How Nemo is built
Nemo is a 3D-printed underwater robot built around one clear tube. Here's the thinking behind it, and how each part does its job.
Five principles
- Every version swims. Get it in the water, learn, then improve. Every version is a real, swimmable step.
- Built for the AI era. The hardware is chosen to think on board: a computer with headroom, a camera worth learning from, and room for an AI accelerator when it's time.
- A long view. Every decision leaves room for where Nemo is going: autonomy, mapping, deeper water.
- Affordable on purpose. Hobby motors, a Raspberry Pi and a desktop printer, not marine-grade price tags.
- Printable and easy to get. If it can be 3D printed, it is. Everything else comes from common, easy-to-find parts.
One tube, two rings, four rails.
Everything that has to stay dry lives in a single clear tube, 90 mm across and 330 mm long. Each end is closed with a plug cap sealed by two O-rings. Water pressure pushes the caps inwards onto the tube, so the deeper Nemo goes, the tighter the seal gets.
Two printed rings slide over the ends of the tube, and four C-channel rails tie them together. Everything else, from the thrusters to the handle, floats and skids, slides onto those rails.
Every connection is the same: a 20 mm tongue in a 20.6 mm channel, held by a single M4 bolt. Small ridges inside each channel hook behind notches in the parts, so lifting and thrust loads go into the plastic rather than the bolts. The rings also press on the caps, so the frame doubles as the thing that keeps them in at the surface.
It's all PETG, printed on a desktop printer with no supports. The longest rails only just fit, running corner to corner across the bed.
Eight thrusters, six ways to move.
Four horizontal thrusters sit in the corners, angled at 45°. By mixing their thrust, Nemo can go forwards and back, slide sideways and turn on the spot. Their thrust lines sit about 235 mm from the centre, so turning is strong and precise. Four vertical thrusters lift and dive, and pitch and roll the body by working against each other.
The ducts follow a classic marine shape, the Wageningen No. 37, which works almost as well backwards as forwards. The propellers were designed for the job in OpenProp. The motors run wet: water floods and cools them, so there are no shaft seals to leak.
| Horizontal | Vertical | |
|---|---|---|
| Count | 4, at 45° | 4 |
| Motor | A2212 | 1609 |
| Propeller | 60 mm, 2-blade | 40 mm, 2-blade |
| Thrust each | ≈22 N | ≈10 N |
Thrust figures are design-model estimates, to be confirmed on a bollard test.
Floats if in doubt. Rights itself, always.
Nemo is trimmed to be very slightly buoyant, about 30 g. If the power or the link fails, it drifts gently back to the surface on its own.
Stability comes from where the weight and the buoyancy sit. Two foam floats ride on the top rail and steel ballast sits low on the skids, so the centre of buoyancy ends up about 15 mm above the centre of gravity. Tip Nemo over and that offset pulls it back upright, with no motors needed.
3D prints trap air in their infill, and that changes as they soak. So the final trim is set in the water, not on paper.
A stack that slides in.
All the electronics sit on one printed stack that slides into the tube. From front to back: the camera, then the onboard computer with the flight controller above it and the power switching below, then the battery, then the motor controllers and a rack of connectors.
The flight controller, a Pixhawk running ArduSub, reads its motion sensors and the depth sensor and turns commands into eight motor outputs. A Raspberry Pi links it to the surface, streams the camera, and is where Nemo's onboard intelligence will grow. Two four-in-one controllers drive the eight motors.
The battery, nine 18650 cells, is the heaviest single block. It sits in the middle of the tube, close to the centre of buoyancy, where its weight balances Nemo rather than tipping it.
Eyes, lights and a feel for depth.
A small camera looks out through the front window. Four 5 W LEDs, potted into lobes on the front ring and angled slightly outwards, light up the scene. Open channels behind them let the water carry their heat away.
A pressure sensor on the rear cap measures depth to within a couple of millimetres. That's what lets Nemo hold its depth by itself, and it's the first step towards knowing where it is.
A thin line to the surface.
Six cable glands in the rear cap carry everything in and out: the tether, the depth sensor, and one cable for each thruster corner. The front two also feed the lights.
The tether carries live video up and commands down, spoken in MAVLink, the common language of drones and robots. It's clamped to the frame with a slack loop before it reaches its gland, so a tug pulls on the frame, never on the seal.
As Nemo learns to think for itself, the tether becomes a way to check in rather than a way to steer. Read about the software.