Cavitation!? Is this why?

Sospikey

Trying to get upside down
Location
Sweden
So this past season I have had some cavitation. Mostly on re-entries or full power starts/turns. I put a pipe on my ski (RN) last winter.
I will upgrade my ski with dual cooling and removed the pump. The pump seal is looking a bit tired. Also, there's a pice of the pump intake duct missing. Is this why I cavitate? Opinions!?
 

Attachments

  • DSC00185.jpg
    DSC00185.jpg
    42.9 KB · Views: 191
  • DSC00186.jpg
    DSC00186.jpg
    65.4 KB · Views: 151
  • DSC00187.jpg
    DSC00187.jpg
    40.6 KB · Views: 124
yup...most likely the area @ 1 oclock on the pump seal in pic 2.....the chunk missing out of the shoe isnt helping, but I'd bet on that seal for the cavving you notice...


if you use a new seal, and silicone it real well....you probably wont have to worry about the shoe though...
 

Dustin Mustangs

uʍop ǝpıs dn
Location
Holland, MI
That looks just like what I found when I pulled my pump this fall (also for dual cooling among other things). Mine were a bit worse (both the seal and the piece missing out of the shoe), but it also sounds like I was dealing with a lot more cavitation. Santa brought me replacements for both!

:bananajump:

if you use a new seal, and silicone it real well....

Does the manual call for silicone on that seal? Mine definitely didn't have any on there when I removed it. Just the ride plate and the grate did.
 

Dustin Mustangs

uʍop ǝpıs dn
Location
Holland, MI
Well, I was gonna get one of the alloy ones until I searched around on the forums a bit. It seems nobody sells one that fits correctly without modifications. In fact, most people seem to be disappointed with the part when they receive it. Then you ad in the fact that I am putting this on a blaster and all the alloy ones are designed for the sj. People 'say' they fit both (and some even say they use the exact same shoe) but the fact of the matter is the factory has different part numbers for pump shoes on the sj vs the blaster. I think I will trust the original engineers that these parts need to be unique. Smaller parts bin equals bigger profit so I think it's pretty safe to say these parts are different for a good reason. Really, the last thing I wanted to do was spend a bunch of money (and time) putting on a part that doesn't fit right just to end up right back where I was before, cavitation central.

Also, pretty much the biggest selling point of the alloy shoes is that once you start making big hp the stockers won't last. I bought this for a while until I looked into it more. Fact of the matter is, a number of aftermarket hull builders use the stock pump shoe on their boats. I think it's pretty safe to say these boats are built around the idea of BIG power. When confronted about this, I saw one manufacturer claim the stockers will stand up to a ton of hp and will only likely break if you hit something. Given I found a broken intake grate when I found my broken pump shoe, I have no doubt this is what happened with my rig.

Big can of worms later, I picked up a stocker from parker for like $55.

:starwars:
 

sjetrider

615 Freeriders are addicted to T1 madness.
I recomend OEM lighter and easy instal. Did you upgrade your prop when you added the pippe? What condition is it in? This could cause cavatation as well.
 

Sospikey

Trying to get upside down
Location
Sweden
I recomend OEM lighter and easy instal. Did you upgrade your prop when you added the pippe? What condition is it in? This could cause cavatation as well.

NO, the impeller was changed earlier, it is in mint condition. No excess play between ring and impeller. In one of the pics you can actaully see the heat that my leakage has induced. The back side of the intake grate is all black! Thinking back I recall touching the intake grate after riding and it was really warm. This is why!
 

just joe

Site Supporter
Location
NorCal
Please post results regarding cavitation when you put a new seal on. Looking at the shoe surface area for the seal, and the seal width, I am suprised that the 'fold' in the seal is causing that much of a problem. You also mention heat, are you certain the prop is not rubbing in the pump, when in the ski (caused by midshaft misalignment)? Check pump bearings?

Heat from cavitation alone seems unlikely, or pumps spinning in free air would heat up, and they don't, theoretically.
 

Sospikey

Trying to get upside down
Location
Sweden
Please post results regarding cavitation when you put a new seal on. Looking at the shoe surface area for the seal, and the seal width, I am suprised that the 'fold' in the seal is causing that much of a problem. You also mention heat, are you certain the prop is not rubbing in the pump, when in the ski (caused by midshaft misalignment)? Check pump bearings?

Heat from cavitation alone seems unlikely, or pumps spinning in free air would heat up, and they don't, theoretically.

My theory is that a small amount of high pressurized water escaped between pump seal and pump shoe. High pressure + small orfice = heat. This would be at 6 o'clock position, where the piece was missing from the shoe. The water passed just by the intake grate. Hence the heat build-up.
I will check bearings/alignment as a pre-caution, but I think I found the main issue already.
 

Sospikey

Trying to get upside down
Location
Sweden
you could just fill that with a 2 part epoxy putty.

Actually, I still had that piece. I Superglued it back. I also JB Welded on the plastic, but wouldn't stick! I did fill in the cavity behind the broken piece with some JB. Then I stared thinking; a new OEM one is about $60....the pump is out anyway... I hate Sh*t that doesn't work properly... :rocketwhore:
I'll just order a new one! :cool:
 

just joe

Site Supporter
Location
NorCal
My theory is that a small amount of high pressurized water escaped between pump seal and pump shoe. High pressure + small orfice = heat. This would be at 6 o'clock position, where the piece was missing from the shoe. The water passed just by the intake grate. Hence the heat build-up.
I will check bearings/alignment as a pre-caution, but I think I found the main issue already.

I don't think that's physically possible.

Water through a nozzle absorbs heat. In a pressure washer, the nozzle will always be colder than the water. It's an endothermic phase change.

I think your heat is from something else. And that something else might be contributing to your cavitation, hard to tell.

You only need a 3% drop in pressure to begin cavitation, which is why it's important to ensure the pump is sealed well.
 

Sospikey

Trying to get upside down
Location
Sweden
I don't think that's physically possible.

Water through a nozzle absorbs heat. In a pressure washer, the nozzle will always be colder than the water. It's an endothermic phase change.

I think your heat is from something else. And that something else might be contributing to your cavitation, hard to tell.

You only need a 3% drop in pressure to begin cavitation, which is why it's important to ensure the pump is sealed well.

Well, the grate attaches to the hull and the pump shoe only. These are glass/plastic, and shouldn't transfer heat that good at all. I can't recall that the shaft was warm or my ride plate either for that matter. And the end of the grate has the "scorched" black look, it is visible in one of the pics. Again, it was a theory, I am open to ideas?!
 

just joe

Site Supporter
Location
NorCal
Well, the grate attaches to the hull and the pump shoe only. These are glass/plastic, and shouldn't transfer heat that good at all. I can't recall that the shaft was warm or my ride plate either for that matter. And the end of the grate has the "scorched" black look, it is visible in one of the pics. Again, it was a theory, I am open to ideas?!


This does baffle me. I've experienced very warm driveshafts from pump misalignment, also warm pumps from impeller rubbing, but there isn't a good conduit for heat to get to the intake grate, as you stated. This i why I am interested in your results with new seal, assuming you put the pump shims back to the same locations and no other changes.

The discoloration can be from warm water (off the dirveshaft) accelerating oxidation.
 

cookerq62

Life's Been Good
Location
Upper Bucks, PA
I chased cavitation probs doing stuff like this. It kept getting worse and worse till the ski was unrideable. I would mess with little stuff like cooling leaks and the pump packing. It would work fine after any change then go back to zero thust after about 100 feet. Turns out it was the mid shaft carrier it was pretty much gone. When I filled the boat with water the missing bearing could keep up with a garden hose. Huge PITA to change. If your boat cavitates the first thing I would recomend doing is filling it with water to see if the mid shaft carrier leaks.
 

gj_fx1

X-H2
in case its not obvious the shoe needs to seal to the hull. The OEM glue was crap.
If in doubt pull it off clean it up and re-seal. (I used 5200). That is assuming the damage insn't something that prevents a seal either to hull or to pump.
 

Kaveman

Born in USA(not Kenya)
My theory is that a small amount of high pressurized water escaped between pump seal and pump shoe. High pressure + small orfice = heat. This would be at 6 o'clock position, where the piece was missing from the shoe. The water passed just by the intake grate. Hence the heat build-up.
I will check bearings/alignment as a pre-caution, but I think I found the main issue already.

The pressure in front of the impeller will be a negative pressure. Thats why if you get a leak it will "suck" in air which will compress unlike fluids.
 
Last edited:

Sospikey

Trying to get upside down
Location
Sweden
This does baffle me. I've experienced very warm driveshafts from pump misalignment, also warm pumps from impeller rubbing, but there isn't a good conduit for heat to get to the intake grate, as you stated. This i why I am interested in your results with new seal, assuming you put the pump shims back to the same locations and no other changes.

The discoloration can be from warm water (off the dirveshaft) accelerating oxidation.

So I put an alloy pump shoe (which was a total b***h to install) in along with a new seal. That did the trick, no more cavitation!:arms:
 
Top Bottom