Cavitation issues (mechanical grinding/vibrating)

[Waternut]

I've been having some cavitation issues ever since I got my new stunt hull and it doesn't feel like any cavitation that I've ever felt before. First off, it sounds like I sucked up a stick or a log when it happens like a grinding sound instead of the engine rpms shooting up and not going anywhere. It's actually hard to pin point when it happens but most of the time it's when the rpms fall almost all the back to idle. No amount of throttle will clear it up but if I back off and let the engine get near idle again, it'll go away.

The pump shoe is in there good and has no gaps. I guess it's possible the pump seal got pinched when I installed it but that seems unlikely. A friend was saying that perhaps the prop is spinning now because of the lightweight hull. Nothing has changed from my last hull except the hull, couplers, and the pump shoe.

 

[blaster800]

Quote : ''''''a grinding sound instead of the engine rpms shooting up and not going anywhere.'''' Check your coupler condition and alignment first , Motor mounts ''broke ,loose''' then I would pull pump look at the shaft splines , Those would be my starting points . cavitation does not GRIND that I know of !

 

[steve-uk]

how are your bearings in the pump?

if the bearings are shot, or you dont have enough grease in there you could be litterally grinding metal

 

[NVJAY775]

how are your bearings in the pump?

if the bearings are shot, or you dont have enough grease in there you could be litterally grinding metal

x2

 

[Waternut]

Well I'm not opposed to taking another look if you think something could've happened but the engine mounts, coupler, and driveshaft all looked fine before I put them in the hull about a week or two ago (this was going on during the first ride after assembly).

I greased the midshaft but I did not regrease the pump. The pump felt smooth before I installed it but I'll certainly pull the pump cone and check it.

 

[NVJAY775]

Kind of a bummer. I'm really looking forward to one of you guys saying great things about the boat. Sorry about the pump... Stay at it.

 

[steve-uk]

Well I'm not opposed to taking another look if you think something could've happened but the engine mounts, coupler, and driveshaft all looked fine before I put them in the hull about a week or two ago (this was going on during the first ride after assembly).

I greased the midshaft but I did not regrease the pump. The pump felt smooth before I installed it but I'll certainly pull the pump cone and check it.

only problem is you cant see the other bearing, you have to take off the impeller and remove the two oil seal (basically destroy them) to see it, at least you can see the one, pack in aload of grease too just to be sure

 

[Waternut]

Well here is what I found. Doesn't look too bad. The grease might be getting a little thin but it doesn't smell burned or anything. I'll load it up with grease and fill the cap but not sure if there is anything else that can be done without rebuilding the pump which may or may not be necessary.

 

[steve-uk]

the bearing looks good

how does it feel? is it smooth?

rotate it and what the balls see how they move

difficult to say on a photo but the bearing does look ok

if you pull the prop you can see if your getting any leakage from the seals.

but seems like your problem could be elsewhere

when you assembled the pump did you silicone each section together and silicone the plate/grate? and silicone behind the midshaft assembly when you mounted it to the hull? just trying to think of some areas where air could get in

 

[Waternut]

I originally rode without any silicone on the rideplate or intake grate. Then I filled those in hoping for better luck but nothing. I did do it but I may not have done the best job sealing the midshaft to the hull. The veins and wear rings are sealed together but not the reduction nozzle.

I'll see if I can find my prop tool to remove the prop. I used to know where it was until I moved and now I'm not sure.

 

[steve-uk]

are you 100% its cavitation and not just the "sound" of you new ski.. could be that too

one of the most common causes of cavitation in the pump seal being old and not sealing

 

[orangefinger]

did you check your splines...its ounds like they might be getting smooth....

 

[Waternut]

Splines are good. It's not the new ski because it only does it occasionally. I think I found the problem. I don't remember the intake grate being worn before...

Here are the videos I made and at 1:30-1:35 you can hear that grinding noise. The second video there is a good (bad) one at about 7:40-7:50. Now after I've watched the videos again and listening for the grinding noise, it sounds like I grind a little after every jump. This in addition to the marks on the intake grate make me believe that the force of the water or vibration of the engine is causing the driveshaft to distort just enough to contact the intake grate.

 

[orangefinger]

second video your steering is off...right ..??

missed that pic...looks mcdog is right post below

Obviously its after you jump

 

[McDog]

Your driveshaft is definitely hitting the intake. Grind it down or switch it out and see if it goes away. From the pic you posted, I'm betting that is the problem.

 

[Waternut]

I put a small shim in between the pump shoe and intake grate to help space it out and I blended out the gouge in the intake grate so it was a smooth transition. After I cut the rideplate, it looks like end of the steering nozzle was contacting the rideplate so I trimmed that as well. Maybe between those two things, this issue will go away.

 

[ncbrock]

let us know how it goes.

 

[Waternut]

Well the issue certainly didn't go away. It might actually be worse now... I'll try looking at other stuff a little later.

What are the symptoms of a midshaft going out and what would it sound like? Mine was smooth when I put it in but it did have a little play in the shaft itself.

 

[kevbo]

You can tell if its cavitation by hopping it out of the water while its vibrating. If it does not smooth out in the air then your problem is mechanical.

 

[Waternut]

Yeah it's definitely not cavitation because I noticed that it was doing it on the trailer today.