Our main idea was to make a very inexpensive and very easy to make underwater thruster. And it's fast, very fast!

Let's see how to make it inexpensive, simple and fast!

Step 1: A Little Comparison

Underwater propulsion requred really huge power. For example the x2 Sport Underwater Jetpack for $3000 has a pair of 1800W motors to get 6mph and at maximum speed it can work 15 minutes only.

Very good and popular motor Blue Robotics T200 has the same power and thrust as our but it costs $338 for 2 motors only (without ESC and others).

We used two DIY motors for $124 together ESC included. At a top speed of 1.85 mph (3 km / h) you can swim continuously for 20 minutes!

Step 2: Main Items

1. DT700 brushless motors: 2 pcs x $14

2. Kort nozzle for DT700: 2 pcs x $40 (and now on Amazon)

3. ESC speed controller at least 40A with BEC: $4

4. Servo tester: $5

5. Slide Potentiometer any from 5K to 30K Linear: $2

6. SLA battery 9Ah: $22

Step 3: Wiring

Here is the connection diagram.

1. Extend wires (AWG 18-16) from both of motors and solder them to the speed controller in parallel. The wires must be long enough so that you can move your hands freely. I made wires 3 feet long, but you can choose the length to suit your height.

2. Solder to ESC two power wires (AWG 16-14) for battery.

3. Open servo tester, delete the rotating potentiometer and solder 3 wires instead of it's terminals (legs). Wires must be long enough to reach your fingers. I made them 4 feet long. Now solder the other ends of wires to slide potentiometer. It should be placed in a housing and sealed with a big rubber balloon or a glove. That gives you full range speed control underwater.

4. Connect BEC plug to the servo tester.

5. Place ESC + servo tester to a small suitable plastic box and seal with an epoxy resin compound.

Step 4: Arm Holder and Others

Here is our version of the holders of motors and electronics. We swam with them all summer and we find it very convenient and practical. Of course you can come up with your own version.

Step 5: About SLA Battery

Previously, we made an underwater robot powered by SLA batteries. The robot made hundreds of dives to a depth of 30 meters and the batteries are still working properly. SLA batteries have rubber caps that play the role of valves for the release of gases when the battery is charged. These caps are tightly pressed with plastic caps and do not allow water to pass even when immersed in great depth.

We used XT60 connectors underwater with a small amount of a thick grease on contacts. It is inexpensive and works perfectly even in sea water. Similar connectors have been working on our ROV for two years and still do not require replacement.

In water two DT700 motors can consume a current of 50 amperes maximum and develop a maximum thrust of more than 7,5 kilograms. This is a very good result, but in order to increase the battery run time, we have limited the current to 30 amperes maximum. To do this, just limit the movement of the slide resistor. We knew it theoreticaly and tested in practice, that at higher current the speed increases insignificantly, but the operating time is greatly reduced. With this current you can swim continuously with a maximum speed of 1.85 mph (3 km per hour) for 8 minutes with one battery or 20 minutes with two batteries.

Battery 9 Ah weighs 2.5 kg on the surface, but in water 1.5 kg only. Usually we use two batteries at once connected in parallel. One on back and one on stomach. Batteries are secured to the strap using velcro. This is very reliable and allows you to quickly replace the batteries with new ones.
Before diving we always try to inhale the maximum amount of air. An adult lungs volume are 4-6 liters of air, that is, our positive buoyancy increases by 4-6 kilograms. With such a large positive buoyancy it is very difficult to dive under water and we need extra weight. Two batteries in the water have a weight of about 3 kg this allows us to very comfortably sink and easily float to the surface.

Step 6: Flying Underwater!

I'm a diver and I often scuba dive, I also like to dive as a free diver, but I have never experienced such fantastic freedom, lightness and the feeling of flying underwater! Imagine that you do not need to swim hard with your hands and feet, do not need to expend energy and air, just relax, click on the button and fly 1-2 minutes under the water at the speed of person walking... You should definitely try it!

Step 7: P.s.

Although most brushless motors can work in water, it is recommended to make some modifications (even included Blue Robotics). Over time (after 100-1000 hours) microscopic damage appears on the wire's protective coating. Leakage of current through these holes can lead to malfunctions in the ESC or RC controllers. Moreover electric current into these holes causes electrochemical corrosion, which leads to a rapid destruction of the wires (after 10-100 hours). To prevent this we should impregnate the wires with epoxy. The engine will operate underwater with increased power (5-10 times the nominal!), so it is necessary to ensure a good heat dissipation. To do this, choose an epoxy with a high thermal conductivity. This is a big and interesting topic, let's take a closer look at it a little later.