Strange prop issue on a Scarab 22. Please shed some light.
 

Hey guys, I have an issue that I'm trying to figure out. This is on a boat that I've owned for a few years but haven't used until this summer. Long story short it's a Scarab 22 with a 7.4, Bravo 1, and Mirage 25P 3 blade.
When I leave the dock the drive is trimmed all the way down. I have verified the drive is as far down as possible. At about 45 mph if I hit full throttle the prop almost seems to break loose. the rpm rises, and the boat does not speed up.
At first I thought it could be a spun prop hub or possibly the engine coupler, but I verified the prop by marking the prop and shaft and marks still line up after I experienced the problem again. I have since experienced the same issue after
doing a u-turn and going though my own wash at low speed, so I began thinking it could be the prop not getting "clean" water. I've noticed this boat creates a large hollow "bowl" of water behind it especially at low speeds and it almost seems as through the drive is not mounted low enough on the boat to process the water completely. Finally, yesterday my wife was driving it at approx 40 mph and while watching the water directly below the stern I told her to go to full throttle. Immediately the engine revved up like usual, but this time I could actually see what was happening. As the engine revved and it felt as though the prop was slipping, there was a fountain of water spraying upwards as if the drive was in trailer trim position. Let off slightly and it caught. Also, could running at WOT with the drive all the way down cause the bow to disrupt the water enough that the prop isnt seeing clean water. (I know I should trim up, I'm just waiting to fix sender unit for gauge.) Sorry for the lengthy post but I'm trying to be as detailed as possible. Thanks in advance for any light you guys can shed, and thanks for reading!

Raise your trim. You can learn to feel how the boat is acting. Start out at mid throttle, maybe 3000 rpm. With a boat your size and a prop with that much pitch you will be moving well enough to un-wet the front of the hull once you are a few mph above planing speed. You'll feel if the prop loses bite, just back off the stick and lower the drive if it happens. Lots of us have no trim indicators. Mine has been broken for a while. When i raise the drive I can feel the boat pick up speed and fly on the back of the hull. Each engine speed needs a different trim angle. And it's also affected by wind, chop, boat load. You can't just pick a trim angle, it requires adjustment on the go.
Too much trim and the boat will porpoise, too little and it plows. Put the nose down and plow at moderate speeds in bad chop or waves. Pick it up to "fly" the hull at higher speeds. I hope the slipping goes away once you add some trim.

Thanks for the response man! I will try what you said and report back. I was just confused thinking that the prop could blow out with the trim all the way down. (had it happen plenty of times trimmed to high so while troubleshooting the slippage kept it down all the way) Makes perfect sense to me now that you explained it. Almost like spinning tires trying to push someone out of the mud with all that hull plowing. I'll let you know how it goes, Thanks again bud!

Has this prop been labbed ? Many labbed props have slip issues. :(

Not sure about the prop. When I asked the previous owner if he was having blow out issues his response was "yes at times, you just need to play with the trim" and " the prop is a lil big for that boat" . It's a Mirage 25P, if I can't get it to quit blowing out using different trim settings, I will try a 23 and see what happens. Again, thanks for chiming in! I value any advice given as I like the boat alot and just want it to function correctly.

i dont see a reason to be going 40 fully trimmed down in a 22 boat. trim up to get that bow out of the water and let her rip. see how much rpm it will turn. and what gps speed your getting. then you can start to look at other props. the mirage plus is normally alot better on these little boats than the regular mirage anyhow.

Trimmed with the drive tucked under the boat is just killing the efficiency of the prop design to ingest and propel through the water

Thanks guys. Yes, I understand the virtue of correct trim angle for various water and load conditions. I generally drink beer while my wife drives and she is still building skill so thats why for now we have run trim down cause its one less thing to worry about, lol... I will take all of your advice and get er on plane and start trimming and see how it goes along with a GPS speed before looking at new props. Thanks again everyone, you have been very helpful.

Ok guys, checking back in after a day on the lake.....the boat had about 30 lbs of gear up front, two passengers and a little over half a tank of fuel.... you can slowly come up on plane from the dock, but if you try to give WOT the prop still breaks loose shooting water in the air. After getting on plane and trimming up till it smoothed out everything's fine up till about