Tether length options - splicing


Have you ever wanted a longer tether (100 meter +) for your Trident?

I had my tether extended from 100 to 150 meter in May 2018 and posted some information about the splicing on the OpenROV Trident Pilots Group on FB on may 30th.

I’m very happy with my 150 meter tether. I very seldom use my 25 meter tether. We did the same splicing operation for another Trident pilot in Norway!
He has not reported back any problems!

The hand reel has a capacity way above the 150 meter.

Tether lengths over and above 150 meter are not recommended by OpenROV Technical dept. due to reduced voltage over longer lengths!

In one instance I experienced a restarting issue after my 90 minutes dive, when I was trying to start and run the unit in my freshwater tank for «cleaning».

Restarting/reconnection of app/unit may be a problem with a 150 meter long tether when the battery is drained more than 60-70%.

Several options for a longer tether exist.

Some options seems rather expensive,

like buying a new 150 meter tether!
Gestimated cost of US$ 450 from OpenROV (List price for 100 meter x 1,5)

One option is to splice your 100 meter tether with your 25 meter standard tether. No extra cost for new tether, but approx US$ 50 for the splice job.

If you think you need more than 125 meter to get down to max depth of 100 meter in strong current, you may look at buying additional 50 meter tether with the watertight connection in one end and splice with your 100 meter tether.
Gestimated cost of US$ 150 for the 50 meter tether and extra US$ 50 for the splice job.

Hopefully OpenROV will come forward with a bargain price for extra 50 meter tether.

Here you have a short description and some pictures (1 - 11) on how we did the splicing:

Kevlar is tied together, wires are soldered together, primer applied to tether ends and then a crimping stocking that have been shrinked in one end and then the stocking is filled with potyurethane casting. Then it will be completely water tight and very strong. We never tested the Breaking strength of the extended tether, but we feel confident that this is not a week point!

We meassured a voltage of 24,47 volts over the 150 meter with a fully charged battery.
We noted during a first test that voltage dropped below 20 volts just before we got communication between the JXD controller and the topside unit and then the voltage rose again.


Missing pictures


Thanks for sharing this! I havent had the need for >100m yet but i feel it’s coming… :smiley:


Thanks Kaare, this is very helpful information.

I am particularly interested to see your voltage reading of approx 24vdc at the topside controller during operation.

I had been led to believe the operating voltage in the tether was likely to be greater then 12vdc when Trident is powering the topside controller so your readings tend to support this belief.


Sorry to write that we did not do any measuring/reading of voltage during operation of the Trident!

AS stated before;

We meassured a voltage of 24,47 volts over the 150 meter with a fully charged battery.
We noted during a first test that voltage dropped below 20 volts just before we got communication between the JXD controller and the topside unit and then the voltage rose again.

Based on the following Information from OpenROV we were happy and completed the splicing ;

«The ROV puts out 32V all the time. If the voltage at the topside ever dips below 16V, then you get a self-reinforcing downward spiral that kills the topside. So you need to make sure that there’s plenty of margin above 16V. I wouldn’t go below 20V. But, no guarantees, as we haven’t done that testing here.»


Hi @Kaare:

Thanks for posting your work here.

Many people have asked for there to be a longer Trident tether in the store, and here at OpenROV we would love to accomodate them. We had a couple of prototypes of 150m tethers made, and we found that they would work with most topside units- but not all. There seems to be an issue with the timing of the way things boot in the topside, and some topsides draw more current than others, presumably due to multiple boot events happening simultaneously. As we mentioned to you earlier, if the voltage at the topside drops below 16V at any point in the boot process, the power supply drops out and the topside does not finish the boot process successfully.

In the long run we want to figure out what’s going on with the various processors in the topside, and revise the boot process to smooth out the current draw. But this is not our highest priority right now.

We are also considering releasing a powered topside, which would allow tether runs of at least 200m.

We could potentially sell 125m tethers in the store. We’ve tested a number of those and they always seem to work reliably. A 125m tether would allow users to reach the 100m rated depth of the Trident as long as a long horizontal runout is not needed.

If users would be interested in either a 125m tether that used the existing topside, or a powered topside that would allow much longer tether runs, please let us know via the forum here, or via the support channel. The more we know about what users need, the better we can spend our R&D funds.

Hope this helps.

-Walt Holm



Thanks for the feedback!

Good to see that you guys have looked further into and tested lengths up to 150 meter! Interesting to hear that results have been different with different units!

I think that most people would be happy with a 125 meter tether ! - especially if they combine with clump weight!

For these guys a watertight connection (extension joiner) between the standard 25 meter and the 100 meter tether would be GREAT!

We suggested this back in February 15 2018 after my very first demo of the Trident in Norway!

See my Post on OpenROV Norway FB.

(Written by Asbjørn Husebø)

Here is a small cut from Asbjørn Husebø impression after testing my Trident:

«There are almost always some current in the water that pulls on the tether and wind to push the boat while anchored. The tether should be 125-150m to compensate for that. If you are worried that people will go deeper than 100m, you can have a software depth limiter. One easy solution for people that have already paid for and received their tethers is to offer an extension joiner to connect the 25m and 100m tether together.»

Hoping to see you offer an extension joiner soon!

Your are also mentionning a powered topside unit for longer lengths of tether!

How can I power up my topside WiFi unit so I do not run into problems due to too low voltage to the topside?

Inge Kåre


Hi Walt

Always a great source of useful information

Just picking up on some thing you said

Would this unit have to be topside?

I understand that it needs to be powered but if it was submersible it may add many options

I for one, have both a 25m and 100m tether (which obviously has submersible connections) if this powered “extender” could be placed mid water it would allow this 125m option (as well as 200m tethers using standard 100m tethers) To be easily implemented

Additionally if a weight could be added to the “extender” it can also act as a clump weight

Just a thought and interested if it could be done

Thanks in advance


Yes, I too am interested in a 125m tether and also a reliable way to join two tethers together with a 32v power injector or powered topside unit when it is available.