# Using Blue Robotics thrusters to propel a diver

Hi everyone,
Does anyone here have had any experience mounting T200 on to a diver for a propelled dive (normally strapped to the forearms)? I’m curious to know if two T200 thrusters would do the job (propel the diver at a reasonable speed) and if so what was the general/setup of this system?
We’ve found plenty of info on the web using other types of thrusters for a similar purpose but not much using the T200. Any lead would be helpful. Thanks!

Hi @odedezra,

I haven’t used a dive propulsion vehicle before, but that seems to be the best comparison here. I found this list of 12 options for different budgets/performance requirements, and most seem to have a speed of 2-3mph (53-80 m/min), neutral or positive buoyancy, 30-120 minutes of runtime (although that presumably includes some time with the thrusters off/on low).

As an arbitrary example, I found the sub-gravity Ecos+ has a max speed of 220 ft/min (~67m/min), which is with 26.5kg f of thrust. Assuming everything else (mass, shape, drag area, etc) stays constant,

\begin{align} F_{max-thrust} &= C \cdot v_{max}^2\\ \rightarrow C &= \frac{26.5kgf}{(67m/min)^2}\\ &\approx 5.9 \times 10 ^{-3}\frac{kgf}{(m/min)^2} \end{align}

I did the same check for a couple of other options and got an average of about C\approx 5.7 \times 10^{-3}\frac{kgf}{(m/min)^2} (to account for slightly different testing/weight assumptions across manufacturers).

Assuming T200 thrusters powered with a bluerobotics battery, a 4S LiPo battery has an average voltage of ~15.8V (16.8V fully charged, 14.8V nominal - stopping point for optimal lifetime), so the easiest comparison is 5.25kgf Full Throttle @ Nominal 16V from the T200 technical details. Two of those would give you

\begin{align} v_{max} &= \sqrt{\frac{F_{max-thrust}}{C}}\\ &=\sqrt{\frac{2\cdot 5.25}{5.7\times 10^{-3}}}\\ &\approx 42.92\ m/min \end{align}

The nominal capacity of the BR battery is 18Ah, and a T200 full throttle current at 16V is 24A, so 18 Ah/(2\cdot 24A) \approx 22.5 mins of non-stop runtime (for two T200s).

A couple of notes:

• thrusters near people/marine life should have some form of guard, to protect fingers and other small living things
• larger thrusters are likely a bit more efficient for this, but with multiple small thrusters you can pretend you’re iron-man
• obligatory comment that fast water, fast ascent/descent speeds while diving, batteries, and electronics in water are all potentially dangerous if appropriate precautions aren’t taken.

Hope this breakdown helps. Would be great if someone actually has experience with this kind of system and can chime in with some extra suggestions

EDIT: updated the runtime calculation to include the number of thrusters (originally forgot to so calculated as though only one thruster would be running).

1 Like

Not used for diving, but most for fun I made an emergency propulsion test for a 10 meter boat
weight 4 000 kg (~8 000 pounds) with one BR thruster. After 1 minute speed tops out around 1 knot.
When diving with underwater scooter I go 1 to 2 knots.
So I guess its worth a try.

2 Likes

I’ve strapped some t200s to my wrists and that exact battery to my back (in a string backpack, enclosed in a 3" enclosure and setup with a thruster commander to run full throttle on switch release) and grabbed a surfboard, and thrusted my way out on a SoCal beach break… they definitely moved my mass better on the board than when in the water. I think a setup of 4 would work well, or running them on a 5S or 6S battery - but at the end of the day the efficiency of a larger prop is hard to beat.

I’ve since done many dives, and would suggest the safest way to experiment may be copying the layout of pod style tow thrusters, which in my experience are the most fun when placed between your legs and ridden - only your feet are in proximity to the blade(s) and your C drag coeff is likely lowest in that position?

Great math breakdown @EliotBR !

2 Likes

Hi Eliot,
Thanks for the very detail breakdown!
If I understood correctly you tried to get to the C (drag coeff?) assuming it is the factor that’s relevant for a diver?
43 m/min is a little less than 1.4 kts which is probably a little too slow, so maybe an addition of two more thruster would be more fun as @anthony-white suggested.
Definitely some more research and trial and error is required!

1 Like

The actual drag force equation is F_D = \frac{1}{2} \rho v^2 C_D A, so the C value that I’ve approximated effectively encompasses drag coefficient as well as the density and cross-sectional area of the person + DPV (assuming they’re roughly the same across different manufacturers) - that is C = \frac{1}{2} \rho C_D A.

Fair enough - double the thrust in the same v_{max} equation (ie assuming the same conditions otherwise) would get you \approx 60.7 m/min, although the battery life would reduce down to 18Ah / (4\cdot 24A)\approx 11.25mins with a single BR battery.

Agreed

1 Like

Perhaps worth mentioning here that the recently announced T500 Thruster is quite a bit more powerful than the T200, and also more efficient. It would likely be a better candidate for this kind of application

1 Like