Technical, Techniques, Hints, and Tips > 3D CAD Design & Printing

Water Jet Drives & 3D Printing

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IKB:
 A few months ago now, I decided it was about time to have a proper crack at a project I have always wanted to do, that of a fully functioning RC Alvis Stalwart, a quick rummage unearthed my old plans for this project from many, many moons ago and a google search for additional reference very quickly unearthed Bob Hinton's article in the July 2011 edition of Model Boats.
One quick online order later furnished me with the article in question, which was a fascinating read as well as a treasure trove of information, as Bob clearly lays out in his article, the trials and tribulations of squeezing a lot of mechanics into a very small space.

Using his article as a guide for parts, as it seemed silly to reinvent the wheel when he had already solved many of the problems and got them to work reliably, I started to search the internet for the component parts. Most of the drive train is based on Tamiya Dual Hunter kits, which are no longer made, but they shared a lot of individual parts with other kits that you can still get hold of, so after a bit of searching, I had all of the land drive components that I required, the water drive parts were another story.

The Alvis Stalwart propels itself along the water using a twin set of Dowty water jet units, in the model version, these were substituted for Graupner Jet Drive unit 2340, unfortunately Graupner have decided to discontinue manufacture of, along with the rest of the range.
Now, there are a couple of places you can get jet drive units from, but they are either too small or way to big, which left me with a bit of a problem, however, one of my regular eBay trawling sessions yielded results, a pair of as new, in the box 2340 units, one swift purchase later had them sitting on my work bench. With eager anticipation I waited for the units to arrive, only to find to my horror, that the impeller housing mating flange on each unit had been badly hack sawed off, rendering the units unusable, fortunately for me, the seller was very good about the whole affair and refunded my money upon return of the items.
This got me thinking, I had a pair of jet drive units, before I sent them back, could I replicate them?
One option would be to mold and cast them in a resin such as fast cast, but this is not always a reliable method for reproducing dimensionally accurate components, another option would be to turn to technology and 3D print new versions.
So this is exactly what I decided to do, before posting the units back, I drew them up in SolidWorks, the 3D CAD package we use at work, then sent the files to be printed at www.shapeways.com

 
A couple of weeks later, my very own jet drive was delivered, (Photo: 3D Printed Jet Drive Parts) I had already designed and machined a basic test bed hull at work during my breaks, (Photo: 3D Printed Jet Drive Prototype) which was quickly assembled and given the all important bath tub shakedown. Below is a link to the video of the first test, apologies for the shaky video.

 
https://youtu.be/CfaUvGJ08gY

 
This test was performed using a 555 motor I had spare in the workshop, a 25Amp Mtroniks ESC and run at 14.4v off two NiMh stick packs.
The results looked promising, especially as I was not using the standard impeller design, but a revised four bladed version I had drawn.
For some time I have been toying with the notion of buying a 3D printer, I have had parts 3D printed in the past at work for projects and always been disappointed by the cost and  time it took to receive the end result.
Like most people, I do not have a lot of space at home to devote solely to my hobbies, my workshop is a standard 6x8' shed in the garden, but this is more than compensated for by having access to a milling machine, lathe, CNC router and laser cutter at work.
However, as awesome as all the kit at work is, it is not readily available to tinker with at home, but a 3D printer I could make room for, so after several weeks of reading up about different types, models, materials and processes, I decided to take the plunge and buy one for myself. As luck would have it, I came across the model I wanted for sale on eBay, the seller was upgrading to a larger machine and was selling at a very reasonable price. (Photo: Lulzbot Mini)
Being able to have an idea, draw, print and test it all in the same day is amazing, it allowed me to explore the difference in performance between a 2, 3 & 4 blade impeller design easily.
(Photo: 3D Printed Jet Drive Parts [Orange impellers by me, white parts by ShapeWays])
This did lead to a small problem, when I took the prototype to the club lake for open water trials, the performance was some what under whelming, I later realised, that I had the four blade impeller fitted, which was more than likely causing so much drag, the motor was just not powerful enough to turn it at the desired speed. Looking further into the problem, the original Graupner units, which have a two bladed impeller, have an operational speed of 18-20K RPM, where as the motor I had to hand maxed out at around 14.5K RPM, hey ho, you live and learn, but that is what I like about prototype work, you are always discovering something new.
One avenue I have been meaning to explore is brushless motors, which are perfect for this application, so after a bit of reading and asking around at the club, I bought  a basic HobbyKing 80A ESC and a cheap ZTW Black Mantis B3660 Kv1900 inrunner brushless motor from eBay.
With a swap back to the twin blade impeller, just to start with, I am hoping to get back down the lake this weekend for more trials.

 

Umi_Ryuzuki:
That's a gorgeous print out. :-)) :-)) :-))


I can not see inside your tail cone, but don't forget, there should be some

stators, or vanes behind the impeller to "straighten" the flow of water.
Otherwise the water will swirl inside the pipe, and disrupt the flow.
I made a pretty good blender my first attempt at a jet pump.
I also tried a four blade impeller vs a two blade impeller, and burned up the motor and ESC.
I also modeled my setups after the Graupner drives.

TomHugill:

That you for taking the time to post this, I love all things jet drive and this really interests me. Out of interest what was the price from shapeways to print this? I've looked into getting parts made but they were always hideously expensive

PeachyPM:
 :-))nice post Alex, keep up the good work :-))

essex2visuvesi:
How do you find the eSun Filament?
Mine seemed to be quite hit and miss as to the quality (the diameter of the filament seemed quite variable)

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