Boat questions

[futura97]

Hye guys

Just wondering if any of you can help me out with a q I have about my boat...

I had a problem over easter with my boat. I was getting excessive cavitation and could not go faster than about 5-8knots without it free reving. I have a suspision it might be the prop, but as luck would have it, I left the spare prop at home while I was down in Mandurah :( . Luckily enough though I was on the esturay and not far from shore, so headed back over and put it on the trailer and came home.

With the spare prop I have (freebie) the size is different and want to make sure I'm not going to do excessive damage before I use it. The old one is 13 1/4 x 17, and the spare is 13 3/4 x 15. I'm wondering if the diameter and pitch would cancel each other out, and also if the diameter is ok being slightly larger. I noticed when I put it on, the tips of the blade are alot closer to the cavitation plate, and that is my only real concern.

The motor is an old johnson 90. I'm pretty sure its early 80's.

Cheers