4-wire theter


Hi all,

this is my fisrt post, so I would like to introduce briefly myself.

I'm Tiziano, from Italy, I'm 25 and I'd like to explore the seas of the Ligurian Gulf, that is where I live.

Ok, really short introduction, so we can discuss a bit about interesting things. :)

In these days I was thinking and reading about the tether, and I'm now studying a 4-wire tether solution.

So, I wold like to discuss a bit with you the idea, just to know if I'm wrong.

The idea is simple, and is nothing really new. I mean: 2 wires for the ethernet and 2 wires for power supply. Just to keep it simple.

First problem: how many Amps are absorbed by the ROV, at maximum? Well, doing some calculations I found something around 3.5 A. I'm right?

Supposing 3.5 A, for a 100 m tether, i was wondering using a PS wire of 16 or 18 AWG. It should be ok, I hope. But... the wire is heavy...

Also, for a safety purpose, I would like to add a nylon rope of 4mm diameter, in order to manually recover the ROV in case of failures, and also to strenght the connection. But, again, this adds weight.

For the moment I'm not thinking about the flexibility of the tether assembly. But it is a problem too!

A possible solution for the wire weight (and maybe for the maneuverability) with this kind of tether, is a scuba diver buoy. Here the thing I'm thinking: http://www.ebay.it/itm/BOA-SUB-SILURO-IN-PVC-PALLONE-DI-SEGNALAZIONE-SAGOLA-/110932259856?pt=Sub_scuba_e_snorkeling&hash=item19d413f410#ht_1816wt_1037

The tether can run from the ground to the buoy, then, with a pulley, down to the rover. In this way, we can reduce the tether weight over the ROV. An other advantage, is that I can run the rover some tenths of meters off the shore and signal the position to the people around me. Last advantage, is that in case of problem, I can pick the ROV first UP, and then BACK to the shore, that is good in presence of underwater stones that can stuck the ROV in case of recovery.

Waiting you for better ideas, and to know if a 18 AWG wire should be ok over 100 m (and over 200 m tether?).




While my electronics are only about 90% complete - during my tests running up the motors it was in the neighbourhood of 3 - 3.5A @ 12VDC at full power on all motors, LEDs.

That being said - one thing to consider on the longer tethers will be the voltage drop at that load... Even at 18AWG you are looking at pretty significant drops.


Thank you for your feedback, Chris.

Yes, the voltage drop is very significant, and I'm thinking about it. If my calculations are not wrong, with 100m of cable, at 4 Amps (overextimate), we should have a voltage drop of about 15V. This for a 18AWG cable i found on RS (it has 20 ohm/km of resistance).

An other problem is that the maximum load is not constant over the time, so we have a voltage over the rover that bounces between two values, in function of the applied load.

I'm saying nothing new, but it is just to share my thoughts of today.


Couldnt agree more - it has been suggested that we use a rechargable battery setup onboard simply to take up the slack under high power operations. It is something we are looking at on #049


If I well understand, the batteries on the rover will be constantly kept under "remote-charge". Right?


exactly - assuming you arent running around at full speed the entire time haha


mmm... but to reach America from EU I need full speed all the time! ^_^

Well, keep us posted about that solution! Very cool!


About the nylon safety line: what kind of load are you expecting? 4mm seems rather extreme.

I'd like to add a safety line to the 2-wire standard tether myself, but was thinking about using fishing line (~0.7mm, ~90kg)

should still be sufficiently stable to haul the whole thing back and won't increase bulk or weight of teher by too much.


I was thinkikg about fishing line too. 90 kg is really enough. I have one in my toolbox that is like yours. The biggest rope was only to give better visibility of the tether. I don't know if it is an advantage or not: it's only an idea.


given the range issues of the 2-wire tether, 4-wire looks like quite an interesting alternative. Thinking of 2x twisted pair here.

use for 10Mbit ethernet directly, optionally using 802.3at PoE+ for (supplementary) power.

it this works as envisiond, you'd get both a stable ethernet link at 100m and ~1.8A/12V.

looks like a reasonable compromise.


Consider using a polypropylene rope because it is lighter than water so will add no weight and might actually add buoyancy to the tether. If protecting the tether line is an issue, polypropylene comes in braided form which could be used as a sheath. Density of polypro is 0.855 g/cm3. I can't find specs for rope size and volume but it would relatively easy to measure or simply to test.




I would use passive PoE, it does not require any circuitry unlike 802.3af/at, though I guess it depends on your expected length of cable.

You can get exterior cat5 used for spanning gaps, ie, hanging in space from a pole to a wall for instance, like old style telephone cables from poles to your home. These cables have an integrated strand of wire, generally steel, to support the cable and allow it to be stressed so it doesn't sag. This could probably be a poor man's all in one tether. Depending on the distance you could probably run 100mbs over it up to about 150m though that would be at the extreme end, probably safer to go to about 125m with good reliability providing you don't use cheap connectors and have lots of patch cables between the rov end and the switch end.