Vandubbi: a submarine made of plumbing

We built a submarine out of plumbing. It is called Vandubbi, it is a wireless remotely operated underwater vehicle with a live video feed, and we exhibited it at Quark, our campus technical festival, earlier this month. This post is the build story while it is still fresh.

The chassis is PVC pipe, chosen on purpose: cheap, strong for its weight, naturally buoyant, easy to cut with hand tools in a hostel room, and when a design turns out wrong, the failed version becomes spare parts for the next one. The frame is also free-flooding. Water moves through it freely, so there is no pressure hull to implode, and the depth rating is set by the weakest sealed component: the electronics box, the camera, the motor caps. The SeaPerch build manual was our starting point, and by the second frame we had outgrown it into our own design.

Thrust comes from brushless motors at the corners, driven through ESCs, with propellers and motor mounts we 3D printed ourselves and printed caps sealing the motors against ingress. An Arduino reads the radio receiver channels and maps them onto the thruster pairs, so yaw is differential thrust: speed up one side, slow the other, and the frame turns without a rudder. The propellers came from the open hardware community as parametric designs, resized and reprinted until the bollard pull, the static thrust with the frame held in place, was worth the amperage.

The control chain, honestly

The receiver hands the Arduino three channels of hobby-standard PWM: a pulse every 20 milliseconds whose width, roughly 1000 to 2000 microseconds, encodes the stick position. The sketch reads each channel with a blocking pulseIn call, applies a deadband, and drives the motor pairs. Our transmitter reads about 1400 to 1470 with the sticks centered, so the deadband spans 1300 to 1500: inside it the thrusters idle, below it one direction engages, above it the other.

Which means V1 is bang-bang control. Each axis is fully on or fully off, the pilot modulates by pulsing the sticks, and the three sequential blocking reads put a few tens of milliseconds between a command and the water feeling it. Underwater, where everything happens slowly and the vehicle carries its own momentum, this is far less crude than it sounds. Proportional throttle through the ESCs is on the V2 list, and the V2 parts list already carries a 9-DOF IMU and a barometric depth sensor to close the loop properly.

DIY waterproofing

Nothing on this vehicle was waterproof when we bought it. The electronics live in a plastic food-storage box, the kind that normally protects leftovers, with the cable runs sealed where they exit. The rest of the toolkit is the hobby-standard arsenal: silicone sealant on every joint that must stay dry, epoxy putty on every suspect fitting, and self-amalgamating tape on anything cylindrical.

Motor waterproofing is its own subproblem, because a motor must let a shaft spin, and a spinning shaft refuses to be perfectly sealed. The SeaPerch tradition pots brushed motors in wax. Brushless motors shift the calculus: the windings are sealed in enamel and there are no brushes to short, so some builders run them fully exposed and treat corroding bearings as a consumable. Our motors began their careers inside taped plastic bags, which the photo above documents without mercy, and graduated to caps and covers we designed and 3D printed for the purpose, with a dry-and-inspect ritual after every session covering what a printed cap cannot.

The cheap ingress test is older than all of it: seal a dry paper towel inside the box, submerge the assembly, and the towel reports in the morning what your eyes cannot see at the seams.

The numbers

Vandubbi V1 runs for 20 minutes on a charge of its LiPo batteries in still water and carries a payload of up to 4 kg. The radio link has a theoretical range of over a mile in line of sight, and we have tested it successfully to 300 meters. The FPV set transmits live video on 5.8 GHz to a headset, alongside telemetry for acceleration, velocity, and battery status. A focused LED on the nose keeps the camera useful when the water gets dark.

Buoyancy is the actual engineering

The glamorous problems in a submarine are propulsion and video. Buoyancy is the problem that fills the evenings, because a payload that can vary by 4 kg will happily flip a small vehicle that was trimmed for neutral.

Our answer is bottles, and the arithmetic is Archimedes at his most generous: every liter of displaced water buys a kilogram of lift. Sealed bottles at the four corners of the frame act as adjustable ballast in reverse, setting the buoyant force, and moving them trims the vehicle to match whatever the payload does to the balance. The 4 kg payload rating is, at bottom, four liters of bottle. Stability comes from geometry: the frame keeps the center of buoyancy above the center of gravity, and that separation produces a passive righting moment, so the bot self-levels after every disturbance with zero battery spent. We simulated the drag on the frame before committing to the shape, then trusted the pool tests more than the simulation.

Station keeping, holding position against whatever the water is doing, has no autopilot on V1. The pilot is the control loop, watching the FPV feed and feathering the throttles. Closing that loop with sensors is high on the V2 list.

The failsafe involves no code

My favorite design decision is the simplest one. The vehicle is trimmed slightly positive, so when the circuit or the battery fails (a wire, a cell, an ESC), the thrusters stop and Vandubbi floats to the surface on physics alone. A failsafe that needs working electronics to save you from failed electronics is a contradiction; buoyancy has no firmware to crash.

Water versus radio

An honest note on the word wireless. Water attenuates radio brutally, and the attenuation climbs with frequency, so our 2.4 GHz control link and 5.8 GHz video are line-of-sight tools that die within centimeters of submersion. Commercial ROVs solve this with a tether. We wanted wireless, and the price of wireless is depth: practical operation means shallow running with the antennas kept thinking about the surface. That constraint is baked into what Vandubbi is for, surveillance and inspection near the surface, where the tethered commercial options are absurd overkill.

Here are fourteen seconds from a 3 a.m. build session in December, complete with the buoyancy bottle and a printed propeller:

What it cost

We were broke undergraduates with curiosity and access to a pool, which turns out to be a workable amount of resources. V1 was entirely self-funded by the team. Our pitch for V2 lists the full bill of materials at ₹34,850, most of it the FPV set, the 3D printed parts, and the radio gear, and includes the sentence “funds will eliminate the key Jugaad which is often the reason of fail.” I stand by that sentence more with every pool test.

The codes and the 3D printable files for V1 are open source in the public domain. Version two is already on the drawing board, and it will be better, because we now know exactly which parts of version one were held together by tape and optimism.