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I have this boat with a 1998 150hp Evinrude 2 stroke on a metal jack plate. It was purchased this way three years ago. I recently went from a four blade 16" prop to a 3 blade 17" Stiletto. Perfect prop but it caused this weird vibration/ hum harmonics from 3000 rpm to 4500 rpm in THe fine folks sent me 4 blade 17" and the problem mostly went away but now it is back. I wonder if it is caused by the jack plate, lower unit, something shaking in the hull or the prop. I can't find anything loose but it is hard to check while underway. Anyone have an answer? Thanks!