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Maximum Tether Size

Hello All,

I’m actully bying a BR2 Heavy conf, and we would like to fit a long length tether on it (like 400m) with power and fiber. After studying all power loss and fiber adaptation to the ROV, we do have a good feeling to make it works.
However, with our power loss calculation, the tether is quite thick, something like 30mm diam…

Is anyone tried to operate the BR2 heavy with this thickness of tether? Is there a maximum diameter that the ROV can handle nice and easily?

Thanks for your repply, and for all community work, it is very appreciable!


30mm tether will not be workable for the BR2 and would be a nightmare to store!!!. What voltage are you trying to send? Sounds like you need to send a higher voltage down the line, see the BR/Outland Tech topside power unit in the BR store

How much water current do you anticipate? Is the bandwidth of a fiber required?

For long excursions, the smaller the tether diameter the better. We routinely run 250m to 300m of tether on an 8 thruster BlueROV2, with 500W power supply and 9.1mm diameter tether without any issues, so long as the currents are low. In the open ocean or with strong tides, deep work is possible by weighting the tether, near the ROV end, with a “clump weight”. However, to fly a BlueROV2 sized vehicle 400m horizontally with a cross current of even 1 knot, with a 9mm diameter, would be highly impractical (requiring very powerful thrusters, many kilowatts of power, and supply voltages over 1 kilovolt. We run 300-400VDC in our tether to achieve 300m distance and the FathomX modules work fine under those conditions (about 60Mbsp). If you can avoid the fiber, it’s certainly cheaper.

Hi Marcus,
Thank for your repply. Indeed we would need to have at least 100VAC down the line… We did had a look to the OTPS but 400VDC is not suitable for us. So we though that it would be more easy to convert 230VAC into 15VDC for the BR than changing our different power supplies.

Hi Peter,
Thank for your repply,
We don’t expect a high current even in open sea, and a kind of donnut is plan to guide the teher through the water column, fortunatly there is no tides in Mediterranean Sea! Work plan is around 300m deep, it is why 400m of tether is a minimum. As I said to Marcus, we would prefer AC current down the line, as our equipment is so far powered between 100VAC and 250VAC.
Even if it’s DC power on your side, do you have an idea of the current drop along your 300m tether when using the maximum power of the BR?
Unfortunatly, we do need high speed datas, as we are upwarding strong vidéos, fiber allows us to have Gigabit connection very appreciable to have full frame vidéos instead of downgraded.

We keep working of finding a good compromise between tether size and power loss, but it’s not easy has we don’t know exactly the maximum power consumption of the BR2 Heavy config. If you guys have any measurments or idea of it?

Thank again,

For information, after multiple tests (change fathoms cards, cable resistance control…), with 200m of slim tether with a BR2heavy, maximum of 35 / 40mbps. So very standard video …

Hello Marcus. Normally, a power supply is sold as a system, so that you purchase both the submersible vehicle power unit (that replaces the battery housing) and a surface power unit that supplies the higher voltage and also includes safety measures like earth leakage (GFCI), Line Isolation Monitor (LIM) and fuses. It can be very dangerous to provide power directly from a generator, unless it includes appropriate safety measures.

To answer the power question, in typically use 15-20A at 15VDC on the BlueROV2 heavy (8 thruster), but if running at 100% gain and pushing very hard, it can draw 60A or more. Our estimates are: 15-200W slow flying, no current, 200-400W normal flying, low current, 400-600W fast flying or strong currents, 1kW maximum for very fast flying or very strong currents. We find our 500W supply is good for most customers, but a few may want to look at the 1kW surface power supply from Outland Technology (available on BlueRobotics.com). Ours or theirs will both run on 115V or 230VAC and have appropriate safeties. We run 22AWG power conductors in our tether (2 total) and a bare earth wire for safety and that would work fine at 400m (about 20% voltage drop with ROV power 500W at 15VDC).

For the voltage drop, the following calculator may be helpful: https://www.calculator.net/voltage-drop-calculator.html

If Gigabit Ethernet is required at 400m, then yes, fiber is your only option, however, even many professional cameras will just record locally and only send HD or even SD video to the surface. I have attached a datasheet for our A/C power supply system. You can email Ramon Gomez Ramon@PoseidonROV.com in Spain or Jeff Conger Jeff@Blue-Linked.com in the US for more details.AC Surface Power Unit rev3.pdf (269.5 KB)

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In the surface power supply we sell ( http://www.deltarov.com/new/product-category/rov-accessories/power/ ), I redress 220VAC and it gives me 310VDC. You can connect an AC power supply on the other end provided that the voltage drop stays above the lower end on your power supply’s VIN.

Going with AC will loose a lot of efficiency.

Depending on client requirements, I sell 8.5 or 12.5mm tether. The larger tether does affect flying.

To give you an idea, using my surface power and my 12.5mm tether, you could use our 750w power supply over 400m.

Alternatively, you could use your 30mm tether and make a junction box/clump weight and reduce the size of tether from the clump weight to your ROV…

Our GigE fiber system is cost effective and we even sell an underwater connector for it. http://www.deltarov.com/new/product-category/connectors/subsea/
Toolkit: http://www.deltarov.com/new/product-category/tools/

Fiber is an investment for sure but well worth it if you are using the ROV over longer tether.


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