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Author Topic: Water Jet Drives & 3D Printing  (Read 18006 times)

derekwarner_decoy

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Re: Water Jet Drives & 3D Printing
« Reply #150 on: April 21, 2017, 04:50:35 AM »

Alex.....you have an interesting issue to resolve ie., “how to eliminate unwanted heat created by the thrust of the impeller being reverse transmitted to the drive shaft thrust bearing and the subsequent heat build up that occurs between the interface of the housing and the thrust bearing”

1. to what temperature does the plastic filament material remain mechanically stable?
2. will the material accept a brass bush being inserted into the housing, during or post manufacture?
3. the brass bush could be spider legged to the thrust bearing to allow air flow between the thrust washer and the bush
4. could sufficient air flow be achieved even with a small finned centrifugal fan mounted to the shaft?
5. could water cooling of the brass bush be an option?....[similar to the water cooling of motor jackets?]

Your CooLRC 4074 motors is listed as a draw of 2800W……..[it also has an option of a water coiling coil]

6. after efficiencies & losses, some of this energy is consumed in providing the rotation power to the rotor
7. some of the energy is the transmitted opposite reaction being the inboard thrust of the rotor shaft and the heat transfer you are experiencing

All of the hypothetical calculations to where this energy dissipates will only confuse you

Manufacture/Print another housing to the latest [as failed] design & install the current rotor & thrust bearing……with a $20.00 digital pyrometer determine, confirm and document  the running temperatures of the thrust bearing, the housing interface and all associated motor/gearbox attachments under full water load conditions ……maintain the full load testing until all temperatures peak and then maintain/stabilise

From there you can progress your thoughts in what and how you will remove this heat from  the housing

Derek
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Derek Warner

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derekwarner_decoy

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Re: Water Jet Drives & 3D Printing
« Reply #151 on: April 21, 2017, 11:36:21 PM »

Alex....just a few points to consider :o

1. manufacture the motor mounting plate from aluminium
2. mount the thrust bearing the aluminium mounting plate [as you had shown a week or so ago]
3. install a needle roller bearing in the housing plate [just next to the point of failure which will accommodate the longitudinal thrust movement]

Lubrication of the free floating needle roller bearing may need further consideration, however the aluminium mounting plate should adsorb the heat gradient created from the rotational thrust [bearing] without any issue

Derek
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Derek Warner

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Re: Water Jet Drives & 3D Printing
« Reply #152 on: April 22, 2017, 01:29:40 PM »

Hi Derek,


A well considered and thought out response as always, I did look at needle roller bearings but unfortunately they are of no use as at the required size they top out at 5K RPM, which is a bit low compared to the 19.5K RPM operational speed of the jet drive, on closer inspection of the thrust bearing, it looks like the problem had arisen from one half of the race, which is pushing against the 3D printed gearbox is a sliding fit on the shaft, where as the other half rotates freely, this was a complete over sight on my part and is explained in the attached image.
The bearing is designed to be installed with one half of the race supporting the rotational side, in my case, this would be the clamp collar on the shaft, the bearing race rotates with the clamp collar relative to the second half of the race which is stationary in relation to the impeller shaft, this half of the race therefore has a larger clearance to allow free rotation of the shaft within, in short I installed the bearing in backwards, the race locked onto the shaft and friction welded itself to the gearbox  :embarrassed:

So a much quicker fix than I thought, I can print a replacement gearbox part, (got to love 3D printing) ready for more testing down the lake tomorrow! 


Alex
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bfgstew

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Re: Water Jet Drives & 3D Printing
« Reply #153 on: April 22, 2017, 03:25:56 PM »

Nice to see you have found the problem Alex, bearings this small are notoriously difficult at best of times but when you are supposed to fit one way, you need to be on your toes to ensure correct orientation.
Look forward to your next run.
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IKB

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Re: Water Jet Drives & 3D Printing
« Reply #154 on: April 23, 2017, 11:23:47 PM »

Hi Guys,


Well, got the prototype rebuilt and on the lake today, on the plus side, the thrust bearing appears to be working, on the other hand, this was one of the shortest runs ever at just under 40 seconds when the impeller exploded!  >>:-(
No idea what caused this, sailing along at half throttle and halfway through the acceleration to full throttle the impeller explodes, now there are a couple of possibilities as to why this might happen:


  • Ingestion -  Even though the intake has a grate on it, not everything is strained out, so ingestion is still a very real issue
  • Torque - I have a lot of power on tap with this setup, it may be that the impeller simply shattered under the sudden change in loading
Whatever the reason may be, it is clear I need another impeller, it may be that the SLA resin I am using is just too brittle to withstand the forces a jet drive imparts, so I will more than likely go back to the PLA impeller which has withstood all kinds of abuse and lived to be abused another day, video from today:


https://youtu.be/-Qb9pNazcqE


Alex
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IKB

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Re: Water Jet Drives & 3D Printing
« Reply #155 on: May 01, 2017, 05:12:08 PM »

Hi Guys,


No testing this weekend as I have taken some time out to redesign the jet unit from scratch, now on the surface it may not look a whole lot different to the old version, but it is all new and there are many small, but important differences, I have started printing the new parts already and hope to be on the lake next weekend.
The majority of the design modifications are centred around getting better flow through the unit, as at full throttle there is a noticeable change in motor note, which I believe is due to the unit attempting to flow a greater volume than the unit is able to support, this just leads to back pressure building up and consuming much more power for little extra forward drive, to help improve flow and hopefully remedy this situation, I have increased the internal bore of the unit to 42mm, this allows room for a 30 x 40mm impeller, the tunnel housing is also 42mm all the way from intake to stator housing, which has a much larger cross sectional area for better flow.
The larger unit also necessitates that the reversing mechanism is also bigger to fit over the larger stator housing, so the entire design has had a size increase of 6mm in all directions, on this prototype version I also printing a section of transom so that I can graft this unit into the existing hull without hacking it up too much.


More as it happens,


Alex
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bfgstew

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Re: Water Jet Drives & 3D Printing
« Reply #156 on: May 02, 2017, 12:48:23 AM »

Can I make a suggestion Alex, the cross sectional area through the pump shoul  be the same volume, looking at your very impressive design it looks as though when the flow gets to the impeller it is getting crompressed and at high revs this will throttle the flow down and increase power consumption with no gain in performance. Take the cross sectional area of the impeller hub and blades and add this to the intake diameter, this will then become the new diameter for your impeller, theoretically this should give full flow at all speeds. Hope this makes sense Alex.
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derekwarner_decoy

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Re: Water Jet Drives & 3D Printing
« Reply #157 on: May 02, 2017, 02:05:20 AM »

Morning stew ....I too have been following %) these impressive designs & redesigns & builds by Alex

I understand and agree that the compression of the volume of water via the rotor will draw additional [motor current], however if this [additional] force is available, the net result is accelerated fluid or the velocity of the discharge water is increased

The secondary side of this difference in diameters or 'flooded suction' is tending to minimise pump cavitation

This cavitation issue is mentioned by Alex in an earlier video...........

Derek
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Derek Warner

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bfgstew

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Re: Water Jet Drives & 3D Printing
« Reply #158 on: May 02, 2017, 08:11:28 AM »

Certainly agree with your points Derek.
Cavitation could be reduced by increasing the intake diameter making the whole intake tapered, increasing pressure into the impeller, taper the impeller (jet turbine style) to increase output thrust even more, the down side is increase in component stress levels? Just a thought?
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C-3PO

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Re: Water Jet Drives & 3D Printing
« Reply #159 on: May 02, 2017, 12:38:21 PM »

I follow this thread and find it fascinating.

Alex could you help with an answer to my question regarding Jet Drives.

I believe the drives need to be "primed" otherwise they are sucking air.

But my main question relates to the vertical positioning of the water jet output - what's best/acceptable to get the maximum thrust from the water expelled from the drive - the drive exhaust submerged under water / above water or something else?

Thanks in advance

C-3PO
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IKB

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Re: Water Jet Drives & 3D Printing
« Reply #160 on: May 02, 2017, 03:19:20 PM »

Hi Guys,


Answers to questions below:


  • Derek & bfgstew - The design decisions I make are based on taking design ques from real life jet units, mixed with my interpretation based on the results I get from lake testing, the large diameter intake ensures a good flow of low pressure feed water into the unit, this reduces cavitation of the impeller, the reduction of the stator housing is designed to increase the flow rate from the outlet, resulting in more power produced from the unit, it is a balancing act, as on the one hand you need some back pressure to help suck water in the unit using the pressure differential across the impeller, but too much will just result in overloading the motor.
  • C-3PO - You are correct, at rest, the jet unit needs to be at least half submerged in order to reliably self prime, once underway, the whole transom tends to become exposed at high speed, so the jet is just exhausting into air, most commercial units usually have a 3-5 degree angle pointing down from the horizontal to point the stream into the water, as this will produce more thrust.
Derek - I don't know how you produce your screen grabs and correct me if I am wrong here, but you may find it easier to use windows snipping tool, as it looks like you are taking a photograph of the monitor? https://support.microsoft.com/en-gb/help/13776/windows-use-snipping-tool-to-capture-screenshots


Alex
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Re: Water Jet Drives & 3D Printing
« Reply #161 on: May 02, 2017, 10:23:18 PM »

Hi Guys,


Progress being made with all of the new components printed for the tear down of the prototype hull, 19.5 hours worth of printing to get to this stage alone, just imagine what an entire boat is going to be like!  %%
I have also printed a new larger reversing mechanism which is curing over the next day or so


More as it happens,


Alex
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hopeitfloats

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Re: Water Jet Drives & 3D Printing
« Reply #162 on: May 03, 2017, 10:45:57 AM »

I think you will find the jet nozzle angle is more to do with trimming  than to produce more thrust althought I guess if the boat is trimmed properly it will perform better anyway. jet racing boats actually have hydraulically operated trim nozzles.  commercial models generally have either a set angle or manually adjusted. better thrust is produced with the nozzle above the water line. the original jet unit was basically a centrifugal pump stood on its end with the outlet under water. when bill Hamilton of Hamilton jets changed it to above water discharge there was a pretty significant increase in thrust.
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hopeitfloats

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Re: Water Jet Drives & 3D Printing
« Reply #163 on: May 03, 2017, 11:34:14 AM »

and the last improvement I have heard of was 3 bladed impellors. apparently increases thrust and lowers cavitation. haven't looked into it any further than reading about it. don't have a lot to do with water jets now.
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IKB

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Re: Water Jet Drives & 3D Printing
« Reply #164 on: May 04, 2017, 11:13:35 PM »

Hi Guys,


Quick update, got some time in the ShedShop tonight and started tearing into the prototype, which gave me an opportunity for a side by side comparison between the old style unit and the new one, as you can see from the photos, I have lengthened the trim tabs slightly, they also have a ten degree down angle instead of five degrees, to try and keep the bow planted in rough water, the stator housing outlet cross sectional area is much larger as is the tunnel inlet, whereas the impeller diameter is only marginally larger in comparison


More as it happens,


Alex




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IKB

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Re: Water Jet Drives & 3D Printing
« Reply #165 on: May 05, 2017, 10:27:10 PM »

Hi Guys,


More progress tonight, ripped out the old drive unit and made good the hull for installation of the new one, this is the major draw back to jet drives, they are pretty integral once installed, so swapping one is not a five minute job


Alex

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IKB

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Re: Water Jet Drives & 3D Printing
« Reply #166 on: May 17, 2017, 03:13:22 PM »

Hi Guys,


Quick update, not a huge amount to share at the moment, I have decided to do a practice run on the prototype, filling, sanding & painting, to give me some idea of the standard of finish I can achieve with the 3D printed hull sections and how much effort it is going to take to do an entire 3D printed boat, so far it is looking good, I put a second coat of plastic filler primer on it lastnight and I am very pleased with progress so far.


More as it happens,


Alex
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