I'm surprised no one has even mentioned
hull speed. I have a 39' twin diesel powered boat - mine has a semi
displacement hull so it will easily exceed
hull speed (call it 8 knots), tops out just over 20 knots. The OP's boat sounds like a planing
hull but the physics are pretty similar.
Around hull speed the engines are under light load, top fuel economy. That's probably the best speed for long engine life. One point that was made by a couple of posters can't be emphasized enough and that is to get the engine running *and up to operating temperature under load* frequently. Do not just go start it and run it at the
dock under no load (there are many post's worth of "why", trying to keep it relevant).
But a well used, well maintained diesel will last thousands of hours regardless of whether you operate at 25% or 75% throttle. WOT is probably fine, for that matter - I've had a lot of diesel equipment fall apart around the
motor at 10k+ hours in industrial applications - but you will use a lot more fuel and the sbmar guy has some good points.
If a diesel fails earlier than, say, 3000 - 5000 hours then it was probably not run much and / or maintained properly. You wanna pay for fuel, go for it. Run it slow? Also shouldn't be a problem, although I do open mine up to ~80% from time to time, WOT occasionally.
BTW, if an engine won't handle max RPMs for short periods of time it's a failure waiting to happen. It is capable of running at manufacturer's full speed - or there's something wrong with it.
As to "why" - it's simple
navigation principles... I can go the ~50 miles from my marina to SF in half a day at hull speed but can get there in half the time if I want to burn some fuel. A boat that can go 25 kts will get there even faster so a three day Bay
cruise in a sailboat (which I also enjoy) can be packed into a day or a weekend.
But we're recreational use and most people will not be "wearing out" their engines.