DIY tether buoyancy

Hi! I am building my own second ROV, and even though I can’t afford BlueRobotics parts yet (maybe I will for the next project), I have copied a lot of design solutions from BlueROV2, and I do mention it often when I present my project publicly, so even though I’m not a customer yet, I hope it is still okay to ask here for advice :smiley:

Since I cannot afford Fathom ROV Tether, and I need 100 m. (I want to stay under $100 for the entire tether), I decided to do the PLC module hack, so I can use pretty much any 2 wire cable, even untwisted, and have 100 mbps data transfer with minimal jitter. So I have many cables to choose from - power cables, telephone cables, speaker cables. As far as I tested, anything works at 100 m. length. Trouble is, all cables sink. I want to achieve neutral buoyancy at any depth (disregarding temperature and salinity variations), but not with floats, since they would introduce a lot of drag and make tether prone to catching on obstacles. So I need to keep it smooth.

What I tried so far:
1. Pipe foam insulation, pool noodles - gets crushed under pressures of around 2-3 bars and loses buoyancy, also absorbs water over time.
2. EPDM foam - also gets crushed.
3. Neoprene tubing - way too expensive, also difficult to get in just the right size to have just the right amount of buoyancy needed for this cable.
4. Non-sinking polypropylene rope - turns out, it is mostly buoyant because of air bubbles trapped between rope strands, which get crushed under water pressure and lose buoyancy.
5. Threading cable through a garden hose / aquarium PVC tube - I tested many different hoses and tubes, and found that ones that have walls thick enough to withstand pressure without imploding are to heavy and stiff. To achieve enough buoyancy the final diameter becomes unacceptably large. Running cable alongside the tube instead of inside it provides extra buoyancy, but makes the tether even stiffer.
6. Using thin walled PVC tube or heat shrink tube over the cable, and replacing air with low-density oil to prevent implosion - requires even thicker tube to reach neutral buoyancy, so much that it becomes unacceptably stiff. Also, water pressure squeezes the parts of the tether which are deeper in the water, pushing the oil to the shallower part of the tether, where it bulges.

As I understand Fathom tether uses PUR foam jacket. I have done some PUR molding before, but I don’t think there is a way I can mold it evenly around the cable for a 100m length. Also, as far as I tested, only the densest and stiffest PUR doesn’t absorb water, which makes it heavy to begin with. Tether flexibility is very important to me, since my ROV is small and needs to maneuver freely.

I would very much love to hear your suggestions on a low-cost solution for tether buoyancy. Like I said, my budget for it is $100 for 100m length. Cable will set me back $30-60, depending on which one I buy (I can buy with thinner copper and jacket, expecting buoyancy material to provide strength, or with thicker copper and jacket so it is strong enough on it’s own). The most important thing is achieving neutral buoyancy at any depth, so the solution has to provide same amount of flotation regardless of water pressure.

Thank you in advance!

Hi @laukejas,

While this forum is partially used for discussion and support of Blue Robotics products, it’s also intended as a gathering point for the general marine robotics community - discussion of DIY manufacturing ideas is perfectly fine :slight_smile:

That is correct - the jacket on both our Fathom tether types is a polyurethane foam, as indicated in the technical details table and drawings.

I don’t have much experience with tether manufacturing technologies, so don’t really have suggestions beyond what you’ve already tried, other than potentially contacting a tether manufacturing company directly and seeing if they’re able and willing to provide something that fits your budget.

Hey, @EliotBR , thank you very much for your reply and info. I am wondering, how exactly is that Fathom tether made? How is that PUR foam molded over the cable bundle? Is it done in sections? Or continuously as the cable is pulled? How does the mold look, how is the cable bundle kept centered? How is the protective jacket applied on top of the foam?
I also wonder about what kind of PUR formulation that is. As I understand PUR stands for polyurethane. I poured some low-density closed-cell polyurethane foam myself, but even the lowest density foam was still way more rigid than what I’m seeing in your tether videos. Is the PUR foam you use somehow different, does it use some additives to make it more flexible? I also noticed that lowest-density closed-cell polyurethane foams still absorb some water over time at high pressures. By my tests, even 400kg/m^2 foam absorbs as much as 2% of water by weight when subjected to pressures of around 10 bars for a few hours. Is this an issue in Fathom tether? If so, does it rely on that yellow jacket to prevent water from reaching the foam? If not, is that eventual absorption accounted for with a little extra reserve buoyancy?

I know this is a lot of questions, and of course I do not think I could repeat the manufacturing of such advanced cable with DIY means, but I am very curious about these details, as they might give me some ideas to consider.

We don’t manufacture the tether cable ourselves (it comes from a supplier), so I’m not certain.

For reference, here’s a photo of our tether. I’ve cut it at different points to show the different materials and sections, and tried to group together the fibers and wires to make it clearer what is what:

From an efficiency standpoint it would make the most sense to me if the foam jacket is applied continuously as the cable gets assembled, so I would naively assume that’s what happens, but can’t guarantee that it’s actually the case. It’s also possible the sheath gets wound around the bundle, followed by a separate stage where it gets wound onto a reel as the foam is deposited onto it.

If it was done in sections I imagine there would be some form of irregularity at the joining points of subsequent sections, which is not the case from what I can tell, so that’s at least some evidence that the jacket is likely applied continuously.

Again, I’m not sure, but I imagine the centering is done by maintaining a constant tension on the wires and fibers as they get pulled through where the foam jacket is being applied.

The foam is the jacket.

I’m not sure, and it’s very possible our supplier wouldn’t share that information with us even if we asked. They make and sell a product to a specification, and how they achieve that is up to them. While I agree it’s intellectually interesting to learn about these things, I suspect that the exact formulations and processes they use are likely “trade secrets” to avoid others underselling them by replicating their work without needing to put in (and pay for) any development effort.

As above, the foam is the yellow jacket.

Given we don’t provide any strict buoyancy values (and buoyancy varies by water temperature and density anyway), it wouldn’t surprise me if some water absorption is expected at depth. It has been mentioned previously by some users as a phenomenon they’ve noticed occurring.

I’m not really sure what you mean by this. A tether is a passive component, so if it’s precisely neutrally buoyant at the start of a dive, and has absorbed some water by the end of it, then it must be slightly negatively buoyant at that point (assuming that the water temperature and composition remain unchanged throughout the dive).

Given the fluid density is not guaranteed to be maintained, the buoyancy of an object in that fluid is already not a fixed property. Presumably that’s partly why minor density changes of a tether are treated as an expected part of operation rather than a design/manufacturing flaw :slight_smile:

Thank you very much for that info, @EliotBR , that is most helpful! That picture you attached is most surprising. I previously thought that PUR foam completely displaces all air between wires in the tether, filling every gap, but apparently it only goes on top and around the sheath. Meaning there is still air inside, between the wires. I find it very surprising that the water doesn’t reach the inside of the cable through nicks/gouges in the cable or the ends of the cable, displacing that air and making tether lose buoyancy. That foam must be quite resistant to damage to achieve such reliability.
Thanks for clarification about the jacket. I had seen cross-sections of other ROV tethers, where PUR foam is white, and is covered with an additional thin yellow jacket on top. I see now that your construction is different. All is clear.

I meant that since there are so many variables that affect tether buoyancy, and most can’t be accounted for since they are uncontrollable environmental variables, that perhaps the tether is intentionally manufactured slightly more buoyant than neutral, so that if it does soak in a bit of water at depth, or if water salinity/temperature is a bit on the lower side of the nominal conditions, the tether would never become negatively buoyant. I say this on the assumption that while completely neutrally buoyant tether is the ideal, it is better to have it slightly too buoyant than not buoyant enough :slight_smile:

There are a number of steps in this process, but very simply for signal wires:

  1. Individual wires are twisted into pairs (sometimes each pair is wrapped with shielding foil)
  2. They are then bundled together (sometimes around a separator), in the case of BlueRobotics tether, kevlar and dacron are added for strength.
  3. A protective wrapping goes around the bundle
  4. The jacket/sheath is extruded around the wire

Generally this is all done in a production line. Steps 1,2,3 are done in one go, then it is fed through the extruder.

The thickness and material properties (such as density) of the jacket material are designed to give the desired buoyancy.

Below is an example of a machine that does this for silicone wire.

I don’t know of any companies that make their own cables in house. It is a specialised process with a lot of variables and secret sauce.

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The fibers in our tether have a water-blocking component (as specified in the technical details), which as I understand it means there’s a reaction that occurs in the case of water ingress that acts to locally prevent water from spreading (e.g. in the case of a nick/gouge). The foam is also quite robust to cutting/damage to start with, and the fine “filler” fibers help to fill the space around the other parts of the cable, to reduce the amount of displaceable air inside.

As stated in the technical details, our tether is designed to be slightly positively buoyant in salt water, and neutrally buoyant in fresh water. I don’t believe water absorption over time at pressure is factored into that, and the exact temperature and salinity ranges the statements hold true for are likely not explicit requirements of the design (and may vary slightly between batches).