Tether density - floating level tapered fly fishing line for neutral buoyancy



Does anyone know the density of the tether (or mass per volume of displacement) from OpenRov version 2.7? I would like to do the same simple calculation on level-tapered fly fishing line. My goal is to weave dry fly fishing line into the tether cables to create a neutrally buoyant tether. I am assuming that dry fly fishing line wouldn’t compress at depth. I could be wrong but I will find out!


Check out my post on this other forum post asking the same question.


So, (hopefully experienced bouyancy calculator people will forgive me) if: <img

Then, we’re looking to add something like 2.63 Grams/Meter of upward force to the tether (for salt water) to make the 5.3 G/Meter tether neutral? I think I will try to measure the force from this stuff: http://www.fishusa.com/product/P-Line-Hydrofloat-Thermal-Fused-Spectra-Line

At the very least, it’s Spectra, so it would contribute a lot of tensile strength to the tether, and it won’t stretch. It also won’t break the bank. I wonder about the compressibility…


That’s great Robert. Thank you very much!

I would really appreciate to know what you calculate for mass and displacement for 1m of that spectra line. Maybe it would be worthwhile to weave 2 lengths of spectra line together in a similar pattern as the rov tether? If you are planning on weaving the spectra into the rov cable then I think it would be a more applicable method.

Thanks again Robert!!!


The numbers I sent are for one tether that I measured. The weights and displacement do vary a little from tether to tether. Just something to keep in mind with your calculations.


Yeah, I want to reiterate Brian G’s point. The tether shipped with OpenROV can vary significantly from bundle to bundle and even within a bundle there is variation. Might be a good idea to overestimate the math on it so that if you are off your tether is a little more floaty than sinky in seawater.

Eager to see how this turns out,



Clarification question for Brian and Zack -

Is the variability in shipped tethers caused by differences in the length of shipped tethers, air bubbles in coiled submerged tethers, or by actual variation in the tether caused by the manufacturing process?

I would imagine that these wires are mass-produced by machinery that doesn’t cause much variability but I could be wrong.




When speaking about the stock twisted pair on the OpenROV 2 series, the variation is from the manufacturer and is caused by variation in jacketing thickness. They are made with mass-produced machinery but there is variation from unit to unit and along the length of the unit.

The actual length of the wire is pretty tight but might also contribute to the variance in weight we are seeing. I’m not sure to what degree.



I’ll definitely weigh mine. I ordered the line I linked to, and i’ll report back here with some estimates!


I can say from my experience with both fly fishing and trying to get my tether to remain neutral that a fly line will be a bit light. Here’s what I have done. I unwound using my trusty drill…a 6mm - 3 strand poly rope and wound the tether back into it. I still had to add a fit of flotation about 3m up from the ROV to ensure the tether stay out of the way. Hope this helps, good luck!


This sounds promising. Looking forward to your observations.


Well it’s been about 50 dives and approx. 75hours of bottom time. It works a treat!