2006 8hp 4stroke Nissan won't hook up

I have a short shaft tiller on my 13' gheenoe that won't push this boat. I've tried every trim position on the motor. It won't open up. Every time I get up to speed the motor rev's and the prop loses bite. I had a 5hp that worked fine just needed a little more speed. I've been told it could be the hub or it could be the shaft is too short for the hull? Need some insight before I throw more money at it.

 

I can't get the motor any lower without a jack plate.  I think it standard 8 pitch.  I believe it's aluminum.  It has 8 stamped on it. And yes it will move, but when wot it blows out.

 

Well, I solved my own problem. Thanks for the insight, though. It always helps if you have the correct prop on the motor. Thanks craigslist. Here's a before and after. I decided to go buy another prop, one that was designed for a Nissan, and not a Yamaha. I didn't inspect the motor close enough when I bought it. There was a gap between the prop and the lower unit itself with the old prop. Then there's a pic of the new prop that's designed for it. No gap. I'm running it tomorrow to see how it works out.

Before


After

 

Motor works great now that the prop has been changed. As far as that prop is concerned it does have 12 splines. I only thought it was a Yamaha prop because of the guy I bought it from. He worked at a Yamaha dealer.