JetSUP: 3D-printed boat lower unit, electro-powered SUP

Once, I recalled having a massive electric motor. I had bought but never used it, and I decided the time had come.

These guys haven't met each other. But I'll make sure that they will be friends :)

I read a manual for the motor that said it consumes 120 Amps. After a brief thought, I bought a 150A ESC that could be delivered to me quickly.

It was a waterproof Flycolor 150A boat ESC. The math summed up in my head: a 150A controller could power a 120A motor, but I was wrong. Why? You will also see it soon.

By the way, the complete process is here (jetsurf.boat):

⬆️ You can watch it in action.

Definition of done

The system is simple:

• A hand throttle to control the motor's speed.
• The upper unit (dry unit) is a box with electronic components that shouldn't be exposed to water.
• The lower unit is a motor and propeller submerged in water.

It should be straightforward, with three simple components. But would it be?

Hand throttle

There are many ways to control an electromotor. I decided to use the most sophisticated approach I could create: a Hall sensor and a magnet.

I made this sketch in Fusion360:

The magnet is placed in a slot on the button. A few rails hold a few springs to push the button back. The Hall sensor is placed on the back of the case.

To avoid interference with water, the hall sensor is coated with epoxy resin:

Assembled hand throttle:

The theory is quite simple:

• ESP32 sends 3.3 volts to the Hall sensor
• Based on the distance to the magnet, the Hall sensor returns 1.6-2.2 volt back to ESP32

The water test showed no issues, so I can proceed with the following parts.

Electronic components schematic

I have this schematic, which has two parts: one submerged and one above water.

• I used two Li-Po 6S batteries.
• The electricity flows from them to the electronic speed control board, which powers the ESP32.
• The ESP32 sends 3.3 volts to the Hall sensor and receives 1.6 to 2.2 volts back, depending on the proximity of the magnet.
• Based on this, the ESP32 determines the speed and sends a Pulse Width Modulation (PWM) signal to the ESC.
• Finally, the ESC starts rotating the electric motor.

3D printed propeller. 3 Blades, 30 degrees pitch

To be honest, I haven't done any math to calculate the number of blades and pitch. I just made a cool-looking sketch and printed it.

Lower Unit #1

For the lower unit, I was inspired by an airplane turbine. I know that water and air are not even close by parameters, but who cares 😁

After a few hours (days) in Fusion360, I came up with this:

It will be placed in SUP's hydrofoil slot:

Lower Unit assembly stage

Step 1: add a motor and route cables:

Step 2: attach the ESC:

Btw, the speed controller is waterproof. I know that it isn't meant to be underwater, but I am really concerned about overheating.

Step 3: Solder cables

I've decided to transfer as much current as possible without overheating wires. So I bought 2 AWG wires.

Step 4: Seal connections with silicone sealant.

Step 5: Test

After a sanity check, I decided that it was time to make an open water test.

Open water test #1

After a short process of attaching the lower unit to the SUP, I was ready:

In a moment, I went into the lake:

But nothing happened.

The motor's shaft just made a propeller's slot completely round, and that's it.

It went from this:

To that:

But luckily, I had designed a slot for a nut, so I could easily fix it.

After a few minutes of adjustments, the sup was ready for the second attempt.

The second try was even worse. With three tiny jolts, I saw smoke from the lower unit:

The Lower Unit #1 failure

It was obvious that I had overloaded the ESC. Why? It was burnt out.

When I say obvious, I mean it:

Lower Unit #2

After a few days of calling myself dumb, I realized that I could try one more time. Unfortunately, there was no option to buy a more powerful ESC. It takes a long time to deliver it, and it costs a lot of money. I decided to go minimal and made this:

A small propeller, two blades, 10 degrees pitch. Should work 😁

After re-printing and re-assembly, I ended up with this:

By the way, in that photo, I was DUMB again.

NEVER TEST THE MOTOR WITH THEATTACHED PROPELLER. NEVER.Especially if blades are facing anyone or anything.

That test went smoothly, but later, I made a fatal mistake. I tested this configuration on a maximum throttle. And that happened:

It could have injured me or even kill, but luckily, it just destroyed my table.

I learned that lesson the hard way.

Open water test #2

After a short preparation, I was ready to go:

And finally, it worked:

Afterword

To be honest, I tested the last setup a few times, and on the third try, I burned out BEC on the second ESC. After killing 2 of the same kind, I see that the original math doesn't work.

Next time, I will try to use something like this 350A controller to drag that mass.

Was it obvious from the very beginning?

Maybe 🙂

Thanks for reading.