If you can't reach the engines specified RPM then your lugging the engine at any rpm, like a car going up a grade in to high a gear.
This is not so.The prop is not a transmission geared to a wheel.
All engine HP /torque graphs will show a different HP at different RPM'S.
Should one prop the boat so the HP required is less than the HP produced (10-20% less) the engine will operate far more efficiently than if propped for full tilt rpm.
Thats how every big noisemaker that needs a fixed CPS operates.
ON a new to you boat simply run to full throttle , note the RPM and pull back 10% or 300RPM , and the engine will not be overloaded.
This is because even a slight lowering of the prop speed required far less HP.
A propulsion engine may create 250 Hp at 2100,
and as a noisemaker the allowed load will only be 175HP s,,1800 standby power ,
160hp at 1500 (Brit 50 Cps) and
125HP at 1200 for cont duty prime power.
All these RPM /HP settings are fine , no lugging , no overloading , just good fuel burn.
Changing props that are "Close" is frequently done to find a sweeter spot for the engine long term operation.
Some are smoother at one RPM , some may shake at a specific RPM , a slight prop change might help with this , the efficiency difference would be hard to measure.
To avoid "lugging" simplty run at full throttle for about a minuet , note the RPM and pull back 10% or 300RPM.
500 RPM pullback on a gas powered boat.
The 10% figure or less if top RPM is under 100! Esso Maru?