M-Jet 60 - 3D Printable Jet Drive for Electric Surfboards 3D print model

Hi, I bought your jet propulsion data and am currently planning a jetboard. I want to use three jets to make the board very powerful. For the battery, I plan to use a 12S Li-ion 44.4V pack. However, I’m unsure about the motors. RClifeon used SSS 56123 500kv motors to build a fast board, but it runs quite inefficiently. He mentioned that with his 44.4V 16Ah batteries, he rides for an average of 3 minutes, which means a consumption of around 14kW! Especially considering that my Jetsurf board with a 100cc engine reaches around 60 km/h with approximately 8 kW. I would like to build the board more efficiently, so I'm considering using 360kv motors. The question now is whether I can still reach my desired speed of 50 km/h with these 360kv motors, which have a no-load speed of 16,000 rpm. Have you ever measured the jet velocity? I’m concerned that while the 360kv motors might generate enough thrust, the jet speed may not be high enough. Do you perhaps have a completely different motor recommendation for me? Additionally, I’d like to know if the jet propulsion system is fully waterproof. I’m considering using a shaft seal instead of the O-ring. Best regards, Jan

Hi, there are multiple considerations to be taken into account. 1) The 360KV motors will indeed run more efficiently than 500KV motors, but they will also produce a bit less power output. 2) The M-Jet 60 on 44.4V powered by sufficiently large 360KV motor will achieve around 20kg static thrust at approximately 40kph exit velocity. Such setup will likely consume around 4000W per single pump, so 12000W in total. 3) Despite using 3 pumps, the exit velocity will be limiting, and I don't think 50kph is achievable. RCLifeon did 44kph with higher KV. I have some different options to suggest. The M-Jet 60 was developed in 2021, and since then the M-Jet designs advanced. The M-Jet 30 and M-Jet 35 are designed for RC boats (30mm, respectively 35mm impeller diameter), but they have features which make them more efficient. You could buy the M-Jet 30 or M-Jet 35 and scale it up to 200%. There are many problems arising if you attempt to change scale of any M-Jet design. All fastener sizes, bearings, shafts, seals, motor mount, servo mount all become incompatible and possibly generating non-existing requirements for dimensions of these parts. Solving this requires a lot of additional effort. The M-Jet 30 and M-Jet 35 come with a STEP file which includes all parts *except* for the first stage and second stage. If you work with CAD, you might be able to modify some dimensions for bearings to make it work. Eventually this can result in a better pump than the M-Jet 60 and achieve your goal. Stepan

M-Jet 30: https://www.cgtrader.com/3d-print-models/hobby-diy/other/m-jet-3d-printable-jet-drive-for-rc-boats M-Jet 35: https://www.cgtrader.com/3d-print-models/hobby-diy/other/m-jet-35-3d-printable-jet-pump-for-rc-boats

Thank you for your response. I’ve given a lot of thought to finding the best solution to quickly build a functioning board without extensive testing. If I use three M-Jet 60 units with either 500kv or 360kv motors, I’d certainly have a working board. I also considered scaling one of your jet drives or designing one myself. CAD is not an issue for me as I’m a mechanical engineer. However, if I scale the M-Jet 30 or 35 to 200%, there are no existing reference values for suitable motors since the same RPM as before scaling is still needed to achieve the same jet speed. Ideally, it would make sense to develop an 85-90mm jet specifically for a surfboard that operates with a power output of around 10 kW, similar to Jetsurf’s 90mm system. However, that would require extensive testing again to determine the appropriate impeller pitch, nozzle diameter, and suitable motor. The clear advantage would be the superior efficiency.

I see. Glad to meet a mechanical enginner. Then calculating appropriate power and RPM for an upscaled pump won't be a problem for you. When scaling up, there might be multiple options to choose from but what I set is constant thrust to weight ratio. If you do the physics, you'll find that RPM scales with (-0.5)th power of scale, power input scales with 3.5th power of scale, torque scales with 4th power of scale, thrust scales with 3rd power of scale and exit velocity scales with 0.5th power of scale. When calculating RPM and consequently motor KV, remember that loaded RPM drops to 80-90% of the nominal Volts*KV value. Reference points (without acetone smoothing): M-Jet 30 25000 loaded RPM 1300W shaft power 5.8kg static thrust 19.4m/s exit velocity; M-Jet 35 20000 loaded RPM 1300W shaft power 6.8kg static thrust 19.6m/s exit velocity. You might ask why the 35 is better - at such small scale, boundary layers on all surfaces are disproportionally more important than at bigger scales. The 35 is bigger which gives more space between the walls where the flow can develop without being hindered by wall friction. When upscaling to 200% this won't be as relevant, and both 30 and 35 might perform the same if both are scaled to 60mm. Of course, a single 85-90mm pump would perform the best.

Thanks for the info! I think the M-Jet 60 is already a solid foundation. I’ve played around with it in my CAD program and made a few adjustments, such as separating the stator and nozzle and thinning the stator fins since I might print it in aluminum. Additionally, I removed the two-stage setup and am using only a single stage. I'm leaning toward scaling the jet up to 85mm and adapting the impeller to fit the motor I plan to use. An 85mm jet has roughly the same cross-sectional area as two 60mm jets. The MP83100 8kW water-cooled outrunner motor seems like a candidate. It’s fairly affordable, though it’s questionable whether it can sustain 8kW continuously. My plan is to use a 16S battery system, around 60V. This setup would only be worthwhile, of course, if an 85mm jet offers significantly better efficiency than two 60mm jets. If the 60mm jet could be optimized with the SSS 56123 motors at 500kv or 360kv to generate 30kg of thrust at 4000W, that would be an attractive solution as well. I might need to dive into CFD simulations to refine the design further. Do you think it’s possible to make the M-Jet 60 significantly more efficient by better adapting the impeller to the motor, thinning the stator fins, and optimizing the nozzle for the motor? Could a 25% efficiency improvement be realistic? I also wonder why your stator fins run the full length to the end rather than just a short segment, like in more traditional setups such as Jetsurf and Jet Ski designs. The M-Jet 35 seems quite different in that regard too.

Alright, sounds good! It is certainly possible to make the M-Jet 60 more efficient by some modifications, reducing vane length is a good idea. 25% is a lot but if you scale it up it should be more efficient from the start already.

Hello, I'm thinking about building an electric surfboard, and I'm wondering if 1 water jet is enough or if two is better, like the rclifeon video about his electric surfboard (https://youtu.be/YsDMwh7Mw-8). He's using two of these but apparently his 500kv motors are too high for 48v because of overheating and bad efficiency.. You've written on the PDF file that the maximum unloaded RPM should be 20k rpm, but is it because of structural limitations on the propeller or because of the water not going through the impeller at this speed ? Because I wanted to print the propeller in aluminum via PCBway SLM, so maybe I can use a higher kV motor to have more thrust ? And so use only one water jet and save money. My plan was to use a sss motor 56123 250kv (or more depending on your answer) with a 72v battery pack. You've also written that a 14k RPM you get 20kg of thrust, is it enough for a jet surfboard ? Thanks if you read this it would help me a lot ! Hope my English is ok ;)

Hello, Using just one M-Jet 60 will not be enough to get you going on an electric surfboard. You need 2 to get up to speed. The maximum unloaded RPM is suggested as 20k to protect the motor. Choosing higher KV and therefore higher unloaded RPM will lead to overloading the motor, because the pump demands a lot of torque. Yes, 500kv on 48V is quite a lot and the setup is probably not running the most efficient way. If you get the impeller printed in aluminum, I think it can technically survive even more RPM, but the motor will not be able to provide enough power. However I think a 56123 250kv on 72V is a great setup, 18000 unloaded RPM is perfect and the motor will have enough torque. You'll just need two pumps. Stepan

Thanks a lot for your good and fast help ! I didn't thought it was to "protect" the motor.. but it's interesting and so I checked your YouTube channel, you're doing a really nice job and it helped me understand things I didn't know or understand correctly. But, even if my motor can pull 6500w continuously and 7800w max in 72volts (I think) to avoid overAmps load on the motor, but it's still a lot, I mean I think, and it's more than rclifeon Wich is 3800w continuously and 6500w max. Does this allow more torque ? So maybe the ability to spin faster the prop, without overheating? I hope I doesn't bother you much because I'm still learning about all of this .. Again massive thank you

Nice!

Hi, M-Jet looks great. I'd like to use it as a bow thruster for a large, though very light, vessel. 20kg thrust would be adequate. A commercially available solution (https://www.jetthruster.com/webshop/-c52919289) runs a single jet through a Y valve to allow it to move the bow in either direction. Are you aware of any such implementation with M-Jet? I wonder how much thrust is lost as a result of the jet having to be routed through 90°?

Hello, I haven't designed anything similar to the solution you posted. Because the pipe diameter is still fairly small, I assume there must be quite some pressure loss both in the pipe and in the 90° elbow. Their solution features a centrifugal pump, which generally produces higher pressure difference at a given flow rate. Thanks to the centrifugal pump they are probably able to overcome the pressure loss in the pipes. However, the M-Jet 60 is an axial pump, designed for lower pressure differences. Attaching any pipes to it might result in poor operation.

Thanks for your prompt reply. I'll investigate further. Two thrusters, facing each direction, with limited pipe work, maybe an option...but one I had hoped to avoid. What immersion depth is required to allow the pump to be primed?

Hi. I am interested in having 2 of this motors installed on a big boat 8000kg of weight. I want it to be able to achieve a speed of 10-15 knots. Do you think i should have the dimensions increased or the mentioned 20kg of thrust would be sufficient? Thanks

Hi, 8000kg of weight is about 80 times heavier than what this pump can push. So unfortunately this is not a good choice for your purpose:)

Hi, thank you for your immediate reply. I will try to find some other solution. Congrats to your great design.