Build Log: Hobbyist RC Submarine — Ballast, Fail-safe Surfacing, Waterproof Servo Bays and FPV

Build Log: Hobbyist RC Submarine — Ballast, Fail-safe Surfacing, Waterproof Servo Bays and FPV

I started this RC submarine project as a compact 1:12 scale hull to explore ballast control and underwater FPV, with the intention of practical, repeatable systems rather than experimental one-offs. The goal was a reliable sub that I could run in local lakes and pools, carry a camera, and recover easily if things went wrong. I sketched the internal layout first so heavy items like batteries and the ballast tank would sit on or near the centre of gravity, which simplified trimming. The build took a few weekends of bench work and iterative testing in a shallow test pool to validate the basic concepts.

Hull construction used a 3D-printed lower hull bonded to a turned acrylic upper section that creates a clean watertight seam, and I kept access to the electronics through a threaded endcap with an O-ring seal. Battery placement and floatation foam were arranged so the dry weight left a small positive buoyancy, making it easier to add ballast rather than cut weight. I kept a parts list and STL files on my project page at https://watdafeck.uk so fellow makers can replicate the layout and modify it to suit their own electronics and servos.

Ballast was the primary control challenge and I opted for a compact variable ballast system using a small bilge pump combined with a dedicated ballast tank rather than fixed weights, because precision trim is much easier to achieve with a controllable system. The tank sits below the electronics and connects to the exterior via a flood valve controlled by a micro servo that either vents or seals the tank, while the bilge pump moves water back out on demand. I calibrated the system by adding small amounts of lead shot and using a counter-sink of modelling wax to tune static trim, then used the pump to make fine adjustments in the water.

Fail-safe surfacing is non-negotiable when you are running an R/C sub away from a shoreline and I implemented three layered protections: a watchdog on the radio receiver to detect link loss, a depth check using a small pressure sensor set to trigger a surface routine at a safe depth, and a simple current monitor to detect water ingress into the electronics bay. The surface routine opens the ballast vent and runs the pump briefly while centring the control surfaces so the sub returns to positive buoyancy, and the watchdog will trigger the same routine if the radio link drops for more than a second. I also tested the system tethered to a float during early trials to make sure the sub came to the surface reliably under all simulated failure modes.

Waterproof servo bays need to be practical to access and robust under pressure, so I designed a removable sealed bay with a threaded endcap and dual O-rings backed up by a silicone gasket, and I routed servo wires through individual cable glands rather than relying on a single glued bulkhead. For control surfaces I used miniature sealed servos mounted inside the bay with pushrods running through a sealed bearing and terminating in a bellows that keeps the outside water separated from the internals. Essential items for a reliable bay include a threaded cap with O-rings, IP68-rated cable glands, and a sacrificial soft bulkhead that can be replaced if it is damaged during a hard impact.

• Threaded endcap and O-rings for access.
• IP68 cable glands for each wire run.
• Internal foam blocks and sacrificial bulkhead for protection.

FPV underwater brings its own set of constraints because radio signals attenuate quickly in water and lighting is poor, so I fitted a small camera in a clear acrylic dome forward of the hull with an internal LED ring for illumination and used a low-frequency analogue video link for short-range viewing, while keeping the option of a tethered HD feed for long runs. The camera housing is neutrally buoyant and mounted on adjustable foam to minimise vibration, and I found that a short mast with a lightweight float for the video antenna works well if you need a surface relay for longer range. Testing the camera for latency and image clarity in the test pool allowed me to tweak the LED colour temperature and gain so the image remained usable in murky water.

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