Old girl on plane; 1970 53’ Hatteras motoryacht

Why speculate on such? Why not just prop them correctly?

As to whether running at 40% load instead of 30% is bad, I don't know, but I feel like running them inefficiently could be bad, forcing them to work harder for less. You're doing that throughout the power curve, not just at WOT. WOT is just where it's easily measured, but at all speeds it's overpropped.

I just don't see why though it's a discussion when one has identified a boat is overpropped. Seems simple. Just correct it.

Yes it can be bad for them as it doesn't match the propellor curve of the motor, you're lugging the engine a bit, and you're running higher EGT's at that lower RPM and have less raw water flow to balance it all out. At cruise you're also losing speed on engines run cruise at load, because a higher RPM at cruise means the engine is making more HP as it has more airflow and boost. What you're suggesting is like running a tractor trailer up a steep hill and in one gear too tall for the rpm, and you're giving more throttle to make up for the lugging. I'd make sure the Detroits make 50 rpm's over rated at full fuel/water load. You want an engine to lead a very long and happy life, make sure it's not over-propped.

Wrong feeling. Diesel engines don't like running for long very lightly loaded, in principle.

In fact, it doesn't even make sense to talk of overprop, unless the engine is ran at 100% load without reaching the rated max rpm, because "braked" by a too long prop.
THAT is the condition where the EGT can go ballistic, damaging the engine - which is the true reason why you don't want to overprop a boat.
But when you run at hull speed any boat whose powerplant could push her at twice the speed, what happens is that you are actually "underusing" the engines (for lack of better wording), even if overpropped - just a bit less than if correctly propped.

All agreed.

She looks great ... lovely old girl.
As for the flag ...well we got abused (by charter guests on another yacht) at Elliot key last weekend when flying our Trump 2020 flags .... we just laughed at them.

You are sticking to the usual reasoning, which I never objected against.
Let's say that we are talking of a boat powered and propped to reach 20 kts at 2300 rpm, and you actually reach that speed (or just a hair under) but making only 2150 rpm.
I NEVER said it's a good idea to keep the boat as it is, if her usage envelope is anywhere from hull speed to a planing speed of 16/17 kts or whatever.

What I said is that if this very same boat is practically always used at hull speed, not only it's not worth bothering to shorten the pitch, but it could actually be even (slightly) better for the engines.
Higher load, hence also higher EGT and overall engine temperature (within reason), at very low rpm ain't a bad thing, because actually the opposite is true: when big diesel engines installed on fast boats are used at low rpm/load, they are running at TOO LOW temperature and EGT.
Not because it's better for them, but because it's unavoidable.

Let's take some numbers, considering a 1900hp Cat C32, just as an example.
At 1300 rpm, it can produce up to a whopping 1000 hp without getting overloaded.
But if you look at the prop demand curve (without forgetting that it's strictly theoretical, and actually different for any hull, gearbox ratios, props, hull cleanliness, and whatnot), you see a value of 343 hp.
In other words, in a typical installation, correctly propped, at 1300 rpm that engine runs at ONE THIRD of its true capacity.
Do you see what I mean now?