If the engine is lugging and can't develop proper engine rpm
due to the load... you need a smaller prop. (pitch and/or diameter)
(make sure its not a problem with the engine)
If the prop is whipping froth and you can't
go anywhere near the speed the prop is TRYING to push the boat, you need a bigger diameter prop. Maybe more pitch
, maybe less. (I'd start with less)
We ASSUME the prop you have is reasonably close (or it wouldn't be the one that got installed)... Maybe one step of pitch
and/or diameter will fix it.
If its not even close... you need someone with as close to the same hull
and engine (+ transmission) as possible to say what works for them.