Introduction: Underwater Wrist Thruster 1.85 Mph!

Picture of Underwater Wrist Thruster 1.85 Mph!

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

Picture of 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

Picture of Main Items

Step 3: Wiring

Picture of 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

Picture of 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

Picture of 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!

Picture of 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.


kylegilbert (author)2017-10-31

This is incredibly cool. Amazing what you were able to accomplish for that amount. Really nice work.

matthewgal (author)2017-10-09


diy_bloke (author)2017-10-06


leshemeli (author)2017-09-08

I am a masters swimmer. You make me think :)

the modder (author)2017-09-07


DIYunderwater (author)the modder2017-09-07

3 km/h

Kris82 (author)2017-09-05

Great idea and I'm impressed that those little propelers can get a person to go that fast.

Alaskan Bev (author)2017-09-05

What happens if you kick your feet up and down as in a regular swimming manner? Does that add to your speed or interfere with the action of the machinery?

Nice question. Yes, we tryed without any success:) But we often use other
styles, for example, if you drop your hands down, you can immerse
yourself in the water to a great depth (it is much easier for a person
to equalize the pressure in his ears if he dives vertically, with his
feet down). It is also very interesting to direct your hands in a spiral and swim spinning. We also tried to mount the motors on the hips, then we can swim with our hands very quickly:)

rennickr (author)2017-09-05

Thanks for posting this! I've looked it through and noticed that just your parts alone total more than $130 and this doesn't include the miscellaneous items. You're getting close to the $169 cost of the T200, if not passed it with the odd extra items. Do you have the STL files for the thruster to bring the costs down a bit?

DIYunderwater (author)rennickr2017-09-05

T200 with ESC x 2 =$388

+battery, wires, epoxy...

HaydnH (author)rennickr2017-09-05

Are the STL files or original design files available anywhere publicly?

DIYunderwater (author)HaydnH2017-09-05

We worked on the Kort nozzle design for a year. The base was developed
in a parametric modeling program, then about 50 different variants were
manufactured and tested before we approached near the Blue Robotics
level in efficiency. 41 g / w (0.09 lbs / w). Perhaps we can do
something new and then publish the file of the first model, but for now we would like to get at least a small return from it. I hope that the amount requested is fair enough, given that this is our design and our work. But we can send this Kort nozzle for free to someone who will make an instructable with our motor.

HaydnH (author)rennickr2017-09-05

Remember to keep in mind that a single T200 costs $169, and it doesn't include the batteries or speed controller. This is much less expensive. :-)

SvetoslavV1 (author)2017-09-05

You have found very nice sliders - USSR made. I wonder where you got these?


claudepotvin (author)2017-09-05

Do you offer this up for sale?

avayan (author)2017-09-05

WOW! This is fantastic, thanks for sharing! I am also extremely intrigued as to how the BLDC is protected from the water (e.g. shorts, oxidation, etc.). I have been tinkering with this idea for a gazillion years and have always wondered how people shield the motor from the elements. I am blown away there is no need to! Although water is a poor conductor, salty water is actually decent. Granted that the winding is made of insulated wire, the magnets are covered in some ceramickey kind of deal, the stator and rotor bodies are usually made of aluminum, the shaft of stainless steel, etc. Not certain about the ball bearings. But I would imagine the BLDC connections (solder points) could face shorting? Thanks for your input!

a-morpheus (author)2017-09-05

I don't see any comment about sealing the, they work just fine being exposed to water? I see some people testing brushless motors submerged,

but it seems like somehow in salt water you'd get problems with the voltage causing corrosion or whatnot.

I looked at your underwater robot instructable and your item and it's still not clear how, or if, the housing is watertight. Is it? How does the shaft sealed in transit from the inside to outside where the propeller is?

Shenzhen (author)a-morpheus2017-09-05

We just making an instructables about:) Brushless outrunner motors are perfect for underwater. Lets wait for a few days and we can show it step by step.

chrwei (author)2017-09-05

instead of hacking a giant servo tester, you can make the servo driver from a 555 timer and a few parts and make it tiny

The Fishing Hobby (author)2017-09-04

Wow, this is awesome! Do you think this could be used for kayak propulsion?

Already made it:)

3,7 mph

2 hours with 2 SLA 9Ah batteries.

Awesome! This is a great idea. Thanks so much for sharing.

dewolfe (author)2017-09-04

Strapon canoe engines?

DIYunderwater (author)dewolfe2017-09-04

The most of factory made canoe engines cannot work deep underwater.

dewolfe (author)DIYunderwater2017-09-04

All the more reason to try them on a canoe with a nice big lead/acid battery which won't impact the canoes speed much. With the Kort nozzle's big thrust you may have a "Two-Fer" there.

About This Instructable




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