Problem - Sudden high RPMs with no speed increase

[FlaviusGlok]

Hey, I'm new and this is my first post. And yeah I'm looking for help / advice.

I have a 2019 Suntracker sport/fish 22DLX with a 115hp Mercury Pro XS.

Basically if I gun the boat at near any speed or trim the revs shoot up fast and the boat doesn't go faster. I can ease up to full speed RPMs if I ease it up slowly. But anything more than "ease" just spins out to like 6000 rpms real quick. My current WOT is 5850 rpm.

I have done a ton of research on this as well as troubleshooting.

I Marked the shaft and the back of the prop with a punch to check for a spun hub. All looked well. I checked this like 4 times just to make sure I wasn't just being unlucky in that the spin just turned back to the starting point. Even so I replaced the hub with a full hub kit and the problem is still there. The old hub looked fine BTW Basically I needed to start ruling things out. BTW the prop is an aluminum Black Max 16x13P

I understand you can suck air if the motor is mounted too high. But it did not do this for the first 4 years so I can't see that to be the problem. Unless it is related to some sort of prop damage perhaps. More on that later.

I have looked under the boat for any obstructions or really for ANYTHING that has changed. I see nothing different that could cause a water flow obstruction / problem. I have not added or removed anything either. I have a lif so I was able to get underneath both pontoons and really get a look at everything. I tapped on the toons to attempt to check for water in them. I could not detect any but that is probably not a great test.

The prop has a few small dings on it that I find hard to believe are causal to this problem. I did some filing on it to smooth things out but that did not help. But this is still an open issue to me. I'll attach a pic. I had to get the pic from the back of my boat. I should have taken on when I had the prop on my workbench.

IMG_2868.jpeg (1011.57 KiB) Viewed 3890 times

The last year or so I have been running it on the low side in regard to gas. Today I added 15 gallons of gas in order to get the level (Stern weight) back to where I was running it. My thinking was that perhaps I am running higher in the water due to less weight. I have not tested that yet because it looked like one of those afternoon storms was headed here. But I have had a lot of people in the boat as well as just myself with no apparent change to the problem. So this is just an attempt to fix anything that has changed.

I have even wondered if the very warm waters we have here could be an issue (~90F). More temp would make cavitation more likely. But even then I don't think that cavitation alone can cause the sudden RMP increase. But I'm still in a learning curve so I dunno. I am familiar with cavitation damage and what it looks like. I worked for Shell and have seen many pump impellers with cavitation damage. The prop shows none of this.

Perhaps there is some exhaust leakage? But I'm not sure how to check that.

[FlaviusGlok]

Uggh. I should have looked at my motor manual closer. Especially with the prop off. I see there is a Bore sleeve that could be a problem.