The Options For Low Cost ROV Tethers

Wireless connections are cool and all, but sometimes you just need a bit of copper. This interesting article on SV Seeker discusses the various ways of making a tether for a remotely operated vehicle (ROV). They experimented with a number of different cables, including gel-filled Cat 5 designed for burial and wrapping the cable in polypropylene rope to keep it protected and buoyant. They also looked at using a single core solid coax cable with an Ethernet to coax converter on either end wrapped in stretch webbing. The upside of using coax would be the length: it can handle over a mile of cable, which should be more than enough for this project. The downside is that they found that the coax stretches under strain, messing with the signal.

The project seems to be on hiatus at the moment, but there is lots of food for thought here for anyone looking to send data over a long distance. I’m looking to put a webcam in an owl box in my back yard, and I hadn’t come across some of the options they cover here, so I’ll be looking into direct bury rated Cat 5 myself.

22 thoughts on “The Options For Low Cost ROV Tethers

  1. I’ve followed Seeker for a while. I suspect it’s not so much on hiatus, but waiting for the other things on the boat to be finished first. In a few more months, they’ll release another video about that aspect of the project, catching us all up on what has happened, little by little, in the last few months.

    1. I also like how they flip the feed for the umbilical.
      The ROV’s used for exploring deep sea and shipwrecks have a spool of cable on the craft itself. The idea being that the larger vessel can feed cable while it descends to the site or object, and then the ROV can start unspooling cable when it starts exploring.
      This was such a lightbulb moment when they explained this, because it allows the ROV to explore pretty much unencumbered because it doesnt need to worry about dragging extra cable around any obstacles or debris, and if the cable does get caught, the ROV just needs to get out of the wreck, and then sever the cable to automatically surface on its own.

      1. The ROV umbilical I have seen was about 25 to 30mm thick. With HV supply, 1^100V for the electronics und 30 or 100kW of 3 phase power for the motors. I think it also was heavily steel reinforced (or kevlar). I think if you want to cut this on purpose, the best way would be to use pyrotechnics (some explosives) like in rocketry.

        1. I had a job offer years ago at the Monterrey Bay Aquarium research institute (MBARI?) to do design and operations on their vessel, called IIRC the Pt. Lobos. The only reason I turned it down was because I would have to assure the safety of power for the submersible, which took a couple thousand volts (4K sticks in my mind) DC through a mile of cable for the high voltage motors. This was beyond my comfort level on steel decks and salt water. Too bad. Talk about a dream job, and they made mostly day trips

          Anyway, you can run a lot of power on small cables if they can take a couple hundred volts.

  2. Check if you can reduce the speed on the coax device some of the newer models use a higher frequency carrier. Also why not use a smooth slug of iron with a string and pull a magnet along the outside of the sheath maybe with some kapton or felt wrapped over the magnet to slide along the outside.

    1. Yeah I’m thinking there’s plenty of transceivers around that are designed to push signal down coax in the most craptastical conditions. I’d have started by screwing around with 75 ohm and cable modems.

  3. It has the downside of not delivering power; but this situation seems like a fiber sort of job. Lightweight and flexible compared to multi-strand twisted pair(especially if dealing with lengths where stretching that changes the twists is relevant), native support for long runs, at least as water/corrosion resistant as copper if jacketed(possibly even if not, but I’d be wholly unsurprised if some obnoxious ‘gradual water absorption ruins total internal reflection’ effect potentially comes into play).

    Potentially also helped by the fact that fiber aimed at 100mb runs is more or less wholly obsolete(so you can either pick up new old stock of that cheap, or take advantage of the fact that everything you can buy is aimed at 1-10Gb and so overqualified if you need to try to salvage a marginal situation by operating at 100mb.)

    1. 1GB is not as overspec’d as one would think…
      A !single! nice 4k camera with a high bandwith stream and you’ll easily saturate a 100MB line, and this is still using lossy compression… now add to this 2 side scanning sonars, maybe even a 2nd camera with a very wide lens for good spacial orientation and suddenly that 1GB line does look more then adequate ;-)

      Also 1GB single mode fiber elements are dirt cheap, media converters are like $30 and less, not to mention the fiber cable itself will be cheaper per length then most metalic cables that include some form of strain relief, you just have to buy the whole spool.

  4. Does anyone make a CAT 6 or coax with a Kevlar thread in it? You could use that for the load bearing. If not maybe run a Kevlar thread or if you can afford it a carbon fiber tape along the length and use shrink tubing at intervals to keep them together.

  5. I worked as a commercial diver for a bit, and our diving line was made of separate elements taped together – a Polypropylene strength member for keeping everything together and for hauling out a diver in an emergency, an air line, a communications line, and a pneumo line (an open-ended air hose) for measuring depth. Might make sense to do something similar on an ROV with a synthetic rope for strain relief and strength, a DC power line and fiber for data. At least with everything separate the chances for water ingress ruining the whole thing are lower, and it’s much cheaper than bespoke combo-cables.

  6. I know how hard it is to make a proper cable.
    I was DJing in the early 2000’s and I needed a one cable that could do a most anything. I liked to keep everything as simple as possible. And boy did this cable worked great.
    I breaded all most a 100′ with 5 cables. It ended taking me 3 days to do.
    It had 1 cable of 16/3 for 120 volts and ground,( I was not concerned about voltage drop it was mainly for the projector.) 2nd cable had 4 cond. #16 shielded,( for audio.) 3rd & 4th were Cat5,( for VGA and spare.) and last #5 was a 3 par. shielded for future.( Ended up putting to video cameras on it with audio.)
    I used this cable for a projector with VGA & comp. and video coming back to me and audio wires for a 800watt amp.( and no I didn’t run the amp off of the 1st cable.
    I had made up different ends for everything I could think of.
    It worked out great and still use it today for when we have movie night outside.

  7. Look for coaxial cable with an integrated messenger line. This is the type typically used for aerial installations in cable systems. It is available in 50-Ohm and 75-Ohm. Amphenol, PPG/PPC have many options available for diameters… RG-59(probably -174, too) equivalent all the way up to 1″ Hard line…. fiber, too.

    The messenger line is an insulated steel cable connected to the outer jacket that is designed to take the strain of any physical loads. It is intended that you utilize this steel cable to secure the ends of the span. This line can be peeled away from the jacket where necessary. It’s tough stuff and you need to start the “peel” with a knife after severing the messenger cable. We ran 200 foot lengths of RG-11 all the time without any problems, including combined ice and wind loads. We picked up a (small) forklift with it in a test of strength(with just the messenger and with just a connector)…

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