3D Printed Jet Boat

[3D_Print_'em]

Hi all,

I have been looking for feedback on multiple forums and came across this one and need some more feedback or suggestions for my 3D printed jet boat.
I am making and designing a scaled down electric jet boat for a school project and am focusing on the engineering and development of the propulsions systems. This involves the intake which directs water towards the impeller and is then driven to the end of the jet where the velocity of the water builds. I just need to know if the design I have made would work for this scale boat and what concepts I have to keep in mind while designing the rest of the Jet pump.
Here is a photo of the intake:

The impeller I have designed is driven directly by a brushless motor and is coupled to a shaft which spins the 3D printed impeller. After seeing how others design an impeller, I made the diameter of the shaft increase towards the end of the jet pump to allow the blade pitch to increase and drive the water out faster. Here is my design of the impeller: It is a two-stage impeller in order to reduce the cavitation of the water around the impeller.

Any feedback or suggestions are greatly appreciated,
Thanks

 

[drewkaree]

Looking at what you've got there, if you are doing some testing, I would try fewer bars on the intake grate, maybe 3 if possible. Smoothing of everything will give you better flow as well, not sure the effects on a micro scale like that, but small changes can make a HUGE difference in our boats, so I can see it helping. I've seen acetone used for smoothing out a finished 3D printed product. If you can't access it due to size, if you can spray it in there and work with some Q-tips, that may work for this. Great Stuff expanding foam cleaner is sprayable acetone in a convenient form factor, and I think you can add a straw to the nozzle tip.

Looks interesting, would be cool to follow your project and see the final result as well!

 

[2kwik4u]

@3D_Print_'em Looks like a cool project.

@drewkaree is spot on with smoothing out that transition on the intake. You want as few sharp corners and as few drastic changes in shape as possible. Smooth lines that allow the water to adhere to the surface of the intake tunnel without disturbance to flow as much as possible. Have a look at your fluid dynamics books to see how sharp corners affect localized pressures as compared to sweeping bends. Less pressure drop ahead of the impeller is best. Same goes for the intake grate. You want to keep the "big" stuff out, but the little stuff will likely flow through just fine. I would at least half that grate count (double the spacing between).

You're on the right track for developing pressure, however you're doing it at the wrong spot. The impeller doesn't develop the pressure differential, the nozzle does. The impeller just generates flow against the restriction of the nozzle. The impeller you have drawn there would be great if you needed to pressurize something very viscous like molten plastic, or peanut butter, but water will just flow around it and not pressurize as you think it will. Have a look at a "double pitched" impeller with a single diameter inner shaft. The Yamaha 6CW is designed like this and has been found to be quite versatile across many boats. I would start with that general design, and tweak as needed to develop flow.

The nozzle is where the magic happens for propulsion. You have a constant mass system with a change in velocity across the nozzle. This generates a change in momentum, which in turn generates the force to motivate the boat forward (at least at any speed). The thought experiment I use is to consider standing in a john boat. Now throw an cinder block off the back, which way does the boat move? Is it because the block pushed against the air? Is it because the block was under pressure? It's because you changed the momentum of the block and the resulting opposing force moved the boat forward. Nozzle does the same thing, just with millions of tiny water molecules instead of cinderblocks. Look into the size reduction ratios on existing nozzles and scale down from there. I think our 155mm pumps reduce down to like 75mm outlets. Just over a 4.2:1 reduction in area, so the existing water should be going roughly 4 times as fast as it came in. That will generate the force needed to move the boat forward.

Good luck and keep us posted on what you learn. Lot of engineering topics covered in what is otherwise a very simplistic driveline.

 

[I_squared_r]

I did this many years ago. I started 3d modeling a yamaha superjet and then I thought "why not put a motor in it?" so I 3d modeled it and put a brushless motor in it. It works great, but it doesn't float. I would have to scale the model from 7" to like 12" or something like that. But I had never planned to actually make it work so nothing was parametric in the model and when I scaled it, many things broke, I tried many ways to make it work, but it always failed. I don't know if the software has changed since then because I did this in 2014. Either way, I lost the model a few years back when my Samsung SSD randomly failed.

I did some 3D printing work for a carbon composites guy in Texas and we spoke about building out of carbon, we came up with a layup, and planned on using honeycomb foam core. We worked on it a little, then decided that the juice wasn't worth the squeeze.

You don't need that intake grate, it's just going to impact performance. On my jet ski the entire tunnel is 3D printed. I didn't try optimizing flow at all. But it has a few vanes on it to straighten water flow. I had designed it where the entire pump section can be removed for future changes, but I lost interest when I couldn't scale the model. I bought the impeller online and surprisingly it had tons of thrust. I imagine it would be really difficult to find this impeller, but maybe you can google it. I remember buying an entire pump for RC boats, I took the driveshaft/impeller from it, and put the rest in the garbage because it didn't work for my design. It also has a brushless motor and in the 3D print I had put a hole in the nozzle so to provide cooling water to the cooling sleeve on the brushless motor.

I keep it at my desk so someday I will get interest to finish it. Just paint it and put mat in the tray area. Handlebar. And put graphics on it like a Yamaha Superjet.

So you avoid my problem. My suggestion is to make a list of parts that you want to use and weigh them. Whatever program you're designing should have something to get a estimated weight for your 3D printed part. Add that all together then calculate what displacement you need so it floats.

Second thought, I designed it so that upper and lower decks can be separated. Reason being, its TINY, and the batteries are located in the gunwhales of the tray so they're impossible to remove without disassembly.

 

[I_squared_r]

Honestly, if you really want to design a jet pump like a pro, you're gonna sink a lot of time into it. I would buy a RC jet pump and bolt it on, done. But if you're a glutton for punishment, you can go over to www.x-h2o.com those guys do freestyle competitions and are some pros that focus on maximize jet pump efficiency. But it's not going to too helpful because torque/hp curves are very different between gas and electric motors. Those guys mess around with setting back the impeller 10mm +/-, pump diameters, nozzle diameters, large intake tunnels, pump cone tuning for adjusting pressure, etc. A lot of it is just trial and error swapping parts and seeing how it feels. Your biggest challenge is balancing all that to eliminate cavitation. And a 3D printed tunnel is far from efficient.. The little lines in the 3D printed part need to be removed because air pockets can form and cause cavitation.

 

[3D_Print_'em]

Thank you all so much for the feedback!
I have made some progress and improvements with many versions made.
I smoothed the surfaces in contact with the water like the intake and hull using acetone and put the impeller in a container to allow the fumes to more evenly smooth the impeller.

I then designed the nozzle that compresses and directs the water at high velocity from the end of the jet.
I also made the intake grate 3 instead of 6:

Even though it had fewer lengths in the way of the water flow, the transition from this grate to the intake wasn't very smooth. So using your suggestions I then just designed the intake without the grate and it is a lot smoother.


I am thrilled with the way that it is turning out. I have received most of the needed parts and will work on installing the impeller and the metal shafts into the jet pump.

 

[2kwik4u]

New grate looks good. Much better.

I'm really curious to see how that impeller design works out. My gut tells me it's not going to do much because of waters low viscosity. Every time I've seen an screw like that it has been for extremely high viscosity (almost solids) materials. Things like PVC plastic pellets and shredded lithium batteries. Those screws use that conical center section to build pressure against the side walls for increasing heat input, or dewatering operations. I suspect you won't generate much flow with water due to the ease of it flowing around the tips of the screw.

Another thought, you can use a flame to polish plastic as well as the acetone. Likely a little faster/easier than letting it sit. Practice on a spare waste part though, as there is a big "feel" to the operation.

 

[TeenGee]

@3D_Print_'em
It looks to me that because the cross sectional area of your impeller design decreases it will increase the velocity of the water as the water is pumped. The impellers in our boats increase the pressure of the water and then rely on the nozzle to convert the increased pressure to increased velocity.
It looks like your nozzle has 4 exit holes. How does the total area of the holes compare to the cross sectional area at the exit plane of your impeller? I think you can optimize that area ratio to get the maximum thrust.
I have attached a little light reading on the subject of Water Jet Propulsion design.
Good luck Mr. Phelps.