How Do Submarines Go Up and Down? Buoyancy, Ballast and Dive Planes
Submarines go up and down by changing their buoyancy. To dive, they let water into ballast tanks, which makes them heavier than the water they displace, so they sink. To surface, they blow compressed air into the tanks (or pump water out), becoming lighter so they rise. While moving, they also use dive planes, small wings on the hull, to angle up or down and fine-tune depth. They turn left and right with a vertical rudder, just like a boat. RC submarines use the same principles in miniature.
By Kyle HilsonUpdated: October 20264 min read
Buoyancy in one paragraph
An object in water is pushed up with a force equal to the weight of the water it displaces (Archimedes’ principle). If the object is lighter than that water, it floats; if heavier, it sinks; if exactly equal, it stays at the same depth (neutral buoyancy). A submarine controls which of these applies by changing its weight with ballast water, while its volume stays the same.
Ballast tanks
| Action | Tanks | Result |
|---|---|---|
| Dive | Vents open, water flows in | Heavier, sinks |
| Hold depth | Trim tanks fine-tuned | Neutral buoyancy |
| Surface | Compressed air blows water out | Lighter, rises |
Dive planes
Dive planes (also called hydroplanes) work like the elevator on an aircraft. When the submarine moves forward, angling the planes tilts the bow up or down so the boat climbs or descends. Combined with ballast trim, they let a submarine change depth smoothly while moving. A submarine that’s stopped relies on ballast alone.
How submarines turn
A vertical rudder at the stern steers left and right, as on a ship. Some submarines also have thrusters for low-speed manoeuvring. Turning while submerged combines rudder and dive planes, so the boat can change both heading and depth at once.
RC submarines
RC submarines use either static diving, with a ballast tank filled and emptied by a pump or piston, or dynamic diving, where a slightly buoyant sub uses dive planes and forward speed to go under and floats up when it stops. Radio signals don’t travel far through water, which limits depth. Details are in how RC submarines work; servos for rudder and planes are covered in model boat servos.
Try it at home
- Put a small bottle with a little water in a tub: it floats.
- Add water until it just sinks: you’ve reached negative buoyancy.
- Adjust until it hovers in the middle: neutral buoyancy.
- A “Cartesian diver” in a squeezed bottle shows the same idea.
Why it matters for safety
Real submarines are designed to be able to surface in emergencies by blowing all ballast quickly. RC submarines copy this with failsafes that surface the sub if the signal is lost or the battery runs low, so a model doesn’t get stuck on the bottom.
Why depth control is hard
Water pressure increases with depth, which can squeeze a submarine’s hull slightly and change its buoyancy. Temperature and salinity also change water density. That’s why real submarines constantly adjust their trim tanks and use dive planes while moving, and why RC submarine builders spend time trimming their models carefully in a pool before running in open water.
Fish use a similar trick: many have a gas-filled swim bladder they adjust to rise, sink or hover without effort.
FAQ
How do submarines go down?
They take water into ballast tanks to become heavier than the water around them.
How do they come back up?
They blow compressed air into the tanks to push the water out.
How do submarines go left?
With a rudder at the stern, like a ship.
What are dive planes?
Small wings that tilt the sub up or down while moving.
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
