Evidence of Cavitation?

[MNdrifter]

While I had my pumps out, I noticed that each blade of both impellers showed wear in 2 places.
Is this from cavitation? Is this happening all the time or can the brief periods when I notice cavitation be enough to cause this?

Cavitation

• MNdrifter
• Oct 5, 2015
• 1

 

[Scottintexas]

I'm no expert but it certainly could be cavitation causing this especially with it showing on the inner blade also.

How often do you feel you experience cavitation and what type, just in a certain RPM range, do you hear a noise or vibration or just a loss of thrust?

 

[MNdrifter]

If there is no debris in the pumps, I never notice any cavitation. I always clear it as soon as I can. It doesn't seem like short, occasional incidents should cause a visible wear pattern. The fact that it is at the hub and outside of each blade makes it hard to picture another cause.

 

[Scottintexas]

I agree that it's probably not you normal weeds in the pump cavitation, is the worn area smooth and just discolored or dimpled ?

 

[MNdrifter]

There is a slight texture you can feel. I would say that the rest of the impeller is discolored and the affected area is showing raw metal. I could understand a worn wear ring causing the outer cavitation. The wear near the hub is confusing.

 

[Scottintexas]

the fact that both impellers show the same wear confuses me, if you had an air intrusion or even weed problem that maybe was causing some cavitation but not enough for you to notice a performance issue that both would sides would not look the same seem slight.

I think I would take it back to the dealer, address why they gave you two starboard impellers with your brand new boat, get them repitched/reconditioned/solas, and then monitor them closely throughout next season to see what happens.

 

[RightStuff]

It looks like a rock or stick or something was stuck in there for a bit and rubbed against the impeller for a period of time. It's very odd that both have the same wear pattern, makes me think it's not something getting stuck in the intake. Unless you sucked up something like a rope and it went into both intakes?

It just looks like something was stationary and the blades were spinning against it for a bit since the wear is pretty consistent between the blades. Probably doesn't take long at thousands of revolutions per min.

 

[Gym]

@MNdrifter, looks to me like sand blasting. If you operate, even for a short time in the shallow water you can suck sand which will cause wear like that. Especially if you back up to a beach or sand bar.

Cavitation doesn't necessarily damage the impeller blades but damaged impeller blades does cause cavitation. Don't forget our intakes draws water from 3 feet down at idle which is much deeper than our draft. As long as the edges of your impellers are undamaged you should be ok. @Scottintexas brings up a good point about 2 right side impellers though.

 

[Ronnie]

I had an impeller from a Mercury powered Sewell jet boat that had similar wear on the hub. It was pitted and raw compared to the rest of the impeller. I sent a pic to Impros and they told me it was cavitational deterioration. That is, the wear was caused by cavitation not the other way around.

Cavitation can be caused by the smallest obstruction disrupting the flow of water into the pump and past the impeller. Maybe it is just one small air bubble developing before the pump but if it happens once every revolution its happening thousands of times a minute and cummilatively it's like hitting the same spot with a sand / air blaster. I replaced the impeller and the wear stopped so Impros opined that a blade or several were chipped or bent just enough to cause the cavitation which resulted in the weed to the hub.

 

[Gym]

@Ronnie, the type of wear in the picture, was caused by physical impact to the impeller not air bubbles. I agree cavitation is caused by the loss of laminar flow over the impeller causing loss of impeller efficiency resulting in perceptible vibration. The pattern of wear suggests, to me, sand abrasion rather than stone, sticks or lake growth. I would be interested to hear from the OP if this hypothesis is plausible based on his boating habits.

 

[Murf'n'surf]

My opinion.... This is indeed effects from cavitation. My guess is that you either slam the throttles down from a standstill quite a bit or run wot a lot. During either of these normal operations there is slight cavitation occurring. The outer blade burns are probably caused from some grooves/gouges in the housing. The inner burns are on the trailing edge surface of the impeller which is common. One question I have is why do your impellers look like such a different color than mine or anyone else's? Are they coated with something?

Remember that cavitation is the impeller beating the water into air and that air causes deterioration of metal, fiberglass or any surrounding material. When air is introduced into the impeller, cavitation occurs but that's not what I see here; imperfections in the impeller and or housing are causing the cavitation.

Bottom line.... Nothing really to worry about at this point. Check your housings for imperfections and determine if you need new ones. If you start to see a lot of metal pitting in these burns you should take action.

I assume your hole shot and top speed are normal?

One way to test if it's from the occasional clog verses normal running is to spray paint the impeller and see what it looks like after a normal clog free day.

 

[MNdrifter]

I'm a very conservative driver. Unless I'm towing a skier, I don't slam the throttles open. I usually run at 25mph. However the last outing of the season I was with another boat and I ran WOT for over 1/2 hour.
There is wear on the housing.
I wet slip my boat in the Mississippi river. The color is a season's worth of scum adhered to the blades. I clean the bottom once a month but don't get inside to clean the impeller. Maybe the marks are so noticeable because the cavitation basically cleaned the scum off.
I haven't noticed any difference in performance. Although my top speed has never been what it should be due to having 2 stbd impellers. But that is covered in another thread.

 

[Murf'n'surf]

I would think there is no issue unless the metal starts to pit. Looking at the picture, I only see the scum being burned off. Looks worse than it is.

 

[waterboy]

The wear on the impeller shown is due to cavitation. In it's strictest definition with pumps, dissolved air in the water comes out of solution in the low pressure parts of the impeller. The effect is basically thousands of mini "explosions" from the air bubbles in these low pressure zones of the pump causing the effect as seen in his pictures and in the exact areas of the blades. We see this all the time in industrial water pumping applications when we have cavitation on high pressure pumps.

As everyone has noted, the loss of smooth flow into the pump due to loss of sealant, weeds, debris or lots of other things will cause a disruption in the smooth flow or loading of the pump. I have that issue right now on my right pump with, I am assuming, loss of sealant in my right pump and will be resealing this winter.

Hope that helps to get to the bottom of what you are dealing with...

 

[Ronnie]

Uh, what he said (lol). Seriously, @waterboy , thanks for the thorough response. Any thoughts on what the OP or potentially any of us should do to identify the cause of and solution such cavitation? I haven't found a lot of info. or pics on this site or YJB on what a properly sealed pump looks like.

 

[waterboy]

@Ronnie thanks. Not a thorough response but cavitation as I understand and have seen in the industrial world do to pumps. As far as our boats, I have had cavitation pop up a time or two on mine. Let me define what I have seen and currently am dealing with and how I plan on fixing it... When I floor both engines wide open. My right engine tach hits 8500 and bounces due to the rev limiter. The left engine hits about 8000 rpm and holds until speed comes up. My right engine is slipping or cavitating. If I back off the gas slightly, I can hold revs at 8000 until I come up to speed. I would define this as slight cavitation, but it is happening. It is pretty easy for all of us to define when you have an engine full of salad, that's bad...

The first thing I look for is loose sealant. My OEM sealant looked like silicone. Silicone is a good gasket material but does not have any adhesion properties. We want to have minimal but some adhesion to what ever material we use to seal the pumps with, because you will likely want to remove that thing some day and a super adhesive will prevent easy removal. Anyway, if there is any loose sealant, it can cause those low pressure zones through the turbulent flow and cause cavitation.

Now when I say loose sealant, I have seen it where the silicone looks fine visually but if you push on it, you can tell it is not adhered to anything as it is loose on the edges. Last time, I dug out this sealant (note that I did not remove the pump) carefully and had a trough or gap to fill. I just did a smoothed over top coat that filled in the gap with new sealant. Problem went away and no cavitation.

Now the argument of what sealant to use. Again another topic where every %$#@! has an opinion of the right thing to use. I researched a bit and centered in on a vulcanized rubber gasket material. I used Permatex The Right Stuff gasket maker. Available at a fine auto parts store near you. I did basically a caulking job and smoothed it over flat with my finger and let dry.

The same engine I repaired this way 3 years ago is giving me issues this fall. I will see if it is a new spot that failed or my old repair. Either way, if I get 3 years out of this type of repair (and don;t have to pull my pumps...) I'm good!!!

Sorry I don;t have pics. I hope my long winded explanation of how I fixed it helps. If I am thinking, I will try to get pics this fall. I know, I know... pics or it didn;t happen Good luck!!!

 

[Ronnie]

@waterboy , thanks for taking the time and putting forth the effort to respond.

@MNdrifter , Sorry about the high jack but I think you should try and identify and fix what is causing the cavitation and not just replace the incorrect impeller and housings or you will likely end up in a similar position down the road.

I successfully resealed one of my cavitating pumps but since there wasn't a lot of information to go on it took me three tries with three different sealents to get it right. I knew from working on jet skis that if you do it wrong you can make the cavitation much much worse. The shop manual is useless by the way, at least it was for me, since it only covered the basics but my boat had sealant from the factory in places not mentioned in the manual and when it failed I experienced the same cavitation that waterboy described. I know you are trying to address a top speed problem but acceleration problems are just as bad if not worse and can prevent you from pulling up a skier or border as well as hurt your get away/get out of the way performance.

 

[waterboy]

@Ronnie Looks like you have been down the road with sealants too. What did you find worked best through your trials with different sealants? I assume too that you pulled the pump shoe and resealed the whole thing too??

@MNdrifter yeah certainly not intended as a high jack but trying to help out to understand the root cause for you. Different implellers are one issue and I guess could cause cavitation. As you have everything apart, inspect the sealant and repair as needed. Once back together, keep an eye on performance to see if the issue goes away. I know you said you are a conservative driver and I would say the same thing for myself, but you do need to slam the throttles to see how things are performing and see if the cavitation goes away or if it is even going on. I know when my boat is doing it, it makes it tough for my big ole' butt to get up skiis as she spools up to speed...

 

[MNdrifter]

All the sealant in my pump is only attached on one side. Just running my hand across it will lift one side.
Boat is going into storage today. Next spring I will be ready with new impeller housings and Permatex to tackle the cavitation.

 

[Ronnie]

I used clear silicone rtv on my first attempt which worked fine but failed a month later. Areas where the silicone fell off or was just hanging loose silicone was easy but some required a little pulling to discover. Next I tried 5200 but either didn't put enough on or put it on wrong. The third time was a charm with 4200 which had lasted 3 or 4 seasons now. I basically revealed and smoothed out all of the joints where the pump shoe and wear ring/impeller housing meets the fiberglass in the intake area. I didn't have to remove the pump shoe thankfully since I've read it is a real pain to do so.

@MNdrifter , is Yamaha picking up the costs of the new housings well? If not are you going with stock housings or aftermarket (plastic or stainless steel)?

Asking be user I'm thinking about optimizing my pumps this off season and currently trying to decide what I'm going to do about the housings if anything. I wish they wee sacrificial inserts like they are on my older seadoo pwcs (they are a lot cheaper and easier to replace). Congrats on the pending new impeller and good luck getting your issue resolved.