Air leak cavitation theory.

[Phildoo]

Since we have all had cavitation issues before, I thought I would bring this up for discussion on determining cavitation problems, wear ring vs air leak.
I have been in the pressure cleaning business for over a decade now. Its pretty much the same general dynamics on the jet drive system we use. You take a water source, compress it thru a smaller orifice to create a higher pressure on the other end, ie..jet drive.
Now, if anyone here is somewhat familiar with a pressure washer system, then you know how to downstream inject chemicals used in the cleaning process. So, this is where my wisdom comes into play. When you inject the chemical you are using(chlorine,degreaser, etc) and you run out of that injection liquid, you now introduce air into the pressurized system. Since ive been doing this so long I know the exact second I have run out of chemical, my employees dont have a clue they did.
The pressure changes slightly and the spray has air in with it. It literally changes the disturbance of the water flow.
We were out one day and docked at a pier in front of a bar, while I was waiting on my friends to show up and board I went ahead and warmed up the engines while tethered there,I put the boat in forward to keep the lines tight. As I waited, other friends hung out there so I stepped onto the swim platform and chatted.I look down and see the jet wash is severely aerated on the passenger engine, the water is completely frothing from aeration. The other was crisp and clear with a nice swirl.
I looked at it pondering and dismissed it as exhaust getting mixed in. But this has been bugging me forever for this one reason.
I know for a fact, that that engine with the frothing wash has a bad carbon seal, its been leaking forever(its on the list).
So my theory is..if you have an air leak, you will visually see air bubbles in the jet wash from your boat. Even after that I have looked at the wash at idle and still only see air bubbles on that engine while going thru no wake zones.
I will take pics of this and post, then repair and take new pics.
It would be so nice to simply look at this and determine if you have an air leak.
Thoughts?

 

[Dr Honda]

I think you will almost always see bubbles in the jet. There are 2 reasons that come to mind.

1) The exhaust is very close to the jet nozzle, (on the skis) and that will mix in.
2) the bailing hoses will suck air if there isn't any water in the hull. BUT, that's not causing cavitation, since it's after the pump.

When it comes down to it... if you have a bad wear ring... the pressure in the pump is lower, and you don't go as fast, and don't get "Out of the hole" as fast. If... the engine reves, and you don't go... that's an air leak.

 

[bigJake]

i see bubbles in the bathtub, my a$$ must be cavitating ?

 

[Phildoo]

Yes. Replace your O- ring.