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Dec 13, 2018, 03:59 PM
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Youngsta's Avatar
Now I'm having posts go missing,,,
or me not pressing the go button,,
I'll pop back later on and rewrite from memory,,
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Dec 13, 2018, 10:56 PM
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Youngsta's Avatar
Hi Oyley
I can't see an ally prop deforming much if anything to bother the performance,
my 28mm props did flex a bit under full load, ,
but as they flexed and changed under load they were then optimized in shape, otherwise you'll have troubles at full tilt,,
I'll be doing less brain computing and using more of the silicon based stuff for my next drive,,
I think the AI still needs us for testing stuff, out there,
I did ramble on a bit from here in the vanishing post,,
about what happens when we become not so useful to the AI,,
and I hope it hasn't learnt about compassion from us,
as we can let ourselves down from time to time,,,,
then it finished off with me telling Oyley that I thought his money was safe with the bet,,
and what a great seasonal gesture it would be to give the £200 away to a poor tramp,,
and then I said, I'll send PM him later with my details and how to post it off,,
so that was about it,,
Time for bed,, hope all is well people
Mark
Dec 17, 2018, 07:14 AM
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unusual_rc's Avatar
Found this explanation, which is quite good!
Remember this information is about real scale pumps in a PWC:


FLOW
Jet pump cavitation is oftentimes confused with the effects of ventilation regarding Personal Water Craft jet pump efficiency. We are looking at several aspects of jet pump security:

1. Aerated water conditions are Rescue Water Craft would operate in such as a surf zone or whitewater in a river
2. Damage to the jet pump unit and its components

Our Rescue Water Craft cavitation occurs when the results of extreme reduction in jet pump pressure on the back side of the impeller blades. This creates a loss of water jet pump pressure.


CAVITATION

It is an interesting process that we cannot see while operating our Rescue Water Craft. We can feel the hesitation of the craft, loading up or stalling of the RWC as throttle modulation is applied at the helm. This is most apparent in white water operating conditions. Or if there is debris that is hung up on the water intake screen or beginning to ‘wrap’ around the driveline.

Water begins to boil at 212 degrees Farenheit. If we reduce the atmospheric pressure low enough, water can also boil at room temperature. These boils can effect jet pump efficiency underway.

As the Rescue Water Craft impeller begins turning through the water drawn into the jet pump at an ever-increasing rate of speed, the pressure on the back side of the blades is reduced, and if that pressure is reduced low enough, the water will begin to boil and form water vapor on the blades. This usually occurs near the outer or leading edge of the blade. There can also be damage to the jet pump guide veins on the backside of the impeller.

FLOW DON'T BLOW
Water vapor bubbles will migrate closer to the center of the RWC impeller blade within the Jet Pump. This is where the jet pump pressure is higher, and the boiling stops. The vapor bubbles will begin to implode against the impeller blade's surface.

This resulting energy release can be so strong that it can begin a process of chipping away at the impeller blade surface, leaving what is called a cavitation burn.

Cavitation can have a lot of different causes. Impeller nicks, dents or different types of damage to the leading edge of any of the impeller blades are often the highest contributing factor. If your Rescue Water Craft impeller no longer cuts through the water smoothly from the water drawn through the water jet, it will cause disturbances in the water flow, and this can result in the effects experienced from cavitation.

Ventilation can occur from the bottom hull of the Rescue Water Craft. At each training course or rescue episode we pre and post-inspect the bottom of the hull for any nicks in the gelcoat or substrate surface.

One of our inspection requirements is to check out the water intake screen, the pump tunnel, ride plate, impeller (both lead and trailer blade edges), pump guide veins to ensure there are no cracks, chips, breaks or scores. We also inspect the pump liner and are sure to fresh water flush after each use.

Typically the keel leading up to the dead rise of the bow is a key area for gelcoat damage from repeated groundings. Chines can catch the edge and fleck off small areas of gelcoat. Ventilation can be suspected to affect the jet pump unit if that area has any surface damage.

source
Dec 18, 2018, 04:09 PM
Registered User
Thread OP
Yep, agree that all of those things are possible.

Obstructions or surface damage or irregularities of the hull in front of the intake or the impeller don't explain why kayaker was still able to make his drive cavitate while the entire hull was submerged and held stationary. (i.e. no possible source of aeration.)
Dec 18, 2018, 04:15 PM
Registered User
Is it correct than one can experience cavitation but not specifically cavitation erosion....I have never experiences erosion on a model boat that I can see...and could it be that we just run for minutes rather than hours on end so just don't experience the erosion.
Dec 18, 2018, 11:13 PM
TTN
TTN
Design & Build <3
TTN's Avatar
Someone install a automotive knock sensor to detect the cavitation like hamilton jet does
Dec 19, 2018, 10:06 AM
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unusual_rc's Avatar
It clearly states in the article that cavitation makes so much noise, it can be clearly heard above engine noise, white water (spray) and turbulent airflow!
So no sensor is needed. And remember these bubbles filled with gas are very tiny in real scale jet drives, so can you imagine their imploding power by making so much noise!
Dec 19, 2018, 12:50 PM
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Thread OP
Yeah and that's exactly what kayaker experienced. A strange loud noise and unloading at high rpm even when the hull was submerged.
Dec 19, 2018, 02:06 PM
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unusual_rc's Avatar
He never stated it was a loud noise as described in the link is posted.
Kayakker simply shared his experiences with us, because he wonders what happens.
Aeration can occur even with fully submerged pumps. Maybe he did have an air leak somewhere, which stopped at a certain environmental pressure (deeper under water).

If cavitation can damage metal (often hardened) impellers, why would much softer plastic survive? Because of the flex one would assume. But cavitation occurs due to the stifness of the metal blades and applied power...
Dec 19, 2018, 02:56 PM
Registered User
Thread OP
Stiffness of the blades doesn't matter. If there's sufficiently high blade velocity and low pressure present at the leading faces of the blades, cavitation will occur.

Plastic blades that flex can reduce this to some degree if they flex in a way that optimizes the pitch for the conditions, but at a certain point cavitation will still occur.

"Aeration can occur even in fully submerged pumps." How? There is no air present to draw into the drive.

The occurrence of cavitation decreases with increased depth. This again lines up with kayaker's observations.

I think we would see cavitation damage on a metal impeller if one were able to run it for a few batteries worth of time in that condition. At the moment kayaker is the only one pushing his drive(s) to that level, and the tests he mentioned were short, so probably no way to know for sure right now.

My offer to toss a tested metal impeller in the SEM here at work for a good close look still stands.
Dec 19, 2018, 05:23 PM
Registered User
Is it so that the water boils and vaporises and hence the bubbles that implode and as such create the damage known as cavitation erosion.

I am still of the belief that our impellers are just not subjected to operation for sustained periods of time.

Also...the softer nature of plastic/nylon impellers are less susceptible to damage as the energy is somewhat absorbed.

Just thinking that my poor kidney stones may have to be blasted to break them up...the two in the ureta anyways....now I believe the 'beam' is cigar shaped and does not blast the stones directly but the urine surrounding the stones and its the cavitation within the urine and those implosions that strike the stones creating fissures and subsequently breaking the stones into smaller pieces that will pass naturally .Now then...what about all that stray waves ...why are they not damaging the softer areas surrounding the stones? Well,there has been some evidence that I read some damage is being done to other areas and some reluctance by some surgeons to use this method in certain areas.
If tests are done is a tub like Mark does there will be plenty of air in that turbulent water,its evident in rapids when full sized jet boats are pushing in the rapids..the engines rev much higher and the drive reduces dramatically.
I don't think we are going to see much if any evidence of cavitation erosion on a model jet pump used in the way we use them anytime soon.
Dec 20, 2018, 01:41 PM
Registered User
Thread OP
Yep, agreed that they likely don't operate long enough in that condition to cause visible damage. And/or that most people are using plastic impellers that won't show the damage.

Kayaker's test was done with an open hull submerged in his local creek though, so no recirculated turbulent whitewater in a tub or the middle of a rapid to cause possible aeration.
Dec 20, 2018, 07:35 PM
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Kayaker's Avatar
If cavitation is boiling water will the RPM that it happens at change if the water is colder? I just tested an impeller with non-overlapping blades on 9s and it made the noise at about the same RPM that overlapping blades do but the water is colder now. Do I need to do these tests in similar water temperatures?

The noise starts more abruptly with overlapping blades so I think I can get this on video next time I’m in the tub playing ‘boats’.

Will cavitation start more easily in aerated water? The aluminum impeller in my Thrasher is looking thrashed after many hours of sucking up rocks. I haven’t looked at it for cavitation damage. It’s about time again to replace the stator bushing; I’ll look at the impeller then. I run it on 6s and think it would cavitate on 8s so I don’t expect to see damage.
Dec 20, 2018, 07:43 PM
Registered User
Not a jet boat but an interesting bit on cavitation several minutes in by a rather exuberant presenter....certainly an interesting boat..three prop shafts with three props on each !! and steam driven,I must get that way out and visit it for a look see.
There is an advert part way through as it is a sponsored video but it does continue after the advert.

The ship that revolutionised naval warfare (25 min 26 sec)
Last edited by Oyley; Dec 20, 2018 at 07:52 PM.
Dec 20, 2018, 10:33 PM
Just Plane Nutts
AirDOGGe's Avatar
Excessive impeller RPMS can cause cavitation via choking, and cavitation causes erosion of metal blade surfaces. But I have never heard of any such erosion issues at this smallish scale level though.

Of course that doesn't mean it isn't happening. Noticeable erosion requires many, many hours/days/weeks of ship or water pump operations to see the effects, while our model runs are measured in a handful of minutes.


This discussion is fascinating and I've enjoyed in a lot, but it all may be a solution seeking a problem.


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