Twisted or straight cables for tether


#1

Hi,

I was wondering if there is difference in using a straight cable or a twisted one when connecting between the home plug adapters? Has anyone tested this?
My tether will be 150 meters.


#2

We’ve done a small amount of testing on this, using both twisted pair for long tethers and straight cable (speaker wire/zip wire). It didn’t seem to make a difference over the length of cable we tested, which was ~300m.

There may be a difference in crosstalk, such as if you’re driving two OpenROVs right next to each other. The Homeplug protocol is sensitive to this, and twisted pair might help here. But if you’re the only ROV in the vicinity, I don’t think it matters.

Let us know what kind of cable you end up using, and how it all works out.

-W


#3

Beyond Walt’s discussion of crosstalk there is also a mechanical advantage to a twisted tether in cases where the cable takes mechanical strain.

When a straight plastic jacketed copper wire is stretched, the copper and plastic both stretch together. When the tension is released the plastic springs back while the copper remains stretched. Now the copper is too long for the plastic jacket and it is in compression. The compression can cause strands of the copper to kink forming a sharp bump than can chafe through the plastic jacket from the inside out.

If the wire is twisted the copper metal is less likely to stretch. Instead the copper presses sideways into the plastic as the twist tightens with no sharp points to cause damage.

Ideally the Open ROV tether should not get stretched. But in the field things go wrong, tethers get snagged, people trip on them, and all sorts of other abuse happens.

SherpaDoug


#4

Thanks for the feedback.
I currently have 100m of straight cable (24awg) that I want to use since I already have it. Just buy some more and splice it together.
After I thread that through the hollow PP rope I have its pretty much neutrally buoyant. At least my first test with a 2m test piece in salt water was. Problem here is that the PP rope stretches a little but the cable won’t. So might be a problem if I need to pull on it too much. We will see how long it holds up.
When the cable eventually breaks I can just change it out with a twisted one or something else I guess.
Twisted cable is quite hard to find actually. At least here in Norway.
One could use a pair from a Cat5 cable I guess, but the insulation on those seems a bit too fragile to me.


#5

Make sure that the rope you use is the same as the rope you tested with.
I tested with a rope I bought locally, but it didn’t come in long spools, so I bought a different rope on a long spool. The second rope was not as dense and did not make my tether neutral.

If your rope is shorter than your tether, when you pull it in by the rope, it won’t pull on your tether, but will slide up until it is tight. It depends on the size of the rope, but for me, the tether was too thin to bunch up the rope. I hope that makes sense.