It Turns Out You Can’t Just Fly A Drone Under Water

The differences between a drone and an underwater remote-operated vehicle (ROV) aren’t actually that large. Both have powerful motors that move large volumes of fluid (yes, air is a fluid), a camera, a remote, and an onboard battery. So when [RCLifeOn] got his hands on a cheap used drone, he reckoned that it could fly underwater just as well as it did in the air.

To his credit, the principle was sound, and the initial tests looked promising. However, we will spoil the ending and tell you it doesn’t work out as well as he hoped due to water leakage. He printed a case with a large panel for accessing electronics inside and an acrylic window for the camera. The panel pressed up against a gasket via the few dozen metric screws along the perimeter. Despite the design being quite whimsical, he quickly regrets the screws as getting inside is tiring on the wrists. He epoxies the hatch to the hull and drills holes to charge the battery to stop the seemingly never-ending water leaks. After its maiden journey, water got inside and fried some of the motor controllers. So for the second test run, he used what limited capabilities it had left.

Despite the project not working out how he expected, it’s a great example of how some reused parts and some 3d printing can make something entirely different. So perhaps next time, instead of throwing that broken drone away, see if it could be given just a bit of love. Possibly the propellers can be combined or make do with only three motors. Or just go the [RCLifeOn] route and make it into something new entirely.

27 thoughts on “It Turns Out You Can’t Just Fly A Drone Under Water

  1. @Matthew Carlson said: “It Turns Out You Can’t Just Fly A Drone Under Water”

    My first reaction after reading that headline: “Gawd. Is Huge Gubbment gonna make me register and pay for a drone-sub license now? Fortunately no; at-least NOT YET… I used to respect – not fear – the U.S. Government. Those were the good ole days.

  2. The big problem of water leakage is solved by water drones that fill the electronics bay with oil. Usually a vegetable oil. They charge the battery by a sealed inductive charger. The motors can be replaced by sealed pumps that move via reaction pumping. Control via wire/fiber as radio does not work in water. Pro uw drones have a ballast weight that drops when the battery is dead = drone floats, like the Ballard ones. It will work on smaller. Ultrasonic control will work if you can buy/create one. Oil is messy, but when sealed properly endure. There may be some online builds. You can use slow motors and horizontal props and fly like an under water plane – using very slow motors. The motors can also be oil sealed or use a magnetic coupler. With truly sealed motor and mag netic couplers I think it is workable, but air will face greater pressure as you go down and could cause leaks.

  3. Easy way is to either fill the housing with oil (as mentioned elsewhere) or encase the electronics in resin (remember to make external access to connectors).

    Radio can be made to work using VLF (very low frequency). However, the antenna length makes this very inconvenient.

    Using hydroponics will add size, cost and complexity to it.

    There’s a reason why Under Water Vehicles either go autonomous or have an umbilical cords.

      1. It does. Buoyancy is related to the actual weight and displacement of the body submerged.

        Also, the type of water alters the Buoyancy, e.g. saltwater is more buoyant than fresh water. So, changing to saltwater environment changes the required ballast.

      2. Oil has a beneficial “side effect” : One of the challenges when designing submerged bodies is the pressure difference. I.e. keeping a certain internal pressure while resisting the external pressure. E.g. at a depth of 10 m the pressure outside is 2 atm as opposed to the internal 1 atm.

        This pressure difference acts upon the hull meaning, that going deeper puts higher pressure on the hull and requires a stronger structure. Filling the body with oil allows the oil to counteract this pressure (oil cannot be compressed like air can), thus reducing the structural strength needed.

  4. I recommend not rolling your own access ports for any marine project unless you’re making tiered rubber gaskets with ZIF-style levers to press a plate onto them. At any marine store, you can buy above-water ports that can fit into any boat hull, just a circular port with an insert that screws in, maybe 45° or 90° of rotation to close. It just puts pressure on rubber to keep water out. A smaller version of that would be more apt for something like this. Design the hull and internals to be watertight, then add an access port.

  5. Acetone smoothed abs for watertightness. Or curing uv layer of uv resin on top of the model.

    Silicone seals can be made by placing cling wrap on top of the high temperature silicone and pressing gently with the object you wanna seal against. Just make sure the cling wrap is as smooth as possible. Let dry and remove cling wrap. Easy.

  6. ‘Aerodynamic’ flight underwater using buoyancy as a replacement for gravity (so ‘lift’ drives you deeper) is a concept that has been used before for other submarines: there are positive-buoyancy submarines that must remain in motion to dive (e.g. DeepFlight’s) and neutrally buoyant ‘gliders’ that cycle their buoyance up and down to generate lift and forward propulsion by ascending and descending.

  7. Finally, a topic I am knowledgeable enough about to actually be able to share some of that knowledge.

    Offshore ROV Pilot/Tech with a little over a decade of working experience… I could tell from the very beginning of his video where all he would fail and have water intrusion. PLA+ or even ABS CAN be used underwater for certain things, but the only way I have ever seen them work in a pressure vessel (which every dry can becomes the instant it goes subsurface) is in very shallow water less than 30’ and after coating interior and exterior surfaces with polyester resin to seal the actual print. Offshore we only use 3D printed parts for brackets for lights and cameras and very light tooling.

    In a wet can, we fill electronics cans with hydraulic oil, the industry standard is Tellus22, and then use a pressure compensator to pressurize that oil to a minimum of 2psi, but normally our working depth requires 10-14psi. Residual air trapped in the system makes this number go up. Comps can be anything from custom firestone truck airbags, to schilling acorn comps with a heavy internal spring mechanism. Seabotix uses surgical tubing pressured up like a balloon and filled with a needle and syringe, and it works great for their small subs. Every oil filed system gets a compensator. Even the shaft seals on the hydraulic thrusters have their own comp.

    Cameras are always dry. The only company making a subsea PTZ camera I have seen offshore is Kongsberg, and it is amazingly useful. That said, it needs lots of light and does not usually get used for more than observing gauges on the actual sub.

    We don’t put batteries on the sub. I say that while adding that our beacons have an emergency (usually pair of D cell) battery, and our electronics boards have various button cell batteries for memory and internal clocks
    . Beyond that, all power comes from the surface. All 3-4Kv of it. It runs hydraulic pumps which power our thrusters. Smaller inspection class subs are still powered through the umbilical. For anything at the commercial level, a battery would be too much of a time limitation when I may need that sub to stay down for days at a time.

    An o-ring channel, o-ring, and some dow55 would have helped this builder with some of his water intrusion issues, but there is really no way around the need for a large number of fasteners to hold down such a large plate. A better option might have been a hybrid of oil flooding the electronics compartment and adding a plate around the edge to spread the compression force exerted by the fasteners. His camera would have gone well inside an acrylic tube which could be sealed easier and then mounted on the front of his housing.

    1. I have done away with epoxy coated petg brewing stuff around 40psi co2 gas without leaks. Acetone with abs or petg coated abs slurry is probably better if you do not care about food safety.

  8. I should add, we never, ever drop ballast. The sub is ballasted slightly heavy, and those lead weights are secured. Anything dropped subsea is a big deal. The only systems that drop ballast are old manned subs, like those used by woods hole.

    Also, radio does not work underwater. VLF is too slow, light does not transmit over long distances, and acoustic is really only used for location and sonar. We put out a field of beacons for larger jobs so we know where we are in the field compared to other assets, but most of the time the beacon systems show a very rough location that might jump 20’ in any direction when we cruise past an asset that reflects the sound waves in some random direction.

    Control is through Fiber optics and very limited RS232.

    1. WHOI are doing some interesting things with Nereid Under Ice (NUI). NUI is run off battery power but still tethered (data tether only, for smaller diameter and extreme range), but can also run fully autonomous, or semiautonomous with an acoustic MODEM – either directly or via another vehicle as a relay (e.g. DriX) to cut down on required bandwidth for commanding and telemtry – or they’re even playing with freespace optics for higher bandwidth (they’ve managed to push HD video over it!).
      Probably not an solution for industry where you want longer operational duties and high power operation, but an interesting capability.

  9. And if you want to cross oceans, like some researchers do, then copper/nickel hulls will keep the barnacles from growing on it. 90% Cu/10%Ni works, but some go higher on the Ni, up to 20%.

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