Running MTU's at low RPM?

All engines designed to be run at planing speeds have the same basic recommendations and generally that is to open them up every few hours to 70-80% load for a brief time but otherwise fine to run at low load. However, many will argue that even that isn't needed. Nothing unique about MTU in any of this. The issue is engines designed to be run at planing speeds.

Also remember that many of these engines were put into sportfish style boats which are designed to run at plane for long distances and then run at low rpm when trolling for many hours. I don’t think it is a problem with these engines as long as you spool the engines up and run then at planing speeds towards the end of a trip when you’ve been running at low rpm’s for extended amounts of time. This will help the turbo heat up, burn off any carbon that’s been building up and generally clean out your engines before you end a trip.

Basic rule of half-hour out of 8 stands for all motors I'm familiar with.

As long as they are running at normal operating temp they ll be fine. In most cases hull speed will keep them at normal temp

Yes but that’s so they can troll at very low speed. And then, being sportfishes, they run flat out back home.

Trolling valves control the amount of oil/ pressure in the tranny. They let the pressure plate "slip" to gain lower troll speeds because of reduced prop rotation. They do not reduce RPM of the engines.

Exactly what Beau said about trolling valves. For what it’s worth, I have never had trolling valves on any of my engines. On a normal trip out to the Canyons, we would do 30-40 hours at low rpm’s before cruising back home. In the thousands of hours I put on various motors, I never saw any concerns attributed to this as the turbo vanes were clean, engines ran great and did not smoke. But I would never put the boat away from a trip with heavy usage at low RPM’s without spoiling it up to burn all the crap out and clean up the engine.

Better to spoil your engines than your children.

(Sorry, I couldn't help myself.)

Something else you may need to think about;
How will the boat ride / handle at slower speeds.
Underwater designs for speed include small rudders and not so tall in diameter wheels.
Not usually the set up you want for (LSD) Low-Speed-Displacement operation.
Other boat items that change from fast to slow; open exhaust ports and E R ventilation.
Also, bow angle my change. On some boats, LSD drives the bow low.

I really think the later MAN, Cat, MTU and other computer driven engines will operate well at low speeds.
My old TI Detroits (Pre DDEC) may not like it though. That is where the idea of running them up under load helps them out some.

Auto pilots sometimes have issues at slow speeds. Had to increase our rudder size to help us.

What ever boat you look at, sea trial it well for handling and behavior at lower speeds. in all wind / current directions.
Fill your surveyors in on what you are doing.

Later engines should be able to offer some gph or even mpg at given speeds. Sometimes the best mpg is not the slowest speed.
Again, trial her well at slow and higher speeds.

On our old boat, exhaust harmonics drive me crazy under certain low rpm and loads.
Bring a DBA meter with you also for your trials to help ensure things are in deed quiet while running slower.
Our ole Bert is quieter and rides better at speed. Just can't afford the fuel burn.