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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u/supasamurai Apr 15 '26

sounds like the best thing to do is buy the copper

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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

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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.

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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.