When I finished reading the fascinating Vladimir Mazhirin’s series about submarines, I suddenly felt a burning desire to build my own. I wanted to build it at home without using turning or other types of machines. I had built RC planes before. In this article, I’ll tell you how to build a simple RC submarine out of household materials, which you can buy at your local household store or Chinese online store.


Theory
A miniature submarine is built based on the same principles as a real one. It has a strong and watertight pressure hull in the middle, which conceals all systems and controls. The hull is located inside the light non-watertight hull, which provides hydrodynamic efficiency and nice appearance. Our submarine is going to have a pressure hull only.

Dynamic diving (during motion) is carried out with the help of diving planes, and static diving – with the help of a ballast tank. How does it work? When a submarine surfaces, it has a little smaller weight than the volume of water displaced by it (the Archimede’s law). In other words, a 3-litre submarine should weigh a little less than 3 kg. After that, the submarine takes some water into the pressure hull. Because the hull volume stays unchanged and the weight increases, the submarine dives. At this moment, pressure inside the hull increases, but not as dramatically as to undermine watertightness.

Construction brings three challenges. They are not related to each other and can be solved separately:
  • Radio control systems (2.4 Hz system do not function under water)
  • Watertightness: the hull, motor shaft, and pull rods
  • Ballast tank

RC systems

Water won’t let a 2.4 Hz signal through. The simplest way to go is using obsolete MHz-range systems (27MHz, 35MHz, 40MHz, 72MHz, or 433MHz). Many hobbyists have such systems and they put them up for sale on forums. A forum member gave me one for free when he read about my submarine project.

Watertightness
We should keep three elements watertight:
  • the pressure hull
  • the motor shaft
  • the hydroplane rods
The pressure hull
It is often referred to as WTC (Water Tight Ctylinder) and has a shape of a 600 mm long pipe with a diameter of 75 mm. It has cylindrical plugs at both ends, and each plug has an O-ring groove. You can buy a piece of pipe at your local plumbing store. We’ll make plugs of several pieces of 4-5 mm thick PVC sheets and order O-rings in China.


Each plug will have six layers. The internal diameter of the pipe is 71 mm, and the O-rings will be 3.5 mm thick. Each ring will consist of an external ring (75 mm in diameter), middle ring (70 mm), and internal ring (65 mm). Coaxiality is as vital as it is difficult to achieve, but rings must cling to each other evenly. We are going to use a 6 mm bolt or pin to center the layers. Drill a hole, draw a cylinder of the specified diameter, leave a margin and cut it out with a hand saw. Perch it on the bolt or pin clamped in a drilling machine and trim it down to the needed size. I used sanding paper.

Glue the rings together on the axis and keep it perpendicular to the rings. I recommend Henkel super glue for PVC parts. Titan does not work, nor does the usual super glue. Fix the rubber rings and check how watertight they are.

Deadwood
It is a unit, which water-proofs the motor shaft. Solder gears into the external pipe’s ends. Insert the shaft and fill the pipe with thick lubricant (Litol). Ishould be refilled from time to time, because water gradually washes it away.

I bought the shafts and gears at AliExpress, and the copper pipes and the lubricant – at a local market. The gears’ internal diameter is 3 mm, and external diameter - 6 mm. So, I used 3 mm stainless steel shafts and a piece of copper pipe with an internal diameter of 6 mm (the external diameter was 8 mm). It is imperative that you buy a special kind of shafts, because usual wire is not symmetrical and may flap.

Soldering will require acid and a third hand ;)

Hydroplane rods
Run the rods through corrugated rubber hoses sitting on 6 mm copper pipes. The rods’ diameter is 1.5 mm. I used a piece of tight wire stripped of isolation.

Ballast tank
This is the hardest part of it. However, we’ll do it in a simple way, I mean using a micro-pump. More specifically, we are going to use a peristaltic micropump, as it maintains pressure and does not require additional valves. This kind of pump can boost pressure up to 1 atm, so it can pump a tank floating 10 m deep. These pumps are controlled in the same way as usual electric motors – with the help of an engine governor and a servo with microswitches.

It is possible to use a rubber balloon, but it may pop. I recommend using a 150 mm syringe, which is known as Janet syringe. The pump will move the plunger. Also, you can fix sensors and control the amount of water flowing into the tank.

To speed up the diving, I replaced a silicon pipe with one having an internal diameter of 4 mm. I use a 6V motor, and the actual voltage is 12V. It causes the motor to heat up a little, but it is far from critical. It went 100 ml / 20 sec.

Components
The submarine has ‘compartments’.

1. Battery and transmitter
2. Ballast tank
3. Pump
4. Servos and engine governors
5. Main engine
The tank should be placed in the middle to ensure that the submarine submerges horizontally (no inclination).

Mounting elements are made of a 5 mm sheet of porous PVC. Next, they are tightened with metal pins, which are lined down the length of the body. It is advisable to pin the rear plug too to provide a solid connection between the motor unit and the hydroplane rods.






Balancing
The approximate volume of the body is 2.8 l. The total weight is about 1 kg, and I had to add another 1.8 kg. The main idea is simple: the volume of the above-water part should be smaller than that of the ballast tank. It can have any weight, but it is imperative that the total weight be 2.8 kg. That’s why above-water elements are made of copper or thin plastic.
Balancing is carried out in two positions:
  • On the surface: move/add loads and fix pieces of foam on the underwater side until there is zero inclination. It is important to fix foam below the water line.
  • Submerged: fix pieces of foam above the water line until the submarine floats horizontally.




What’s next?
  • telemetry: voltage, current, depth, orientation
  • rescue: a buoy and water detector
  • torpedoes, missiles
  • 3D-printing parts
On my home page you can find:
  • links to items to be purchased in China
  • drafts of this particular submarine
  • more pics and vids highlighting the process
Thanks for reading!
I'll do my best to answer all your questions.
This is the translated version. You can read the original Russian article here.