I bought a 2013 Bennington 20' pontoon boat with a 2013 Yamaha 90 4stroke on the back from my inlaws. How do I know if the prop is the correct pitch and size. When running at 4500rpm it runs and pulls great but if I bump it up to 5000 rpm or more it acts like it starts to cavatate. The rpm's go up and speed drops. Is this normal. I was on a mercury 60 4stroke this weekend and it ran 5500 and pulled hard and seemed to run faster then my 90. Any thoughts?