Merry Christmas and Happy Holidays!
This forum post will (soon) be turned into a guide, but as it has been cooking for almost a year I wanted to get the design and info contained here out in the world! I plan to refine some instructions around the kill switch, so stay tuned and please feel free to ask any clarifying questions!
Introduction
Diver propulsion vehicles (DPVs) typically are large, heavy, torpedo shaped devices, often called “scooters”, that can greatly increase the distance covered in a dive. This article does a great job discussing the typical comparisons that go into evaluating a DPV.
But what if they didn’t have to be awkward to carry and use? What if they could be more efficient, affordable, and customizable? And what if it didn’t blow the water right back at your body??
This DPV design weighs about ~8 lbs in air, and can easily propel one (or more!) divers or swimmers at speeds of up to ~1.5 m/s!
We get a lot of questions about T500 performance, and with many certified divers on staff at Blue Robotics you know someone was going to build something like this eventually! After a solid initial effort, subsequent testing and iteration in Hawaii has led to the design and parts list that is shared here today freely! You can find the full CAD models in Onshape, feel free to adapt the design as you see fit!
By sharing this, we hope the community tries it out and this design can become an open-source platform that enables many different marine applications. Unlike Blue Robotics vehicles, this tool really puts the user directly in the action! Blue Robotics doesn’t intend (currently) to make this into a product, kit-based or otherwise, but we welcome all to build one and have some fun!
There is of course inherent risk with a device like this, so please solemnly remain cognizant of the risks inherent to scuba diving and ascending too fast - don’t get bent!
Additionally, when snorkeling / free-diving without a tank to breathe from, it’s possible to descend deeper, or travel farther than you may feel comfortable or able to make an unpowered ascent or return from. Using it for surface or shallow-depth propulsion without breathing gas at depth is safer in many ways, but one should always keep the risk of shallow-water blackout in mind. Exercise caution, and always have a buddy along!
Please refer to the safety section of this guide before using the DPV.
Design description
The frame started as a t-slot aluminum frame with a large foam buoyancy block, and has evolved into a single aluminum sheet-metal folded part. This piece can be ordered from vendors like oshcut for an incredibly affordable price - it can vary but expect less than $70-100, including shipping! It holds the two thrusters and enclosure in a lightweight and fairly ergonomic fashion.
A 4” watertight enclosure holds a single 4S 18AH battery, providing incredible endurance (~20-80 minutes) and easy hot-swaps in the field. A thruster commander is mounted alongside its potentiometer, which is coupled to a rotary control on the outside - composed of the Blue Robotics switch and vent plug! This lets a user easily control the throttle to move at a continuously adjustable speed - a feature most DPVs don’t provide!
A magnetic reed switch is used as a safety cut-off (kill) switch - it’s held in place with a finger, and attached to a user’s wrist in case the unit is dropped at any time. It’s a good idea to use an energy meter with a display, to track battery voltage and overall current capacity consumption. This way you always know how much “fuel” is left in the tank!
The estimated cost of a unit is approximately $2700, of which $2500 is in the Blue Robotics store.
What you’ll need (BR) ($2500 + tax, shipping)
- T500 x2
- 2X BasicESC (or BasicESC 500)
- Thruster Commander
- 4s18AH Battery
- 4” Enclosure
- Vent plug
- On/Off switch
- Short standoff x2
- 2X 4” clamps (includes required bolts for mounting)
You’ll also need
- Reed switch (and magnet, included)
- 10mm magnet
- Power meter
- Sheet metal folded cross-bar (STEP file for oshcut: Sheet metal frame - sheetmetal frame.step (122.3 KB))
- 3D printed internal mount
- 3D printed shaft coupler
- 3D printed outer dial
- 3D printed safety magnet holder
- 3D printed switch slide channel
- 3D printed hand grips
- Superglue
- 8x M4x8mm T500 mounting bolts
- 4x M3x12mm Grip mounting bolts
- 2X M3x10mm Thruster commander mounting bolts
ZIP of 3D printed parts (Printables page coming soon!)
DPV printed parts.zip (4.6 MB)
Assembly
- First mount the Thruster commander to the internal mount using 3x M3x10mm bolts.
- Attach this piece to the o-ring flange with 2x M3x10mm bolts, noting the alignment with respect to the locking-cord tab.
- Install the 5 hole endcap and ensure the 10mm penetrator aligns with the internal mount - you can check by looking through the hole the potentiometer will fit.
- Install the Vent-plug bolt in the hole that aligns with the potentiometer in the end-cap plate, don’t forget the o-ring under the bolt head! Securely tighten the backing nut from the inside with a penetrator wrench.
- Place a drop of super-glue on the threads of the standoff, and screw it into the other to make an extended rod.
- After putting a couple of drops of superglue on the internal hex of the 3D printed adapter, insert the standoff and make the threaded hole flush with the end. Superglue this assembly into the vent-plug, and set aside
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Disassemble the nozzle off of one T500, and replace the propeller with the opposite-hand (CW with CCW)
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Mount both T500’s to the aluminum frame with props rotating upwards on the inner edge to cancel out the overall torque - just like the BlueBoat. Use 4x M4x8mm bolts for this connection.
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Mount the base of both enclosure clamps to the frame, on the same side the thruster mounting tabs on either end extend towards. This is the front of the vehicle.
Now it’s time to assemble the electronics enclosure.
- Mount the Bus Bar that came with the Thruster commander to the top of the printed mount, and connect 10 AWG wires from here to a female XT90 battery connector. If a power meter is being used, this should be spliced into this length of cable.
- Connect both ESC500s power wires to the bus bar, with jumpers present so that both ESCs receive the correct polarity voltage.
- Connect the signal wires from each ESC to the Thruster Commander - you can use the left or the right channel, or put them on the same side since no steering input is being used.
- Push the reed switch into the thruster commander mount and attach the leads to the switch input of the Thruster commander.
- Insert the potentiometer into the printed thruster commander mount, and connect it to the throttle input.
- Secure 2x blanking penetrator blanks in the closest position to the speed control knob, and drill the farthest two holes out to 14mm.
- The end-cap plate with installed switch penetrator bolt can now be attached - before bolting it down, attach the printed potentiometer coupler by threading it onto the end of the standoff.
- Tighten the vent plug with standoffs fully into the switch bolt, so it can’t be twisted further - this is the maximum throttle position. Twist the potentiometer to the fully clockwise position, and adjust the thread engagement of the potentiometer coupler so that when the two parts come together they are aligned.
- The end-cap plate can now be secured with the included 6x M3x10mm bolts - don’t forget to install the single o-ring used by the face seal between the plate and the flange!
- The second o-ring flange can be assembled with the blank plate - this cap is the one that will be removed to install / swap the battery.
- You can use the T500s with the cable length they come with, but shortening the cable can reduce weight and increase the electrical efficiency of the system. The WetLink Penetrator gland can be unscrewed, releasing the cable from it. The outer jacket is stripped back so that it is only present for the desired length. If the rubber compression seal is in good shape, it can be reused as this penetrator won’t be subjected to the extreme depths that a fresh seal requires!
- With the T500 cables routed, they can be installed in the 2x M14 holes.
- Lay the tube onto the clamps halves on the frame, and secure in place with the other halves of the clamps and the included 8x M4x10mm bolts.
- The end caps can now be inserted into both ends of the 4” enclosure tube. This design does not use a vent-plug, which means you’ll have to compress the end-caps in when feeding in the locking cord. A little internal positive pressure can only help!
- With some super-glue in the pocket, attach the 3D printed speed-control knob to the exterior of the Vent-plug - this makes it much easier to rotate with just a spare finger!
- Remove the end cap plate bolt that is closest to the reed-switch, and install the printed switch slide channel. Install a 10mm diameter magnet into the safety switch and attach a bit of string - put a bowlin loop knot in the end so you can turn it into a slip-knot bracelet that won’t come off when pulled on!
- Finally, install the 3D printed hand grips with 4x M3x12 bolts, threaded directly into the printed part.
First Test
- Before connecting a battery, verify the wiring according to the diagram below
- Remove the flange with the blank end-cap plate, and while holding the power connector from the DPV, insert the battery with its connector facing you. You may have to rotate it to accommodate the power monitor and ESCs.
- Connect the battery - you will hear the ESCs through the thrusters start-up and initialize, if the safety-reed switch is not in place - this is because the Thruster commander initializes the ESCs when the switch is open. If the switch is in place, you won’t hear the final initialization tone until the throttle is rotated to the center position.
- When the switch is closed, the thrusters will spin - make sure to have the reed-switch approximately centered, and adjust it to stop motion (with the switch in place.)
- Rev the motors, and check that they each push air away from the frame towards the side without the enclosure present. Don’t let them spin in air for very long!
Safety
- Follow local regulations.
- Avoid boat channels. Use a surface float (towed)
- Don’t get fingers in the propellers! Don’t get line in the propellers!
- Remove the on/off safety switch magnet if putting anything near propellers - suction can be quite powerful!
- Avoid using it alone!
- Don’t discharge 4S batteries below 12V - start to end usage when battery voltage approaches 13V.
- Avoid sand at all costs - this blasts the coatings on the T500 motor, and can negatively impact both efficiency and thruster life span.
- Monitor mAh consumed, vs. total capacity (15 or 18AH depending on the battery used) - check when you check your air pressure! It’s good to end the adventure with an amp-hour or two left in the “tank.”
- A good cruising power consumption is 125 to 250 watts. The speed increase from 300 to 700+ watts is minimal, as the effect of drag is exponential.
Scuba Safety (you should know this already!)
- Don’t ascend too fast!
- Don’t go too far!
- Use a compass, don’t get lost!
- Never dive alone!
- Always wear a dive computer, and heed its ascent rate warnings!
Go have fun!
Operator Checklist
- Battery charged and cells in balance
- Battery connected
- Both locking-cords installed
- On/Off switch tested, both motors turn, slight breeze felt in expected direction
- Adjust throttle to 0, or just above idle
- Remove on/off switch, attach to wrist
- Gear-up and get in the water with the DPV!
- Check for leaks
- Turn it on and zoom onwards!
- Disconnect battery when you exit the water and are in a dry environment
- Don’t let the battery voltage get below 12V! End usage once voltages approach 13V, the drop off after this point will occur more rapidly.
Future design dreams
- Imagine a unit with a 6S Mega battery!
- How can the magnet attachment for the safety switch be improved?
- What about a purpose built PCB, with integrated power monitoring, display, and inputs - an upgraded Thruster Commander!
- How can the frame be improved? Camera mounting points? Integrated buoyancy for neutral or positive behavior?
- How can the grips be more ergonomic, and accessible for more user’s hand size or grip method?
In the Wild
Here are some youtube videos illustrating this DPV in the wild!
