Oooooops, He has an Owens 27 .... a stinkpot.

Vibrations in high speed props can arise from various causes: bent
prop, prop out of balance, shaft-whip due to engine misalignment, bent
propshaft, missing or severely worn cutless bearing, etc. etc.
A flat-plate rudder on a powerboat 'can' develop vibrations
(excitation of natural frequency) but that is extremely rare on a
powerboat so check the above first, starting with the engine
alignment.