Merc 115 optimax races (as in neutral) at high RPM's

[erndog105]

New to me 2008 20' tracker with 115 optimax. Dealer did a good job checking it out and making it ready for me. At 2000 - 2500 rpm engine runs ok. When it is pushed up to 3000 + engine acts as if it is in neutral and races up to 5000 +. This is new. Not sure if it is engine trim (which I really do not fully understand) or something serious. Any suggestions to check out?
Thanks

[r&s20]

sounds like in prop may be slipping. this is easily checked by taking prop nut off and scribe line on face of hub and on prop itself (match marking) replace nut. run motor (under load) then pull prop not off if marks don't match then hub is slipping