.....when I reviewed the engine survey the Cummings specified RPM of 2600 was not met. The boat was only able to reach 2500RPM at full throttle (measured at the flywheel with tachometer)
I'm curious what that means and if it's something to be concerned about.
Assuming during the engine tests that the engine achieved its maximum governed rpm with no load on it, what you're seeing is the load effect of the propeller on the boat. Just as a car cannot reach specified WOT rpm going up a hill in high gear, so a boat's engine may not be able to reach WOT rpm in gear because of the prop pitch.
The fact a boat can't achieve the manufacturer's listed WOT rpm under load is not necessarily a cause for concern. Some boats, like Grand Banks during the 60s, 70s, and 80s at least, came from the manufacturer "overpropped." It was like getting a car with higher gear ratios.
The advantage is that for a given rpm the prop moves more water and thus you get more speed. For cruising at lower speeds with low to medium power engines, this makes for a more efficient operations because you go farther on less fuel.
BUT.... there is a limit to how much you can get away with. Too much pitch on the prop and the engine may be working too hard all the time.
And there are other types of boats, or other types of operations, where you don't want this overpropped or "cruise prop" setup. This is partriculalry true of boats that by design are intended to be run faster than the typical cruiser's 6-8 knots. In these applications, horsepower is more important, so the props are pitched down.
It's a pretty complex relationship, engine power + rpm + prop diameter + pitch, and there's really no one-size-fits-all combination or explanation.
The very rough and general rule of thumb is that the prop should be pitched to allow the engine to reach is specified WOT rpm. The fact that the boat you were testing did not, and the WOT rpm was 100 or whatever less than the book figure, would indicate the boat is slightly overpropped.
That is not a bad thing at all for some boats and engine/prop combinations. I suspect--- but I don't know-- that a Nordic Tug could benefit from overpropping unless the owner wanted to run it at the higher but less fuel efficient speeds the NT is capable of.
If you want more information on how this prop pitch-WOT rpm thing works, I suggest you talk to a reputable prop shop in your area. We did when we were told our old GB needed new props, and we learned a hell of a lot in the process of getting our prop situation squared away.