How Do RC Submarines Work? Diving, Ballast, Radio and Building One
An RC submarine dives in one of two ways. Static divers take water into a ballast tank (with a pump, piston or gas system) to become heavier than water and sink level, then push it out to surface. Dynamic divers float slightly positive and use dive planes and forward speed to drive under, popping back up when they stop. Everything electrical sits in a sealed watertight cylinder. Radio is the hard part: 2.4 GHz signals barely penetrate water, so hobby submarines use lower frequencies such as 27 or 40 MHz and stay close to the surface.
By Kyle HilsonUpdated: October 20264 min read
Static vs dynamic diving
| Static diving | Dynamic diving | |
|---|---|---|
| How | Ballast tank floods / empties | Dive planes and speed |
| Can hover at depth | Yes, with trim | No; surfaces when stopped |
| Complexity | Higher: pump or piston, valves | Low |
| Safety on failure | Needs a failsafe to blow ballast | Naturally surfaces |
| Typical | Scale and advanced subs | Toys and simple kits |
The underlying physics (buoyancy and trim) is explained in how submarines go up and down.
Ballast systems
- Piston tank: a motor-driven piston draws water in or pushes it out; precise and self-contained.
- Pump and bladder/tank: a pump moves water in and out; vent valves needed.
- Gas system: compressed gas (propane-type in some kits) blows water out of a soft tank; quick surfacing.
Most submarines also have fixed lead ballast to trim them so they sit just at the surface with the ballast tank empty.
The watertight cylinder
Motors, receiver, ESC, battery and ballast system live in a sealed watertight cylinder (WTC), usually an acrylic tube with o-ring end caps. Shafts and pushrods pass through seals or magnetic couplings. The space around the WTC floods freely when the sub dives, so the outer hull doesn’t have to be sealed. A tiny leak in the WTC is the most common failure, so builders test it in a bucket before every season.
Radio under water
Water, especially with minerals or salt, absorbs radio waves; higher frequencies are absorbed faster. 2.4 GHz systems lose contact within centimetres to a few decimetres, which is why many sub builders still use 27 MHz or 40 MHz radios, which reach a few metres deep in fresh water. Some use a towed antenna or a buoy. Saltwater makes the range much shorter.
Failsafes
- Signal-loss failsafe: blows ballast or reverses the piston when the signal drops.
- Low-battery failsafe: surfaces before power runs out.
- Leak detector: surfaces when water touches sensor pins in the WTC.
- Positive buoyancy: trim so the sub floats if everything stops.
Control surfaces
Rudders steer; bow and stern planes control depth and pitch. Many builders use a pitch controller (a small gyro or accelerometer board) to keep the sub level, as long subs tend to nose-dive. Servos must be waterproof or sit inside the WTC with sealed linkages; see model boat servos.
Building or buying
- Start with a dynamic-diving kit or a toy sub in a pool.
- Scale subs are usually sold as hull kits plus a WTC kit.
- Test the WTC dry, then in shallow water on a line.
- Trim in a bathtub or pool before going into a lake.
- Run in clear, calm water where you can see the sub.
Where to run
Choose clear, calm water without strong currents or weeds, ideally a club pond or a pool if allowed. Never dive in murky water where a stuck submarine would be impossible to find. A float on a thin line is a cheap insurance for the first dives.
FAQ
How deep can an RC submarine go?
Usually a few metres, limited mostly by radio range.
Why don’t 2.4 GHz radios work underwater?
Water absorbs high-frequency signals within very short distances.
Static or dynamic diving for a first sub?
Dynamic; it surfaces by itself when stopped.
What’s a WTC?
The sealed cylinder that holds the electronics.
More on this topic: the complete RC Boats guide covers the basics behind this article.
About 20 years in RC. Mostly boats in the NAVIGA Mini Eco Expert class, with many self-made parts; also a King Motor Baja 5B, a home-built electric plane, home-built nitro planes he has flown, and an RC helicopter. More about SwellRC
