How do you avoid overloading mechanical engines?

Interesting comments, thanks for your contribution.
I guess the main reason why I never noticed any EGT drop with the RPM increase might have something to see with the fact that I was referring either to planing or semi-displacement boats.
In both cases, once over the hump, the drag increases exponentially with speed, so the typical power demand (hence EGT) is bound to raise constantly with the RPM increase.
The behaviour that you describe is probably more typical of displacement vessels, where the power demand still increases with speed, but in a more linear fashion.

Out of curiosity, do you have any specific engine in mind for which the builder declares the higher max power at anything below the max rated RPM?
I've seen plenty of engine charts, from several builders, and indeed in many of them (particularly the electronic ones) the max power can ALSO be produced starting from a few hundreds RPM below the max.
But I never came across one where at max RPM the output was even slightly less than at any lower RPM.
I'm talking of full load curves of course, not prop demand, where by definition max output=max RPM.
On the other hand, if by "most engines reach max horsepower at a lower rpm than max rated rpm" you actually meant max RELATIVE horsepower, i.e. output per revolution, then I fully agree of course.

Yes, I was referring to your comment.
No idea about which engine model your graph is referred to. Judging by the RPM range, possibly some automotive stuff?

I just re-checked several specs sheet and I still can't find, among the most popular marine diesels, a single one which at the max rated RPM doesn't produce the maximum horsepower.
Most of the electronic ones are tuned in such way that, at full load, their max horsepower is available already before the max RPM, for instance around 2000 rpm, and then the curve remains perfectly flat up to the max 2300 rpm.
That's what happens for instance with the Cat C9, C15, C18, C32.
The C12 can even produce its max output starting from 1700 rpm, but it also remains flat from there all the way up to 2300 (2500 for the C9).
Other similar examples are all the MAN CRM, MTU 2000, FPT Cursor, Cummins Quantum, VP D12/13/16... You name it, really.
For all of them, the output at max rated rpm and 100% load is always the maximum they can produce.

Anyway, the fact that for instance a C12 can already produce 705 hp anywhere between 1700 and 2300 rpm does not mean that you should ever use all that power at anything less than the max rpm, in a boat.
In fact, if your prop would demand 705 hp already with the engine spinning at 1700 rpm, the engine would be able to deliver it, but such condition implies that you are pushing a way too long prop, or that the bottom is badly fouled.
And by definition, you would be unable to increase the rpm and the boat speed further, because that's already the max output you can squeeze out of that engine.
Now, I agree that in such condition your EGT at 1700 rpm would be even higher than in a "normal" WOT condition, i.e. 2300 rpm and 100% load.
But this is clearly an exception, due to a wrong prop and/or other anomalies.

Sorry, I don't have any Yanmar specs sheet, so I struggle a bit to understand what you mean.
Got any detailed specs/curves?

Thanks, all clarified.
Fwiw, I fully agree with the explanations from CR.

Yes and no.
It's theoretically correct to say that you can run an engine at full load also at less than the max rated RPM, but only as long as it's a momentary condition due to some particular reason.
For instance, while cruising at a constant 1850 rpm in a long and rather high ocean swell, you could see the real time consumption going up and down significantly, while riding the waves uphill and downhill respectively.
But if you are cruising at 1850 with an engine whose rated max rpm is 2500 (As I recall it is on the 671, but I might be wrong), and you see that it's constantly burning the fuel that it's only supposed to burn at full load, then there is definitely something wrong.
In fact, if at anything less than the max rated rpm the propeller is already demanding to the engine the max power that it can deliver at full load, by definition you will be unable to increase further the rpm and the speed, so you are indeed running a setup which is bound to overload the engine for some reason - typically, either too long prop or fouled hull/prop.

From my part, no problem at all to use this thread also for something not so strictly EGT-related.
Actually, even the title (and my OP) was meant to include whatever it takes to better understand and control engine load, EGT being just one of the elements to consider.
Nothing against evil whims either, btw.
Anyone who can't stand a bit of that, should rather steer clear from boats and boaters!

PS: no objections from me also to change the title to "HP / Fuel / RPM / load curves" or anything else more appropriate.
But I think this is something that only an admin can do.

Well, I did try, in #52.
But happy to hear any other thought you might have!

All agreed, CJ.
That's not so relevant vs. the question from Capt Fred, though.
The only thing that can be safely said with regard to EGT, when talking of an engine which at 1850 rpm can burn either 10 or 20 gph (depending on propeller demand, hence load) is that EGT is bound to be significantly higher when burning 20 gph.