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.

From first hand experience, I learned that there is a third caveat with which one must be concerned. In a prior boat that had high-horsepower J&T 892s, with Alison gears, I burned out a transmission because one of the gears was slipping. A great deal of pressure was required to keep the gears from slipping, but too much pressure and the engine's rear main seal will blow out (with gear oil escaping into the engine's oil pan). I ended up creating my own micro-processor based digital display gauge to indicate slipping on either engine.

Now you tell me. I certainly couldn't find one but didn't mind making my own. And trolling valves (where a high degree of slip is expected) are a sufficiently different application (from my need to confirm that there was to measure even slight slip) that a trolling valve slip monitor would not have performed as well.

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.

Got the numbers:

• Port 138 degrees and 214 psi
• Stbd 129 degrees and 223 psi

Fairly sure these have been consistent for two years at cruise. Engines were synchronized. Both engines at 164 degrees. Port engine runs 1 gph higher at cruise 70-80% load. Anything In the temps that merits further investigation?

All the data is from the DD engine displays, MTU Series 60. Are the RPMs shown on these displays considered accurate?

On one yacht, sadly no longer with us, she had counter-rotating Baudouin diesels. To go astern you had to shut them off, re-fire them to run backwards, and then go in reverse.

Thank you modern gearboxes.

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

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.

That’s exactly my situation. I‘m done wondering and worrying about it.

Thank you Capt J!