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Nov 27, 2019, 12:43 AM
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Quote:
Originally Posted by Kayaker
I am about to try a 4092 motor with a 180 ESC so the little 410mm boat is now up to 2.7kg/6 lb. I havenít put it in the water and may not float. I canít help myself. The jet drive was begging for more power. I should at lest be able to get numbers for this jet drive with a good size motor. Iíll be testing a 7 blade and 11 blade stator also. Iíll post the results here.
Hey Kayaker, keen to hear how you go on the 7 & 11. Good luck on your flotation!

I'm not sure how the 11 will go, on this small 35mm size I think I should have extended the bearing housing hub-cone further back and cut the vanes even earlier, there's a lot of very skinny spaces there where the vanes meet but I did the 11 more as an experiment. I'm pretty confident in the 5 and 7 vane ones and did the 3 vane render but that will bit of a test too, I'd expect the 3 vane one to perform similarly to the FJD stator but could well be wrong!

Sundogz says the FJD constant pitch is a feature but I'm not convinced by his reasoning.
To my mind, his constant pitch is only effective if you assume backflow near the bearing hub which could happen, but I'm more of the opinion that with litres of water clearing the unit every second, a shallow central pitch is more of a hinderance. We'll get some clues with your prints but entry angle and shape are also important... and block the nozzle to a smaller diameter and everything changes again!

I've ordered a 5 & a 7 myself, but of my V3 version which has more hydrodynamic looking vanes but a slightly straighter entry angle with a thinner start, though it's still not sharp like the FJD.

Further changes are waiting on me getting up to speed with freecad which is almost completely different to openscad, but I'm still trying to do the same things!

I also want to do a variant on the 35mm stator with holes lining up with the stator fixing, and further stator variants with hex nut pocket for easy bearing extraction and swap out. Still waiting on my own motor, ESC, new radio set and batteries and a later order for shaft and coupler. Meanwhile I've been experimenting with some ABS prints and vapour smoothing, I find thick layers give great adhesion but shallow high precision layers end up more porous. For ABS this is OK as the acetone infiltrates and bonds the ABS tightly as it evaporates. So I have some stators slowly evaporating out their acetone, looks like it will take a few days at least for the acetone to vaporise off enough for the ABS to be in a normal state of hardness.

I've also started to work on a CAD construction concept for my new hull which will be another aggressively stepped monohull, but I'm planning to 3D print it in watertight sections.

Graham
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Nov 27, 2019, 02:55 PM
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Kayaker's Avatar
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With a 4092 motor, 180 amp ESC and the batteries the 410mm boat doesn't float. I made a float for the boat to sit in so I can do some tub testing to see how this motor works with this jet drive.

The 4092, 930kv motor is running on 9s (33.3 volts) = 30,969 RPM in this 35mm jet. My first test is with a 46.5 pitch steel impeller pushing water through a 4 blade, 26mm stator and squeezing out a 22.5 mm nozzle. As expected this was too much pitch and squeeze but I had to try. Low voltage cut off kicked in at 4 seconds from voltage sag with 4400mAh, 9s, 65c, two year old Graphine batteries.

The amp draw was 90 at 33.3 volts = 2997 watts = 4.02 horsepower.

Iím cleaning and tightening the battery and motor connections. Iíll replace the 46.5 pitch impeller with a 39 mm pitch and start testing Graelís stators. The Shapeways prints of the 7 and 11 blade stators are cleaned up, ready to test and look great. My river and tub are frozen, theyíll thaw in a week.

Quote:
Originally Posted by grael
I also want to do a variant on the 35mm stator with hex nut pocket for easy bearing extraction and swap out.
I am gluing both bearings in place with epoxy and they still come out easily with an impeller shaft end stuck in and wiggled. My problem has been with bearing pockets getting lose over time.

I expected the stator bearing to wear out fast but this is not the case. Water lubricated? I was going to drill a hole in the stator cone to grease the bearing through the nozzle and using a set screw plug. No need.
Nov 28, 2019, 01:22 AM
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Ah... I wondered what was keeping you off the water! Ice will do it

9S ! I'm shocked!
pushing the envelope there, especially with 30,969 RPM!
Seems like it would be a happier boat with 3S less and 20,000 rpm.
Nov 28, 2019, 06:07 PM
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Kayaker's Avatar
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I would like to see how many RPM this jet drive can spin before cavitation so I’m trying the bigger 4092 motor. If it doesn't cavitate spinning 30k with the 39mm pitch impeller then this jet drive can push a boat very fast. This might need an innovative hull to handle the speed.

I’ve been running the 35mm jet with a 4072, 1800kv on 4s to get 26,640 RPM. No cavitation at this RPM but this motor is a bit small and actual RPM when pushing water could be under 20k.
Last edited by Kayaker; Feb 16, 2020 at 08:27 AM.
Nov 29, 2019, 08:48 PM
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Quote:
Originally Posted by Kayaker
I would like to see how many RPM this jet drive can spin before cavitation so Iím trying the bigger 4092 motor. If it doesn't cavitate spinning 30k with the 39mm pitch impeller then this jet drive can push a boat very fast. This might need an innovative hull to handle the speed.

Iíve been running the 35mm jet with a 4072, 1800kv on 4s to get 26,640 RPM. No cavitation at this RPM but this motor is a bit small and actual RPM when pushing water could be under 20k.
I think cavitation is most affected by impellor tip speed and water pressure in that region. Water pressure is affected by how fast air pressure at your at your local water level can resupply the water you are removing from the intake area.

Power to weight ratio is improved by spinning the same weight motor faster. Going to a high vane count, using mixed flow, more stages all help to increase the exit pressure and with a smaller diameter outlet, your exit velocity. If the same amount of water is exiting twice as fast, it's giving you more thrust. So to get more motor power with a limited RPM, you need more current, more motor cooling. Or gears- which seems to be a backward step with brushless!
More torque with similar weight through more motor diameter.

I've for a long time felt that the intake is the best place to get motor cooling through conduction, an aluminium intake fixed to an outrunner end is perfect for dissipating excess heat.

As far as a new boat design, I've been working on one.
Early days, but I'm working on a hull design using chunky scales. Openscad design so it's program generated.
The idea is to break up the whole of the boat's underside into discrete lift/release surfaces to break the boat free from the coanda effect (water following curved surfaces), and introduce lots of low friction air pockets. The attached image gives a quick idea of how it works but I need to work the formula for tile placements to ensure correct overlaps while getting the general 3D shape I want. A typical planning hull is reliant on big flat surfaces but I don't actually need them for this boat, I can mix the hydroplane effect from the tile steps with conventional hull low speed design stability, and of course I need to factor in edge angles for best high speed behaviour. Low speed there is more friction to break away onto the plane but with the power we have available I'm not concerned. Also, as speed increases the tiles start forming airpockets higher up and this naturally works down to the underside with speed increase.
Feb 16, 2020, 08:19 AM
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Kayaker's Avatar
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Stator testing


I’ve been testing Grael’s 4BDN stators with a FJD jet drive. The 11 vane, V1 stator is just what I was looking for. The stator vane root has more pitch than the outside for less disruptive flow. This stator puts the water into a very straight column. This makes the thrust push in a more linear direction instead of a general forward push like most stators in RC jet boats give.

The boat goes straight more predictably and turning is more accurate and predictable. This helps me aim the boat better when jumping waves and rocks so hopefully it will land in deep water instead of on a rock. The straight column of water from 11 vanes with a progressive pitch gives more authority to steering at low RPM. Most of the steering in rapids that I do is at low RPM. This 11 vane stator seems to give better traction in bubbly water and have more thrust in green water. Grael’s stator has improved how my three dimensional thrust steering works for better control in rapids. Thank you Grael.

The only things I don’t like about this stator is if a boat has a hoping problem feeding it very straight thrust can make it hop more. The straight column of water shoots farther often getting me wet standing next to the action.

The jump at the end of the video I overshot the landing because I was testing an impeller for the first time that gives more thrust than I’m used to. Oops. Landed on a rock, broke the steering, boat headed down the canyon. It’s very hard to get down on foot because of cliffs and big rocks. I went to “plan B”, a whitewater kayak and found my jet boat in about a half mile and my wife waiting a mile down canyon (class 4) at the nearest road access.

Check out the video.

https://youtu.be/sNyQeSrbCtE
Last edited by Kayaker; Feb 16, 2020 at 05:17 PM.
Feb 16, 2020, 11:54 PM
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Quote:
Originally Posted by Kayaker
Iíve been testing Graelís 4BDN stators with a FJD jet drive.
https://youtu.be/sNyQeSrbCtE
I love the jump right at the end though what a pain to loose your steering like that!
How is your new impellor different?
Do you think the 4BDN stator is giving you noticeably more pressure for the nozzle stage compression?
I can redo those STLs at some stage, I need to add a small cone in the centre to move water out of the very narrow inside spots, it looks very draggy in the middle.
Feb 18, 2020, 02:09 PM
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Kayaker's Avatar
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Quote:
Originally Posted by grael
How is your new impellor different?
I’m testing different impeller shapes for my style of RC jet boating in rivers.

RC jet impellers are mostly scaled down from full size jet drives. These impellers have torque needs that match the torque curves of gas engines. I use an electric motor to power my jet drive so I want an impeller that matches the straight line torque curves of electric motors. Overlapping impeller blades with an axial jet drive do this.

The impeller I’m testing now is for instant thrust like you can see in my last video. It’s plastic so it spins up faster than steel. It is a normal 39mm pitch but with a lot more overlap on the blades giving better thrust at slower speeds and giving better boat control at all speeds with an electric motor.

Plastic overlapping blades need to be thick enough to not bend much (1.6mm blades sanded down to 1.4mm for a 35mm impeller). I added a big radius to the blade root. This is a good way to reduce bending without making the blades even thicker, reducing the water canal even more. With this much overlap the blades can’t bend or it compresses water making amps go way high. Non-overlapping plastic blades can bend and change the pitch of the entire blade so no water compression so no problem.

The impeller STL file included here has 36mm blades to fit a 35mm bore. I file the ends of the blades to get a tight fit for faster re-priming. I have also included a “screw” if someone with limited CAD skills wants to experiment with different amounts of overlap.
Last edited by Kayaker; Feb 18, 2020 at 04:04 PM.
Feb 19, 2020, 12:45 AM
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Quote:
Originally Posted by Kayaker
Iím testing different impeller shapes for my style of RC jet boating in rivers.

RC jet impellers are mostly scaled down from full size jet drives. These impellers have torque needs that match the torque curves of gas engines. I use an electric motor to power my jet drive so I want an impeller that matches the straight line torque curves of electric motors. Overlapping impeller blades with an axial jet drive do this.

The impeller Iím testing now is for instant thrust like you can see in my last video. Itís plastic so it spins up faster than steel. It is a normal 39mm pitch but with a lot more overlap on the blades giving better thrust at slower speeds and giving better boat control at all speeds with an electric motor.

Plastic overlapping blades need to be thick enough to not bend much (1.6mm blades sanded down to 1.4mm for a 35mm impeller). I added a big radius to the blade root. This is a good way to reduce bending without making the blades even thicker, reducing the water canal even more. With this much overlap the blades canít bend or it compresses water making amps go way high. Non-overlapping plastic blades can bend and change the pitch of the entire blade so no water compression so no problem.

The impeller STL file included here has 36mm blades to fit a 35mm bore. I file the ends of the blades to get a tight fit for faster re-priming. I have also included a ďscrewĒ if someone with limited CAD skills wants to experiment with different amounts of overlap.
That's very well thought out kayaker. I'd be interested to know how the pressure figures work out for different amounts of overlap.
Feb 25, 2020, 02:10 PM
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It would be interesting to check thrust at different throttle end points to get a thrust curve to show the difference between non-overlapping and overlapping blades. This would be a good way to prove to people that what I’m seeing on the water with my boat is true.

On the water is the best way for me to test boat handling like in my last video. The improvement in boat handling with overlapping blades is easy to see when the boat is making complicated moves. The boat is much more predictable so it can make the moves and I can go faster. For jet sprint boat type racing this is a big improvement.

https://www.youtube.com/watch?v=sNyQ...ature=youtu.be

Adding a lot of blade length and overlap the impeller is not quite as fast. The impeller with the most overlap in the photo seems a good compromise between highest speed vs more accurate steering.
Last edited by Kayaker; Feb 25, 2020 at 02:40 PM.
Feb 26, 2020, 03:32 AM
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Could it be kayaker, that there is good efficiency and pressure at higher boat speeds, but with lower boat speeds there's less efficient intake filling and a lot more slippage?
So the combo of 11 vane stator and 3 blade prop with significant overlap is more slip resistant resulting in more pressure when the boat itself is moving more slowly.
Now I'm wondering, even on the 35mm pump, if you should be running even more blades on your impellor?
Here's a 7 blade one I've attached, I'm not sure if Shapeways will pass it though in versatile plastic or their steel/brass process, depending on blade thickness.
I get about 0.82mm measuring the blade thickness in meshlab. Easy enough to change at my end.
Feb 27, 2020, 02:07 PM
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Above ĺ throttle overlapping and non-overlapping blades have about the same amps, thrust and water pressure. Non-overlapping blades may have less friction making them a wee bit faster.

Good timing on the 7 blade impeller file, I added a cone and sent it to Shapeways with an order of impellers I want to test. Iím most interested in yours and a PWC style impeller. Iím including the STLís here if someone would like to try one:
Feb 27, 2020, 04:33 PM
Jetdrives R Us
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Kayaker, what does PWC mean? I noticed it 'bites off more than it can chew', so to speak as the blades start at a narrow nose section and continue to the wider hub . Water can't be compressed, but this design tries to. What was your intention?

On a mixed flow impeller, the blades would get narrower toward the hub and the intake chamber corresponds.
Feb 27, 2020, 07:53 PM
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Quote:
Originally Posted by sundogz
Kayaker, what does PWC mean?
I donít know what to call this type of impeller. A lot of Personal Water Craft jets use this type so I call it a PWC impeller like the SeaDoo in the photo. The Proboat River Jet uses one like this. It seems like a cross between a mixed flow and axial drive to me. The blade pitch varys from front to back to make up for the cone hub unlike a mixed flow that has a cone wear ring to match.

Iíve broken two impellers in the last two weeks. The thicker stator vanes of the 4BDN V1 didnít brake. Impellers are easier to replace so this is a good trade.
Last edited by Kayaker; Feb 27, 2020 at 09:17 PM.
Feb 27, 2020, 10:19 PM
Jetdrives R Us
sundogz's Avatar
Quote:
Originally Posted by Kayaker
A lot of Personal Water Craft jets use this type so I call it a PWC impeller. The Proboat River Jet uses one like this. It seems like a cross between a mixed flow and axial drive to me. The blade pitch varys from front to back to make up for the cone hub unlike a mixed flow that has a cone wear ring to match.
The River Jet impeller has a non-tapered hub - the blade OD tapers but the blade pitch is constant. Here is the Riverjet impeller next to the mixed flow MickBeez impeller. Yours appears to have a tapered hub, but constant pitch blades, am I correct?


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