**Hull.** Shape inspired by the Akula-class submarine. The outer hull is 3D-printed in PETG/PLA: 240 mm diameter, 5 mm wall, a 2 m envelope. It is 16 prints: 8 sections, each split into upper and lower halves. Status: printed, still rough.

**Waterproofing.** The outer hull is free-flooding by design. The dry volume is a separate PVC pressure shell, 200 mm in diameter and about 1.35 m long including its end cone. The generator sits in its own steel section next to it, not inside the dry shell. Design depth 10 m. Status: dry shell in design; sealing is not designed yet.

**Ballast.** Pneumatic ballast: an acrylic tank of about 1 liter floods through bottom vents to dive, and high-pressure air from a small aluminum receiver (8–10 bar) blows it clear. At the surface, a 12 V compressor refills the receiver through the snorkel. The full system also includes a moving-mass trim actuator and control fins. Status: in progress, not tested. The first test is on a bench, and the boat does not dive in it.

**Propulsion.** Two counter-rotating propellers, each driven by its own brushed DC motor (Class 795, 12–24 V). Status: design; parts not yet bought.

**Sonar.** ARGUS-6, a rotating active sonar built from scratch, using 200 kHz piezo transducers (HC200E27TR-1). Status: in progress. An early single-channel test board showed a repeatable return in a bucket test; range not confirmed yet.

**Power and charging.** Batteries: two Li-ion packs (21700 cells, 576 Wh nominal together) plus a LiFePO4 backup pack (960 Wh nominal). For charging, I'm experimenting with a small two-stroke gasoline engine (Zenoah G260PUM) driving a water-cooled brushless motor built for electric hydrofoil boards (Maytech MTI85165, KV200), which works as a generator. It never powers the loads directly. It only charges the batteries, through three separate charge channels. On the vehicle it would run only at the surface, breathing through a snorkel, with engine heat going into the water through a copper keel-cooling loop. Status: the engine-generator setup runs on a test bench in my apartment. So far it makes about 400 W electrical (about 25 V at 12–15 A at most). Under load, the generator's back torque seems to hold the engine's RPM down, so next I'm adding a belt reduction between the engine and the generator. It is not on the vehicle yet, and the generator control boards are specified, not built.

**Electronics.** Main computers: a BeagleBone Blue for real-time control (ArduPilot) and a Radxa 4D for ROS 2 and vision. Custom PCBs designed in code with JITX, including an STM32-based power distribution board. Nodes talk over CAN. Communications: Wi-Fi and LTE at the surface. Underwater, an acoustic link (specified) and a short-range optical link (in design).

**Software.** Planned: ArduPilot (ArduRover in boat mode) on the BeagleBone Blue for real-time control, ROS 2 (including power and battery management nodes) on the Radxa 4D together with vision, and a Cockpit-based ground station. Status: planned, not running on the vehicle.