25' pontoon prop help

[99v92c]

Bought a 1980 maurell 25' pontoon with a 79 evinrude 85hp with 13 1/4x17 prop. With the engine lowered to the lowest hole it seems like the prop either cavitates or ventilates. When the boat starts picking up speed it sounds like the prop comes out of the water and revs really high. When I lower the throttle it settles down. I was told that it has too much pitch for the boat. Any help would be appreciated

 

[1960 Starflite]

It is too much pitch for that combo, but.
I bought a similar setup. 22ft Toon - 85hp Merc. 13 x 23p pitch. No cavitation or slipping. The engine would only turn 3500 rpm. I figured it out, finding that the previous owner had mounted a transducer with a large mounting bracket to the bottom of the engine pod. This parted the water stream in front of the prop. Thus the owner had to put an aggressive prop on to get a bite.
Long story but saying "anything in front of the prop"?

 

[99v92c]

No the only thing in front of the prop I've noticed is the bottom of the engine pod seems to divert the water quite a bit. The faster I go the more it diverts the water and that's when it seems to cavitate or ventilate. I can only get about half throttle before it over revs and loses bite

 

[1960 Starflite]

I don't know anything about Evinrude engines, could you have a "short" shaft and needing the "long" shaft version?
Hopefully someone else will chime in. I'd post a picture of mine but the 85 Merc is off.

 

[99v92c]

Omc short shafts are 15" if I remember right. Mine is the 20" shaft