Originally Posted by johneck
The design point will not necessarily hit the rated power at the rated RPM and design speed. This is due to the limiting torque of the engine. The propeller torque demand curve must always be below the engine torque limit or the enigne may lug. So the bollard torque may impose a limit.
As Ad Hoc stated propeller power demand is ~RPM^3, thus propeller torque demand is ~RPM^2. All diesel engines have a decreasing torque near rated RPM. If you don't reach rated power, you will have more torque available than at the rated power. If you lower from rated RPM to 90% of it, your propeller power demand drops almost 40%, torque demand drops over 20% and your diesel can output a few % more torque and maybe 95% of the rated HP. Thus missing the rated RPM by 10% is a huge error in propeller selection.
Bollard pull or getting into plane can be a problem for some planing boats, not for displacement boats.