prop question for a pontoon boat

[curtis45]

I have recently purchased an old playcraft 24 ft pontoon boat with a evinrude 115hp motor. ( Model 1155393M).

My question is: What is the max rpm for this old motor? I am getting 4,500 rpm's at wide open throttle. I am used to seeing 5,500 rpm's at wide open.
I am running around 24 mph.I have played with adjusting the tilt of the motor up or down via the pin and holes in the motor mount.
Should I drop a prop size? Not sure what size is on it now.
Thank you.
Curtis

 

[Doh]

That is a Pontoon Boat's nightmare. Finding a small enough pitch prop. Rule of thumb, you need 5"s less pitch than what you have now.