Generally speaking your engine(s) should be able to reach 4000rpm to 4500rpm at wide open throttle (wot). Even though most boaters do not run their boats at full throttle most of the time, the engine(s) should be able to reach this rpm range at wot to insure that the propellers are not overloading the engine(s), or that there is not some other problem.
If your engine(s) does not reach 4000rpm-4500rpm:
Check the throttle linkages to make sure that the throttle is opening completely.
Make sure that the tachometers are calibrated correctly.
Make sure the fuel filters are not clogged.
Check to see if the flame arrestor is dirty or clogged.
If the engine(s) are operating properly then the propeller(s) may be overloading the engine(s).
To increase rpm, propeller pitch must be decreased.
When operating in a following or quartering sea, bring the trim tabs all the way up to prevent bow steering.