First, make sure that your engine will rev up to its maximum rated rpm + 200 or so, in NEUTRAL. If it won't do that, then you have throttle stop or other more serious problems.
Then take it out, warm it up and run it for a few minutes at wide open throttle in gear. It should be able to hit maximum rated rpm plus maybe another 100 rpm. If it hits max wot rpm + 200 then it is underpropped. If it hits less than maximum rated rpm then it is overpropped and you are a candidate for a prop repitching job.
The Mainship 390s were delivered with Cats, Yanmar 6LPs, Yanmar 6LYs, twin Yanmar 4LHs. All have different maximum rated hp rpms.