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Aug 16, 2019, 12:17 PM
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Build Log

1:10 Jetsprint with 3D printed 2-stage pump


Hi guys, I would like to share this project with you. This is my scale jetsprint replica.

RC Jetsprint Boat with 3D printed Jet Pump (2 min 11 sec)


I have built some jetboats before, my first jetboat had a KMB 28mm, but I wanted better performance so I bought a 3D printer and started to design myself.

I see many single stage jets on this forum and I have also designed some single stage pumps but this is my first 2-stage, and the performance difference is just huge.

-27mm 2-stage produces the same static thrust as a 30mm single stage (I have measured it)--> Saves space, gives better scale look

-Primes much faster, in the video below you can see how quickly it catches the water again after the boat jumps (Btw the jumping is a problem I dont know how to overcome so far)

-Handles better, I dont know how to describe that, but it feels completely different than a single stage pump

By the way, I have tested and measured various stators during the development and have found an interesting thing:

The larger stator outlet you use, the more static thrust you get without increasing the amp draw. I would love to understand why this happens in terms of physics. However, the pump handles worse, primes later and I think it decreases the top speed (unfortunately I cant measure that).
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Aug 16, 2019, 11:21 PM
TTN
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Looks awesome! I really like what you've done with your printed mold. I'm lazy and just go for the printed hull approach but I can see the advantage of having a fiberglass hull - strength and low weight?

Regarding handling: It looks like you're not far off it just needs to be trimmed: What I mean by that is, print yourself a set of nozzles with the exit nozzle pointing upward 1 degree, 2 degrees, 3 degrees, 4 degrees. Start with 1 degrees up trim, drive it around, do some cornering drive it for a minute get a feel for it, shift the battery forward/back a bit if you like to test. Then go up a nozzle degree. You'll find there's a point where it stops bouncing and rides flat.

I'm not sure if you've seen my thread but I also have a 2 stage pump on my boat and it goes great. I definitely notice that the acceleration isn't has hard and off the line as my other single stage boat, but the water exit velocity is a lot higher (throws water almost double the distance). I can also confirm that the 2 stage pump has fantastic priming properties even if one of the impellers is severely damaged (front impeller missing 1 of 3 blades, rear missing most of the other 2 blades and still worked really really well)

Great work printing it in polycarb. Its so hard to print.. I usually just go with either PLA or ABS depending on what I happen to have on hand.

I like how you have made the pump simple and strong - I find my self struggling to keep it simple and not over engineer it haha.
Last edited by TTN; Aug 16, 2019 at 11:49 PM.
Aug 17, 2019, 02:45 AM
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Thanks!
Actually, I have found some inspiration in your project.

I have never tried to print a whole hull so I dont have comparison, but fibreglass is quite simple and cheap to work with. The weight of the hull alone (no electronics, no pump, no rollcage, no cover hatch) was under 300 g, it depends how many layers of fibreglass you use. The strength is really nice, I have even driven across some rocks without any problems. I esitmate the strength to weight here to be much higher than a printed hull. The most difficult thing about the mould is to join the two halfes. You obviously have to lay fibreglass onto the bottom and top part separately to have space to work in. Then you screw the halves together and lay fibreglass onto the joint, which is hard to get to.

Thanks for the tip with nozzles! I have thought about this option before but was not sure if that would have big effect enough, so I will definitely give it a try.

I read your blog since the start, I am glad you went for the 2-stage too.

To the polycarbonate, I have a prusa mk3 with custom built enclosure. The temperature around the print has to be at least 40 degrees, otherwise it warps immediately. I have also found out that the layers easily separate if you print it under 280 degrees.
Aug 17, 2019, 08:44 AM
Jetdrives R Us
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Yes, very nice indeed. Michael and I had planned on making a two-stage pump as a continuation of the 3D FJD project. The first stage keeps the second stage loaded so (as you said) it keeps it primed. I'm interested in how you are measuring static thrust? We've found that a larger stator opening lowers the thrust.

And I am interested in if the suggestion TTN gave you on keeping the boat from hopping (to tilt the nozzle upwards) works. The commonly accepted method is to trim the nozzle down to keep the nose planted, so this is interesting indeed.

If trimming doesn't work, what does seem to work well is to use a ride plate which (for all intents and purposes) lengthens the hull, or to design a little bit of hook onto the rear bottom of the hull. Or both for 'problem' hulls.

The makers of the Thrasher jet boat also use a one-piece shaft (I've always thought it was so you'd have to buy their proprietary motor) which can be easily done using an outrunner motor. I can see the benefits of that in your design. Good job! Sdg.
Aug 17, 2019, 09:27 AM
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Thanks for your tips.
To measure the thrust, I put the boat in a pool and tie the stator to a rope. The rope leads to a 3d printed lever which pushes on a scale. It should be fairly accurate, I think the inaccuracy cant be larger than 100g.
I have actually tested various stators with my previous boat with single stage pump, but there is no reason why this one should work differently.
With 30mm pump, I have tested exit diameter from 15 to 20 mm.
The 15mm produced 1900g and the 20mm 2400g thrust, all with about 30 Amp draw on 4S.
I am not sure why my results are different than yours, it might be because of the specific combination of the impeller, pump and motor.
I would be very interested to see which pump to nozzle diameter ratios have you tested and what were the results.
Last edited by Steven_M; Aug 17, 2019 at 09:33 AM.
Aug 17, 2019, 10:50 AM
Jetdrives R Us
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As a rule the more thrust you're producing, the higher the speed. That is why the question. Yes, a fish scale is a good way to get those measurements. I am only repeating results that were gleaned early in our project. Maybe Kayaker will speak on this with more first hand information.
Aug 17, 2019, 01:13 PM
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Quote:
Originally Posted by sundogz
As a rule the more thrust you're producing, the higher the speed.
This seems obvious at the first glance: The drag of the boat increases with velocity squared. As soon as the drag equals the thrust, the boat stops accelerating. Therefore, as you said, more thrust=more top speed.

This definitely works for prop boats.

But when talking about jet drives, I have an idea that makes me doubt about this rule, let me explain:
The velocity of the water leaving the nozzle (I mean velocity aganist the boat, not total velocity) is always the same, no matter what speed the boat is going: lets call this the exit velocity.

But what happens when the boat achieves the same velocity as the exit velocity is?
I think the boat loses the thrust. (I might be wrong here)
It would make no sense, if at some point the exit velocity was lower than the boat velocity. Then the water stream would go forwards instead of backwards.

However, if you decrease the nozzle diameter, you increase the exit velocity, so the point where boat velocity=exit velocity=zero thrust becomes higher, therefore you should get more top speed.

To solve this problem, it would be necessary to clarify, if decreasing nozzle diameter and therefore increasing exit velocity increases thrust or decreases thrust.
I must say, my measurements might be wrong and I do not want to argue with your results, I just hope this theory is understandable.
Aug 17, 2019, 02:15 PM
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? I think you are saying that you can decrease nozzle diameter and that will increase velocity/thrust (speed) up to a point - after which you will lose velocity (thrust) and with it, speed. I agree with that. But if that's not what you said, then my apologies. This is what makes sense to me anyways.
Aug 17, 2019, 03:16 PM
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Ok, I see that what I have written is confusing, sorry for that.
The main point was that the boat can never travel faster, than the water leaves the nozzle. The pump always has to accelerate the water backwards in order to propell the boat forwards.
Decreasing the nozzle diameter increases the velocity of the water stream, therefore the top speed limit of the boat is higher.
The question is, if smaller nozzle and higher exit velocity means higher or lower thrust (force).
I will probably try to measure that more precisely again and use more different nozzle diameters.
Aug 17, 2019, 07:03 PM
TTN
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Quote:
Originally Posted by sundogz
And I am interested in if the suggestion TTN gave you on keeping the boat from hopping (to tilt the nozzle upwards) works. The commonly accepted method is to trim the nozzle down to keep the nose planted, so this is interesting indeed.
Its counter intuitive but it's giving me results. I talked to a marathon jetboat race driver and he mentioned that whatever the boat, they usually settle at 3 or 4 degrees of upward trim which I thought was interesting, since I found myself landing on 3 degrees too. No idea why it is like that - at least its a easy thing to test.
Aug 17, 2019, 10:49 PM
Jetdrives R Us
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Quote:
Originally Posted by Steven_M
Ok, I see that what I have written is confusing, sorry for that.
The main point was that the boat can never travel faster, than the water leaves the nozzle. The pump always has to accelerate the water backwards in order to propell the boat forwards.
Decreasing the nozzle diameter increases the velocity of the water stream, therefore the top speed limit of the boat is higher.
The question is, if smaller nozzle and higher exit velocity means higher or lower thrust (force).
I will probably try to measure that more precisely again and use more different nozzle diameters.
Yes, smaller nozzle/higher exit velocity means higher thrust - to a point of no returns where it will start dropping (with the same motor). It will take more and more power to go beyond that point. Think pressure washer.


Quote:
Originally Posted by TTN
Its counter intuitive but it's giving me results ..... - at least its a easy thing to test.
Yes, it does sound counter intuitive, that's why I'd like to hear from another if they get the same results. What's going through mind is that maybe it holds the nose up, stopping the hopping/hammering. Interesting.
Aug 17, 2019, 11:15 PM
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I'd like to add I can't recall whether the driver said 2-3 degrees up or if he said 3-4 degrees.
Aug 17, 2019, 11:16 PM
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Has anyone actually analysed why the boat might be hopping in the first place. Before putting forth their solutions and remedies to rectify the problem. Because the solution invariably lies within the problem. 🤔
Aug 17, 2019, 11:41 PM
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Thread OP
I dont know what the problem exactly is.
According to the advices here it seems that having the nozzle 0 degrees is the actual mistake.
That is also what I will try to fix at first.
Unfortunately I wont be able to run the boat in the next week to test that.
Another problem might be that the boat is scale-wise highly overweight: scale weight would be around 850 g and that is what I was planning, but actual weight ended up being 1150 g. (I have never built a scale boat before so I forgot to count little things like rollcage weight, cover hatch weight, servo mount weight etc...) I dont know if that has any effect on the hopping.
Aug 18, 2019, 11:47 AM
Registered User
Take this with a grain of salt, but back in my stand-up PWC days, porpoising was almost always an issue with too much weight rearward. Try moving some weight toward the bow and see if it doesn't help. If nothing else, it's very simple to try.


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