency check between the delta temp in/out the cooler that rtrafford suggested makes very good sense.
Well worth the 3 minutes it takes (if that), imho.

End of the day, we all must do our due diligence aboard to put to rest issues that bring us concern. If you have an issue that is raising a flag in your mind, you're doing your job as a captain or an owner or a mechanic if you chase down the concern and vet it out. Period. And there are always ways to vet these things out without spending big dollars or using pneumatic tools. If it's in your head, don't merely dismiss it.

That's my philosophy. Like it or hate it.