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.
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.
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.
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.
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.
That's a gorgeous print out.
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.
How do you find the eSun Filament?
Umi_Ryuzuki - I modeled the Graupner unit as closely as I could, I have made a few minor tweeks to better match my setup, but the stator fins are there and I get a nice straight flow from the nozzle. Interesting what you say about the four blade impeller, I have also found this in my trials, what method are you using to create your impellers / housing?
I was building them up and then casting them out of polyurethane.
Your impeller designs are interesting, with the curved leading edge.
I seem to recall that most jet ski and other impellers are more squared off.
Take a look at how the rc jet sprint boat guys have built up their pumps.
I look forward to seeing more of your results.
First a quick reply, Umi_Ryuzuki: My impeller design very closely follows the original Graupner one, as blade design is something I know very little about, the one I copied had a curved leading edge, just like you see in the photo.
First run, brushless motor with 11.1v Lipo, initial lakeside pump prime tests looked promising, but when sent to open water, the rpms flat lined in a very disappointing way. At first I suspected a loose coupling or impeller slip, brought back in for investigation.
Third run, just to prove that the jet unit was fine, I swapped back to the original 555 on 14.4v Nimh and the results were much better than the last trial with this motor, the twin blade impeller being easier to turn, let the motor achieve a higher RPM and therefore pump more. We did notice that a lot of water seemed to being pumped for not a lot of go, surmising that the boat was sitting to high in the water, I shifted the batteries back to point the nozzle down and hey presto, instant speed gain.
Fourth run, happy with progress so far, except for the flat Lipo, I was a bit stuck, you see, I am in the middle of changing from Tamiya connectors to XT60, the Lipo having XT60 and the Nimh loom having Tamiya connectors. Fortunately a fellow club member had some choc block of suitable size to hand and I was able to bodge the Nimh loom onto the brushless ESC for a full power test. This was fine, except I was now running at 14.4v, instead of 11.1v, giving a top end of 27360 RPM on my 1900Kv motor, nearly 8000 RPM above what I actually needed, oh well, this is what testing is for! Into the lake it went and took off like a scalded cat, on the plane, with only the back fifth of the hull in the water... I am going to call this a successful test.
So, "what now?" I hear you ask, well, as much fun as this has been, as I originally stated at the beginning, this has been a tangent of my actual project, namely, an RC Alvis Stalwart. I will carry on working on the design of the drive unit, with a view to printing my own, from scratch, in ABS.
Glad you've finally had a successful test!
It must have been pretty nippy lakeside today I'm guessing it must have been one of the diehard scale chaps that came to your rescue!
Very impressive stuff, I would love to see a video at some point. Out of interest are the 3d printed parts available to buy on shapeways?
Sorry, the parts are not available to buy on Shapeways, I have done this all very much for my own amusement and although different to the original Graupner units, they are still a complete rip off, so not sure where I would stand legally if anything did come back at me.
I tried to take a video using a GoPro attached to the top of one of my other boats, unfortunately, the POV is to low to get any good footage, next time I down the lake, I will try and get some better video and post in this thread.
Great pic Alex, although for a moment there I thought your Go-pro was on fire!
Next stage is under way as I type, I have split the jet housing into two halves, as it won't fit in the printer as one piece... well, it will, but not in a nice way. So with this I plan to print and build a complete jet unit from scratch, without any bought in printed parts, more updates to follow soon.
Finished the first stage of my home print jet drive, need to include some tabs / lugs to help locate the two halves, but apart from that, I am really happy with it.
IS that ABS or PLA?
Good, ABS will take paint well as long as its primed properly.
True, and would be perfectly acceptable for a display model or gentle use, but I know IKB and I've seen how he handles his boats! only joking Alex. if you've got the option of ABS I'd personally go with it, the cellulose primer you have at work will bond nicely with it.
Another new year, finally got back into all things boating for the first time this year and I have to be honest, it has not been the greatest start to the year.
Down at the lake today, the conditions were good, bit cold and windy, but the sun was out, so decided to take the prototype for a spin and capture some clearer video. First run was good but ended abruptly when part of the intake grill broke free and was ingested by the jet drive unit... not a pleasant sound But on closer inspection, no damage had been done, so on to the second run of the day, which... well, I'll let you all see for yourselves, thanks to my good friend, "PeachyPM" for waving the camera and enthusiastic commentary.
So, with waders donned, the boat was saved and it is currently drying out on the radiator, if I'm lucky, some of the electronics might survive!
But it was not all bad, the first sail of my second hand Yorkshireman tug went much better than expected.