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Dec 20, 2018, 11:26 PM
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Much the same as my line of thinking there AirDOGGe….
I am of the frame of mind that cavitation may be happening anyone seeing the effects as cavitation erosion?

I recall,as a young Salty Seaman that we used to hang the ships log from the poop deck of the ship I was on...we were probably doing maybe 7000 miles around trips..UK to Bermuda,various ports up the Eastern coast of USA to New Jersey..back to UK via Ireland.
The shops log was what looked like a torpedo with fins..this used to be spinning away whilst at sea and read and noted at regular intervals..I never saw one replaced on a single trip and when they were the senior officers got their hands on them and got some engineer to polish out any damage and have it turned into a table lamp !!
I do however recall seeing them marked up,scratched,indents and what looked to me like pitting...some indents I was told were potentially teeth of sea creatures...damage from debris etc.....and the pitting from cavitation true this was I have no idea but for sure...we don't cover several thousand miles with our toy boats.
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Dec 25, 2018, 02:33 PM
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unusual_rc's Avatar
Originally Posted by Kayaker
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 Iím in the tub playing Ďboatsí.

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 havenít looked at it for cavitation damage. Itís about time again to replace the stator bushing; Iíll look at the impeller then. I run it on 6s and think it would cavitate on 8s so I donít expect to see damage.
Interesting question about water temperature!
The water is boiling for sure, yet it does happen at completely different temperatures than an ordinairy cup of tea.
One would assume that in warmer water the pressure drop (decrease) to make water boil will come earlier. So indeed cold water will have a cavitation limiting effect.

Aerated water means it does not meet one of the most important requirements for cavitation: a pressurized environment with no room (leaks) for gas coming in between the lower pressure areas!
In aerated water both jets and propellers lack traction because of this. So no low pressure cavities can form in the first place.

Interesting to read there is almost no difference in rpm when the noise and disrupted spray start to come in between an overlapping and non overlapping impeller!
It still sounds to me were are talking about the so called "water column lock", which prevents the impeller to puch more water through the pump. Some water is reversed back between the blades, which creates turbulent (violent) water flow.
If one would apply a tremendous amount of energy to this impeller, cavitation could occur if the plastic impeller was not shred into pieces beforehand.

Nice steamboat!
Remember how the first propellers looked when mankind stepped over from peddle wheels to this new form of propulsion
Sep 12, 2019, 01:51 PM
Registered User
Some info, may help or may cause more confusion.

In my professional life I apply motor drives to municipal water pumps. So I don’t have expertise in the pumps per se, but I have witnessed a lot of technical discussions by some really smart pump engineers.

First point
1. When a pump cavitates, it creates a small vacuum zone. This zone actually pulls the air out of the water. ( I’m pretty sure this part is a gross over simplification).

2. A steel or iron impeller are the most susceptible to cavitation damage. On a microscopic level the cavitation actually removes microns of metal. ‘This action is very similar to EDM. Electrical discharge machining. In municipal water they will specify a bronze impeller if intermittent cavitation can not be avoided. Bronze is much less prone to cavitation damage. They only reason they don’t use it exclusively is because of the cost.

My guess is the plastic used in these impellers is probably not capable of cavitation damage. Any damage seen on an impeller is probably the result of small particles in the water. If you think about scale of a grain of sand to the pump unit it is like a small rock in a full size unit.

After 25 years experience as a start up / application engineer I have learned one phrase that should never be used. “that’s impossible” I have seen the impossible happen too many times! In each case the reason the impossible was possible.... is because there were facts discovered later that were not taken into account.

On a different note,
has anyone made a custom impeller for a pro-boat jetsprint? I just ordered a brushless kit for mine, my intention is to keep the boat pretty heavy in the water and have enough power to plow at reasonable speed. There is a small dam / water fall near me and I want to be able to run upstream, in a controlled manner versus dancing across the water. This is necessary because often there are lots of small kids playing in the water there, and high speed skipping might be a bit dangerous!

The first time I took my boat there was a Labrador retriever, luckily on a leash... I have never seen such a look of torture and despair as he watched my boat going back and forth!

So if you don’t have a rescue boat to retrieve your speed boat.... get a Labrador! Lol
Nov 02, 2019, 05:05 AM
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After having run into the issue alluded to in post one on page one, I feel a bit vindicated although I'll never get back those moments of my life spent trying to explain things to someone so determined to not understand, that they are even contradicting things they've learned on their own thread and promptly forgotten.
Looking at this and other threads, more power at top RPM, more torque, additional stages, more vanes, more exit constriction for faster exit speed? Doesn't seem like there's a magic fix for cavitation.
Nov 02, 2019, 03:39 PM
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Kayaker's Avatar
I haven’t found a magic fix for cavitation but I’m learning how to avoid it.

Here are ways I found to avoid cavitation on the 45mm FJD. I’ve tested this jet drive in my tub using 5, 6, 7, 8 and 9 cell battery voltages to see how fast I could spin the impeller with a 930kv motor before cavitation starts. Holding the boat in still water it cavitated just as it reached full throttle on 7s and easely on 8s with the 930kv motor. With the boat moving fast on open water it didn’t cavitate on 7s but did on 8s.

On 7s the unloaded motor could be spinning at 24,087 RPM. The data from my Swordfish ESC points to cavitation starting at 17k in static testing the 45mm jet. The big difference between these numbers suggest this 45mm jet drive could use a larger motor than this 40x92mm but that’s a different subject.

A non-overlapping blade impeller slips in the water more than overlapping blades so achieve a higher RPM when loaded. This means non-overlapping blades need a little lower kv motor or lower voltage to stay below 17k, RPM where cavitation starts. This also means overlapping blades can make a boat go faster.

An impeller blade with a squared off trailing edge cavitates at about the same RPM as an angled trailing edge. An angled trailing edge re-primes better than squared but costs 7% more watts.

I just starting to work on understanding cavitation for the 35mm FJD because I needed a high volt ESC that I now have. Testing has started. Nothing worth saying yet. With this jet drive I can see if the number of stator blades has an affect thanks to Grael.
Last edited by Kayaker; Nov 02, 2019 at 04:24 PM.
Nov 02, 2019, 08:37 PM
Registered User
I'm very keen to hear how your stator tests work out Kayaker! Pity you haven't got a 3D printer. I'm going to have to order a set of bearings, shaft etc and build myself a boat but I've printed everything to go in it!
Nov 03, 2019, 12:43 PM
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Kayaker's Avatar
I had a front jet bearing get noisy at 40 hours use on my 35mm FJD. Boca has my favorite bearings on sale as a ‘kit’ so I got four for $18 USD, regularly $11 each. These are 5x13x4mm with ceramic balls and 440c SS races. These are specially designed for off-road or excessively dirty driving environments:
Nov 03, 2019, 12:49 PM
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Kayaker's Avatar
Shapeways is sending a print of the 7 blade stator to me this week, should I also try the 11 blade? Do I remove the last 1/3rd of the 11 blades after I get the print? Why did you make a 7 and 11 blade stator and what should I be looking for in performance?

Printing stators is sure a lot easier than making them with bits of aluminum and stainless steel.
Last edited by Kayaker; Nov 03, 2019 at 01:06 PM.
Nov 04, 2019, 12:31 PM
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Kayaker's Avatar
Here are 5x10x2mm jet drive water seals for the 35mm FJD, $2.55 USD:

Here they are in the UK:
Nov 07, 2019, 01:55 AM
Registered User
Originally Posted by Kayaker
Shapeways is sending a print of the 7 blade stator to me this week, should I also try the 11 blade? Do I remove the last 1/3rd of the 11 blades after I get the print? Why did you make a 7 and 11 blade stator and what should I be looking for in performance?

Printing stators is sure a lot easier than making them with bits of aluminum and stainless steel.
Hi kayaker, I would have started with the 5 or 3 blade stator to have a more comparable performance but It will certainly be interesting to see how the 7 goes.
I mainly rendered the higher blade count ones because it was easy and I wanted to see that more authentic look. The 7 and 11 blade ones will be difficult to polish in the area where the vanes meet, I'm slowly chipping away at doing an even better vane but it's painful. I have several approaches, the one that worked so far, I design an 2 dimensional cross section of the vane at the outside radius and then extrude it up, narrowing it as I go. Last night I tried re-writing one of my fin generation programs to do a whole vane in one iteration but it's difficult getting it consistently following the parameters I want. The problem with the extrusion method is that I have to use controlled step sizes all around the perimeter or when I extrude it, and trim bits off for the hub and tapering outside, the triangles that form it don't line up well with the water flow. With the one-shot generation I can easily subdivide these over the radius to control the smoothness as fine as I want. Then when I combine all the vanes with the rest of the stator... my CAD program spits the dummy and only produces the cooling outlet stump! Not a programming error, but a rendering error! I have yet another idea to cure it though, I think I have to install each vane with a fractionally different axial offset to avoid illegal co-incident faces. Or assemble them with a gap, but meshing into an extension cone to the hub cone.

Anyway, back to your purchase of the shapeways prints, easier clean for the lower vane count ones. Better match for lower KV motors for the high vane count ones, especially with my smaller nozzle options. Remember I did the 3 vane render with a 1 deg rotated vane merge for slight thickening.

And I attached some of my 3D print designs... there might even be some FJD parts in there!
Last edited by grael; Nov 07, 2019 at 02:07 AM. Reason: Add images

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