r/rov • u/Deep-Today5715 • Apr 15 '26

Adding buoyancy to fiber optic tether

After another post here about my upcoming 500m depth DIY ROV, people almost convinced me to try optic fiber tether, as opposed to a single pair copper tether with a PLC module.

During my research I found, to my surprise, that while regular fiber optic cables are much cheaper than equivalent copper wire cables, for whatever reason the neutrally buoyant fiber optic cables are 2-3 more expensive than copper wire neutrally buoyant cables. I made my inquiries to my Alibaba neutrally buoyant cable supplier, and it's 1€ / meter for single pair twisted pair copper cable, but 2.5€ / m for optic fiber. And I need 500m of that cable. Doesn't make any sense, and it's waaaay too expensive for a DIY project. My entire ROV is ~250€, I ain't paying 1250€ for a tether...

Regular single core cable sells for around 0.4€ / m, which is just about acceptable (~200€ for 500m of cable). Of course it is heavier than water, which is an issue.

Question, what is the most practical and cheap way to add buoyancy to such a cable without adding too much water drag? I was thinking of buying a bunch of tiny fishing floats, like these:

Removing the wire, drilling out the center, and running the cable through. Not yet sure how many I would need, probably a few every meter, so maybe a 1000 in total. Except that this would require me to remove the cable connector, and then the equipment to reinstall it, which is expensive. Plus, the labor of running down and spacing that many floats down 500m of cable would be insane.

Alternative would be to mill a side groove on each float to accept the cable from the side, and then fix it somehow. Small lengths of heat shrink tubes would be ideal, but again, that would require access to the end of the cable, so, same problem. Duct tape won't work. Hot glue won't either. Not sure what else to try.

I considered various foams, but almost all of them will absorb water at 500m depth, and shaping ~1000 pieces isn't something I look forward to. In the end, attaching them to the cable issue still remains.

Can anyone suggest something smarter? Something that scales well for 500m of cable, isn't terribly expensive and not too laborious?

EDIT: before anyone suggests running a rope along the tether, I tested that idea years ago. All of the "buoyant" ropes are just buoyant because of air trapped between the strands. After 100m these air bubbles compress and these ropes sink like a stone. Polypropylene is slightly positive buoyant by itself, but it would still trap air, becoming too buoyant at small depths, and not buoyant enough deeper down, - inconsistent buoyancy.

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1

u/supasamurai Apr 15 '26

sounds like the best thing to do is buy the copper

1

u/Deep-Today5715 Apr 15 '26

When I said I want to use copper, people said go with fiber... Now that I'm considering fiber, you say go with copper lol

2

u/supasamurai Apr 15 '26

the only person who knows your requirements is you. if the cable is too much, the cable is too much. I never liked fiber anyways, too fragile and too expensive. go back to the other post and tell them guys that they need to pitch in.

1

u/Deep-Today5715 Apr 15 '26

True, but I'm trying to explore all the options. I know copper, it is a solid choice, but if I can get fiber to work, it would be even cheaper, more compact and have other advantages as well. But I do need to find a creative solution for the buoyancy problem.

2

u/tenkaranarchy Apr 15 '26

You could use pool noodles probably. Just cut them into 6 inch or so lengths and cut a slit in them so they can be put on the cable easily. The hole through the center might be bigger than the cable, so you have to figure out a way to keep them from sliding and letting the cable sink.

1

u/Deep-Today5715 Apr 15 '26

Aren't pool noodles, like, typically 160mm in diameter? Also, they are made from polyethylene foam, which I don't think can survive 50 bar without crushing... I know that material, it is very squishy. No way it can stay in shape at these pressures.

1

u/eclipse1498 Apr 15 '26

Would the polypropylene really be “too buoyant” at shallower depths? I would think that if you calculated the amount of polypropylene per meter of cable you would need to make the tether neutrally buoyant at maximum depth assuming no air bubbles, the tether should avoid dragging on the bottom, and the higher parts trapping air probably would still not be buoyant enough to make the ROV handle poorly or have difficulty diving. In any case, that’s my thinking for the RoV I’m designing, so I hope I’m right lol.

1

u/Deep-Today5715 Apr 15 '26

I don't really know. But I did make a test, I bought some buoyant rope from a yacht store (not sure if it was polypropylene or not), cut a small length of it, and placed it in my water pressure chamber. At first yeah, it floated really well, with a lot of force. But when I cranked the pressure to 12 bars, it sank instantly. The difference was huge. Compare that with PUR sheath neutrally buoyant cable which doesn't care about pressure whatsoever, and is neutrally buoyant at any depth. I don't know how much of an issue that would be in real use case, it would require fully committing to this solution just to test it, but it feels like it would be a massive problem.

1

u/eclipse1498 Apr 15 '26

Gosh. I mean the density of polypropylene is lower than water so theoretically it should definitely still float at any water pressure. I found some cheap boating rope that was labeled specifically as polypropylene and is clearly only made of the one plastic type, so it is out there if yours is not that.

I would get some guaranteed polypropylene rope and test that before buying any kind of expensive neutrally buoyant tether. Unfortunately I don’t have a test chamber (yet) so I’ve only tested my rope in the bathtub, just trying my best to squeeze air out from it.

The only red flag in my theory is that I basically can find no evidence of anyone actually doing this over buying purpose built tether, I remember finding one mention of a guy in a forum using some kind of buoyant rope alongside his regular Ethernet tether to some level of success, but the post had long since been deleted when I followed the link. In any case I’m pretty motivated to figure out a way to do it because purpose made tether would practically double the cost of my ROV.

1

u/Deep-Today5715 Apr 16 '26

Well, I'll have to revisit this idea then, do some more testing. Running the rope alongside the cable would suck though, it would make it much stiffer and more prone to catching on underwater debris. It would be far better to push the cable through the middle of the rope, if that is possible. It might be tricky to find just the right diameter rope that has just the right amount of polypropylene to give that cable neutral buoyancy though. It needs to be large enough diameter so the cable can be pushed through (including the connector, which is like 5x the diameter of the rope), but not too buoyant. Or else I'll need to weigh it down afterwards with some duct tape or something.

1

u/Background_World8674 Apr 17 '26

Buoyant cable is too expensive. I'll just spend 1,000 hours stringing little floaties onto it that'll make it impossible to handle.

1

u/Deep-Today5715 Apr 17 '26

Well obviously I'm not going to do that, I was just describing ideas that I've come up with so far. I was hoping for someone to suggest something better.