DIY twin-waterjet tow boogie (3D-printed turbines)

Build, Repair & Mod
1

Still a work in progress, first real run should be next week, but I wanted to start the thread now. Here’s the story so far.

Where it started

A year ago I rode a waterjet ferry (62km/h !) and spent the whole crossing staring at the water blasting out the back. That did it: I wanted to build my own turbine, fully 3D-printed. This is where it got me.

What it is

A tow boogie, two 80 mm waterjets, mixed-flow pump turbines designed in CAD, checked in CFD and printed. DIY battery + PCBs. It’s mostly an engineering project I do for the fun of it.

Board
Board1920×2560 639 KB

CAD
CAD2078×632 175 KB

Turbines and CFD

Two 80 mm mixed-flow pumps, designed in CAD, run in OpenFOAM, iterating on the rotor/stator geometry.

Design point: 4 m/s inflow, 2 N·m on the shaft, ~4500 rpm gives about 100 N (10 kg) per turbine, so ~200 N (20 kg) for the pair. That thrust number is what sized the whole drivetrain.

diag_A1_vectors
diag_A1_vectors1560×720 178 KB

The rotor adds swirl, the stator removes it and convert it back to pressure, the nozzle convert that pressure into jet speed (thrust)

diag_E3_swirl_section
diag_E3_swirl_section1320×600 65.8 KB

I learnt so much about waterjets by designing, simulating and iterating!

Motor and ESC

  • Flipsky 56115 (continuous 2Nm) / 150KV
  • Mini FSESC6.8 Plus

Battery

Home-made pack sized to slide into a 75 mm PVC tube:

  • 10S7P, plain NMC Li-ion, spot-welded nickel strip
  • 3D-printed holders, flower layout (1 centre cell plus 6 around)
  • threaded rods clamp the stack into a rigid cylinder

montage_battery
montage_battery1920×1114 223 KB

montage_xt60_passthrough
montage_xt60_passthrough2032×1179 444 KB

Hull

A Decathlon Olaian kid bodyboard. I CNC-milled the foam to cut the bay for the tube, motors and electronics, then dropped in an aluminium T-slot cradle for the two jets.

CNC
CNC1920×2560 994 KB

Electronics

I made 2 PCBs:

  • a PCB inside the tow handle (transparent PVC tube) at the end of the rope, with two 3D magnetic sensors and an accel/gyro
    • throttle comes from a moving magnet + 3D magnetic sensor
    • second magnetic sensor for a dead man security
    • steering come from a gyro AHRS. The bar tilt drives the yaw rate of the board.
    • Small TFT screen to displays some info
    • 18650 cell + USB-C charging
  • a PCB on the board, UART connection to the 2 VESC, ESPNOW protocol between the 2 PCBs.

Later, I want a small wrist unit, GPS-watch style, so it can come back to me on its own by GPS.

control_bar
control_bar2001×587 101 KB

First trials

First powered test in the pool, one turbine, tied down so I could measure it: about 300 W in for roughly 6 kg of thrust, and the jet out the back is very satisfying. Good first data point, it lines up with the CFD.

essai_piscine_6kg
essai_piscine_6kg1920×1350 270 KB

Next steps

Finish the wiring and sealing, first real run next week (I’m waiting for the PCBs), then find out whether a finless board steered by differential thrust is actually rideable off-axis (I’m pretty sure it is not).

10 Likes
2

Wow. Very cool project!!! How are the series of the battery linked? Simply by pressure? Or is there some hard connection? Interesting design.

1 Like
3

very interesting. I will follow that one…
congratulations!

1 Like
4

Nice !!
Not easy but you will have more thrust (at the start) if the rear jet is below the waterline. You can also think about directing the flow in order to have a directional boogie. god damn it’s awesome ^^

5

Each battery element (1S7P) have a nickel lug on each side. Once assembled with the 3 rods I can solder all those lug pairs together. So no, it does not rely on contact pressure.
But I started a side project to make the same kind of batteries without solders. Everything will be plugged with custom PCB discs in between the cells. The goal is to be able to travel with the empty casing, then buy local 21700 cells and assemble the battery. But more on that later. Once my waterjet tow boogie is working!

6

From my understanding, what count for the thrust is only the flow of water (L/s) and the ejection speed (m/s). Once the water exited the nozzle, everything that happens after does not matter. Ejecting water above the waterline just makes the pump pump a little bit more because it was to pump the water up ~10cm. But it better fitted my design, and the rope anchor point is lower than the trust plane. So this will prevent the board from tilting up.
About directing the flow, I hesitated, but in the end I preferred to stick with something mechanically simpler. No moving part (apart from the shafts/rotors). And both turbines are counter rotating, so there is no torque felt by the board.

7

Great project! I think steering with the controller on the rope has no significant advantages. As soon as you are on the rope you can easily steer with your board. It is very intuitive. Steering needs to be on a wireless remote. +gps following in the best case. Like the other ideas in the project!

8

Battery assembled and first buoyancy test done:

IMG_8728.HEIC
IMG_8728.HEIC4032×3024 4 MB

It’s tail heavy, by design, so that the waterjet is fully submerged at rest. It can roll easily and then it stays upside down.

I temporarily plugged a RC remote to do some tests:

IMG_8730.HEIC
IMG_8730.HEIC2016×1512 1.29 MB

  • Max motor phase current : 30A (each)
  • Static pull force : 23kg
  • Power : I don’t have telemetry yet :frowning:

So far so good!

Next steps:

  • Pool noodle to improve roll stability
  • Custom PCB (arriving tomorrow) for remote control + telemetry
  • Waterproof casing for the PCB
1 Like
9

Looking good! What are your plans for water proofing everything? This makes me a bit worried :grinning:

IMG_4535
IMG_45351206×2622 416 KB

10

Hope it is working out that way.

My guess is, once it starts moving, it will ride with the nose up and suck in air.

For my “normal” boogie I have to balance it nose heavy so it’s almost level at speed.

But maybe with your design it will “suck” itself down.
Interesting.

11

I think that most boogie have a propeller axis way below the rope axis (in Z). So the resulting torque has a tendency to lift the nose.

With this board it’s the opposite, the rope is attached 2cm lower than the waterjet axis. So the torque will slightly push the nose down.

But there is only 1 way to find out: real test tomorrow at the sea!

2 Likes
12

The plastic bag is of course temporary. I’m awaiting my custom PCB to arrive, then I will 3D print a small enclosure and fill it with silicone. The liquid one (A+B) that cures overnight.

Regarding the XT60 I’m hoping it will be fine with just a bit of grease.

1 Like
13

What do you think is missing from bodyboards with standard motors? What is the goal of the project?

14

You’re right that if it was just for me, to take off with my foil, a simple mast + motor + prop would work fine.

But here are my goals:

  • An excuse to experiment with 3D printed waterjets
  • For my kids (5 and 7) to play at the lake without the danger of a spinning prop.

Bonus : without a mast, it’s flat and easier to store / carry around.

1 Like
15

Out of curiosity I did a static pull today (just got a cheap load cell delivered).

With 50% battery and 6000W of power the force/ load is about 60kg. This a single 6 inch prop.

I can get up on foil with 50% throttle which is measured to be 30kg. This is a 32L board, 680 foil but doing a prone start and I’m about 95kg.

I haven’t thought too much about it, I guess static load is important for deep water starts and getting on foil, but once up having a reasonably fast and efficient propulsion is important.

With 22kg on a big board and foil and if you are light weight then should work fine.

IMG_6816
IMG_68161512×2016 1.73 MB

3 Likes
16

Thank you for this test, wow 62kg is huge! But 6000W too! I’m not aiming to have that much pull, but 30kg seems achievable if I increase a bit the motor current.

From my previous eFoil project, I know I could takeoff with ~1800W (72kg + 105L board / 1070cm2 foil). But I never measured the pull force.

This morning I did some first tests in sea water with my kids (with a RC remote, my custom control bar is not finished yet). They had a lot of fun:

20260718_090431_IMG_8746_frame1948
20260718_090431_IMG_8746_frame19481920×1147 182 KB

RC remote (Radiomaster TX15 ELRS with telemetry : RPM, duty cycle, GPS speed, current and voltage):

IMG_8762_half
IMG_8762_half1466×1466 199 KB

As I predicted, the harder you pull, the lower the nose gets, to a point where it sinks (with adults on the body board). I should either move back the battery to change a bit the CG, or raise the rope attachement point. That going to be the next test!

And this is my tow bar with my custom PCB inside:

  • PVC tube + caps
  • 18650 Cell
  • gyro to control the yaw
  • magnetic sensor for the throttle
  • small LCD for both batteries level + speed + link quality

IMG_8760_half
IMG_8760_half1920×1440 283 KB

Firmware ongoing, hopefully next week it’s working without the RC remote.

1 Like
17

I wouldn’t change anything yet.

The riding up happens with speed, not with pull.

Up to 15 km/h my boogie is rock stable no matter how hard I pull. It’s once I get over 20km/h it gets a bit unstable and I cannot get beyond 30km/h on flat water.

Before I added a lot of weight to the boogie that limit was about 18km/h.

1 Like