Cavitation or some issue with prop

[TL7]

Yesterday when pulling a rider out of the water, the boat would take off as normal and then it would momentarily "rev up" and feel like the shaft is spinning but the prop wasn't. This would last a second or so then return to normal. It did it a few times, and we came home. Today, we had a group with us and it started doing it again, only worse. I went to the dock, pulled the boat and changed the prop. Different prop, same result. Called the dealer, and he said it sounded like cavitation. Sounds reasonable, but with two different props?

Background - it's a 2007 Outback. Memorial Day weekend I dinged our prop. Stock prop is an OJ 13x13, we took it and had it refurbed. I also bought an OJ 13x12 from Skidim, for better holeshot. This is the prop that I had on yesterday that first started "slipping". So today I figured maybe it's just not the right prop and changed it out with the original 13x13, but it did the same thing. Once I told the dealer that it was doing it with two separate props, he said we may need to look at the rudder or shaft. I don't mind taking it in, but I wanted to check with you guys to see if it was something I was overlooking. I've never changed a prop in my life, so part of me is second guessing if I even put the props on correctly. If they aren't "positioned" correctly, can this cause what I'm experiencing? I basically put the key in, slapped the prop on and tightened it until I couldn't tighten anymore.

I need to note that this is the 4th time I've taken the boat out with the new prop, and yesterday was the first time it acted up. The first two times it ran flawless, even weighted down with quite a bit of weight. Since it first did it yesterday though, it has gotten progressively worse until we took it out.

Any ideas are welcome. I plan on taking in to the dealer on Monday just to give it the once over unless something is figured out by you guys. Thanks in advance!

[TL7]

Follow up - a few people on wakeworld mentioned to check the tranny fluid. The tech at the dealer mentioned this as well, but I didn't remember to check when I got home. Apparently if it's low this causes it to "slip gears" so to speak. Turns out it very well may be the tranny. Checked the fluid and it's not even registering on the dipstick. Doesn't smell burnt though, so I guess if I add back to full and monitor it it will be ok. Is it normal to use transmission fluid or should I be concerned with a leak? I didn't see any fluid in the bilge area. I have to be honest the tranny fluid isn't something I've paid much attention to since we bought the boat. Guess I need to add that to the list now.

[gcnettl]

Hopefully you can find the leak. Is this boat a V-drive?

[TL7]

It's a direct drive. I will give it a thorough inspection tomorrow.

[TL7]

Another recommendation from wakeworld - "Check the coupler to drive shaft connection
if the nut loosens the key may work loose."

Can someone point me in the right direction on this one?

[maxpower220]

First, I think it would be impossible to put a prop on wrong. The tapered shaft eliminates that as long as you install the shaft.

You symtoms sound like transmission. Check the level. If you have not checked it before, it could have been low for a long time.

Behind the transmission, the output shaft connects to the prop shaft. There are (usually) 4 bolts that connect the shaft.

[Fastest1]

It is the fluid level. When low they act just like a prop that is cavitating. Except it will get more and more frequent with less throttle (and less fluid).

[Woody929]

Wow, holy thread revival batman

[Fastest1]

Nothing like being left holding, huh?
I didnt even look at the date but I did experience plenty of slipping yesterday.

[mmandley]

Yea old thread but still a good one to remind people about. People tend to forget about the V drive and trans unlike the engine. Same in cars.

I have had little bits of the cavitation since new. Normally only when weighted, happens a second or so here and there.

If you can plan on it when it happens that means something is not right. Check fluids, as other mentioned, change the fluid. True Cavitation should only happen under load, its a boats equivalent to spinning tires.

Engine power exceeds the props ability to pull water past it, creating air bubbles in the water and essentially prop slippage.
This is generally when you need to run less weight, or get a bigger prop.

Continual slippage or when the boat is not weighted tend to lead to trans, V drive or other hardware issues.