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[derekwarner]

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

[derekwarner]

Alex....just a few points to consider

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

[IKB]

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 

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

[bfgstew]

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.

[IKB]

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

[IKB]

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

[bfgstew]

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.

[derekwarner]

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

[bfgstew]

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?

[C-3PO]

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

[IKB]

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

[IKB]

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

[hopeitfloats]

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.

[hopeitfloats]

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.

[IKB]

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

[IKB]

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

[IKB]

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

[IKB]

Hi Guys,


Slow but steady progress at the moment, the painting is coming along nicely with the top coat applied now, I am going to cut back the paint on the hull and then polish it to get it as smooth as possible, the finish doesn't stand up to close scrutiny, but I more than happy for this test phase having sprayed it with a rattle can during my breaks at work.
I am also working on a new drive setup, reusing existing parts, I want to see if the large 500Kv Outrunner will spin the jet unit when driving through a step up gearbox, if it works the way I think it will, then this should be a much less power hungry system when compared to the current unit.


More as it happens,


Alex

[IKB]

Hi Guys,


So finally got the 2nd GEN drive installed fully into the prototype in time to take it to Kirklees open day and had a fantastic time, weather was great, gentle breeze to keep us all cool and then I tried to actually sail the jet boat, at first everything seemed to be going well, new trim tabs are doing a great job and it is much more stable whilst cornering, just gently cruising around, testing the steering and listening for any untoward noises, but there were none, so decided to open it up and then it happened, some idiot had forgotten to tighten the grub screw that holds the impeller to the shaft, the impeller walked forwards and chewed up the leading edge of the blades on the inside of the tunnel.
One rescue later I had the unit stripped down and back together in relatively short order, so back onto the lake we went, got up to about 2/3rds throttle, all was well and then a dead stop, this time the unit had ingested something, bird feathers I think, with that cleared out, back out again only to have the same thing happen again, at which point I decided to put it away for the day and sail my puffer.
This is the only real problem with the Kirklees lake, it looks quite clear, but there is lots of stuff floating about and a jet drive is essentially a very powerful hoover, I even had issues with my puffer getting stuff wrapped around the propeller, even the Kirklees rescue dingy's electric outboard suffered the same fate.


It is very easy to focus on the negative, on the flip side of all of this, we had a cracking day, lots of good sailing, made some new friends and when the jet boat was working, the new unit appeared to be performing very well indeed, it does however desperately need some spray rails fitting, as at speed it just vanished in a haze of spray.


Video of the first run: https://youtu.be/_B2X4-uHDKw


Alex

[derekwarner]

 ..... "this time the unit had ingested something, bird feathers I think" ...

1. Is this what happened at 1:16 minutes Alex?
2. It also appears to be a very wet at most speeds....
3. Will the orange polymer accept a paint color change?

Derek

[IKB]

Hi Derek,


Answers to your questions:

• The video shows the first run where the impeller came loose, the feathers was a later run
• Yes there is much more spray now, I am putting this down to a combination of a much smoother hull allowing water to slide past and a greater trim tab angle keeping the bow planted
• Yes, the orange PLA parts will take paint, all of the bow is the same material, why, you not like the orange?

Alex

[IKB]

Hi Guys,


So finally getting around to posting an update of a test I did on 23/07/17 at the club lake, same basic setup that I was running at Kirklees, but with the addition of spray rails and a second stern mounted camera observing just what the jet nozzle is doing / reacting, the additional weight of the second camera was making the prototype list to port, so additional ballast was added internally on the starboard side, I got the hull to sit level in the water, but the water line was much higher than I would've ideally liked, but with so much power on tap, this is not really an issue.   


The first run went well, priming is not an issue with the raised water line, the new spray rails did a really good job, with hardly any water on the deck, as opposed to last time, where it was awash pretty much constantly, the new trim tab angle is also working well and in conjunction with the spray rails, the boat sits very flat in the water; the only issues I had were the steering servo mount got ripped off the bulkhead it was attached to, (which is why I end up on the lakeside at the end of the video) due to the larger nozzle requiring more effort to turn it and the unit shifting a greater volume of water, I was also having an issue with the prototype rolling to starboard at full throttle, now this could be down to unbalanced loads, or the spray rails not being exactly the same, (they were just glued on at the side of the lake, five minutes before the test run) whatever it is, I need to have a closer look at the setup, I am also going to remove the second camera, which will give me a better understanding of how the new unit behaves compared to the last one.


I still intend to try my old 500Kv Outrunner with a step up gearbox, I am also going to see if I can improve upon the actuation of the reversing mechanism, as well as redesigning the steering linkage to be a two bar setup.


Lastly, I have been trying for a while now to produce a video which shows both camera views at the same time, after much struggling I gave up on using the free software that I had and after playing with a trial version of, Movavi Video Editor, took the plunge and bought a copy for £35, which is much more economical than paying £20 odd a month for Adobe's cloud system! 


Video of the first run: https://youtu.be/AkTWQfZgA4g


Thanks to fellow BMBS member richald for the photos from the day


Alex

[PeachyPM]

He he, i do like the end of your videos chum! 

[IKB]

Hi Chris,


I may of crashed into the lakeside, but do you notice how my boat still works and is not in two pieces like some I could mention? 


Alex

[IKB]

Hi Guys,


Quick progress update, I am back in CAD, this time working on improving the steering and reversing mechanisms, as the MK2 unit exerts too much pressure on the steering linkages, not to mention if you try to engage reverse with the unt running, the force of the water passing over the reversing flap holds it in the down position, this leads to the first problem, the reversing mechanism in real life would be operated by a hydraulic ram mounted on top of the steering nozzle, so why not do miniature hydraulics I thought? You see RC trucks & diggers using mini rams, how hard can it be?
Well, it's not very hard, but it is stupidly expensive and the rams don't come in the sizes I require, so after much pondering and looking around the internet, I came across a couple of YouTube videos of people using liquid in Lego Technic Pneumatic cylinders to create a hydraulic system, why not? I thought, if I take two cylinders, link them both together and actuate one with a servo, the other, should in theory copy the motion of the first, this is the idea, I have parts coming in the post and I am modifying the 3D printed files to accommodate the new system


More as it happens,


Alex