Another prop question

[k5guy38]

So I got our tritoon done and on the water. It's 1975 20' harris flote boat with a 1976 Johnson 85hp. Took it out for a run with the prop that was on it. 13.75x13. I don't have a tach but I can tell anything over 3/4 throttle is over revving the hell out of it now. There aren't many choices for props for my motor and I haven't found a good chart to use. Do you guys have any ideas of what I should go to.

[Bryden24shp]

Is it the prop or is the motor too high on the transom? Check the engine height first, buy a cheap ole tach or use an old dwell tach to find your RPM's, then we can go from there. Hate to waste your money by sending you in the wrong direction.
I have all the tools to help, but without the right info, I can't a whole lot...

[k5guy38]

Ya im sure the motor is deep enough. It's not a cavation issue. Im gonna order a tach. But i don't want to blow this thing up in the mean time lol.

[rancherlee]

That should be the correct prop for that motor, Just about every pontoon I know of with an old crossflow V4 runs a 13 pitch prop. There is no way a 2.0 gear case, 75hp (prop rated) should be over revving with a 13 pitch prop. If the prop is old the rubber hub maybe shot and spinning/slipping under load OR the prop is venting. Don't rule out venting, there is so much turbulence on a 2 log pontoon that air always seems to find the prop no matter how deep the motor is running. That was the case with my 20' when it was still a 2 log.

[k5guy38]

The prop is only half a year old with a new hub. The tritoon did raise the entire boat but I also compensated for that when I built the transom mount. If the prop was sucking in air would that make the water behind the motor be foamy or bubbley ? My old motor used to suck air or cavitate so I have a idea of what it feels like and sounds like. It gets up out of the hole awesome and then tops out at 21mph which I don't care about speed.

[rancherlee]

Mine has a strange "cavitation" since adding the 3rd toon. I'll loose about 3-4mph and the RPMs get close to redline if I trim out too much and I need to back WAY off the throttle for it to grab again. Not sure if you have trim or not but you could try moving the trim pin all the way in.

[k5guy38]

I will try to get pic's tonight. I have it so the front of the motor is tilted up a little so the lowest pin.