A little over a year ago, on a particularly drizzly day, I had a wild idea: what if I could 3D print in place my very own sea scooter? I wanted to challenge myself with two key design constraints you won't see anywhere else: the entire hull must be a single, one-piece print, encapsulating all the electronics and a battery. And, crucially, the hull must be hermetically sealed, with no holes for drive shafts or electrical connections. This project would take me 500 hours of design, building, and testing.

Overcoming the Challenges

The first challenge was a big one: how to make the 3D print completely waterproof. I built a pressure rig to test different print settings and coatings, eventually settling on ASA plastic with a flexible epoxy clear coat. This combination held up to four atmospheres of pressure, which was exactly what I needed.

Next, I had to figure out how to drive the propeller without any holes. My solution was to use a magnetic gear. The gear consists of a two-pole diametrically magnetized inner rotor, an iron-filled concentrator built into the hull, and an outer rotor with eight magnets that also has an iron backing. The motor sits inside the hull and connects to the inner rotor, transmitting power to the propeller on the outside through the magnetic field. It’s a completely contactless drive system.

Another challenge was controlling the scooter without a physical throttle. For this, I used magnets and Hall effect sensors. I designed a wireless throttle and a master on/off switch that sense the proximity of a magnet on the outside of the hull. I even managed to incorporate this clever mechanism into the scooter's handles as it was being printed.

Powering and charging the scooter was my next hurdle. I built a custom 26V lithium-ion battery pack using eight 26650 cells and a compact battery management system. I designed the pack to fit snugly around the drive motor inside the hull. For charging, I used two off-the-shelf inductive power transfer circuits in parallel, which allowed me to wirelessly charge the battery at about 1 amp.

The Final Build

With all the subsystems designed, the final challenge was to fit them all inside the hull and print the entire thing in a single, continuous process. This required some serious CAD work and careful planning. The plan was to pause the print at specific points to insert the electronics and hardware.

First, I paused the print to insert the iron bars for the magnetic gear's field concentrator. Next came the "super widget"—the entire drive system, which had to be carefully lowered into the hull and secured with tabs and screws. I then inserted the wireless charging assembly, which I connected to the battery pack and glued into place.

There were a few tense moments during the printing process. At one point, the weight of the drive system caused the build plate to sag, but I managed to save the print with a manual Z-offset adjustment.

In the end, it worked! I had a fully assembled sea scooter prototype, still attached to the print bed, that was running smoothly. After giving it a coat of epoxy, it was ready for its maiden voyage. The water was cold, but the sea scooter was a success. It moves through the water, logs data, and works exactly as I'd hoped. While it may not be a speedboat, it works! And that's all I was hoping for.