Leaking penetrators - what have I done wrong?

I just completed my ROV and tried a few, shallow dives. Unfortunately I noticed that a couple of the cable-penetrators are leaking. Not much, a drop every now and then, but of course it is not supposed to be like this.

I am using the BR cables, penetrators and the Marine Epoxy.

I try to attach a picture from where you, hopefully, can see the penetrator and a drop of water.

Any idea on what i have done wrong?


Cable and penetrator surface preparation is extremely important when potting the penetrators.
I always cleaned the inside surfaces of the penetrator with 91% alcohol and used Kicker to insure that the CA used to seat the cable jacket in the penetrator barrel cured.
I also lightly sanded the individual wire insulation with 600 grit wet/dry sandpaper and thoroughly cleaned with 91% alcohol.
Then I used a 25 gauge tip on my syringe to ensure that I was able to get the epoxy around all of the wires in the barrel. I also potted only one end at a time to ensure a good bond and even dispersal of the epoxy in the bores.

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Thanks for the feedback!

Seems like I have to be even more thorough when potting the penetrators…
I did clean the cable jackets and the inside of the penetrators, but I don’t think I cleaned the insulation of the individual wires after stripping the cable jackets.
I will replace the two leaking penetrators, clean every part even more thoroughly and use some sandpaper.


Hi niclas,

I would also check your outer cable jacket for damage all the way up to the thruster as i have had a similar problem when this is damaged water can travel up the cable.


This, is one of the most common reasons for penetrators to fail. See it all the time. The blue robotics cables are nice, doubt they would break on their own, but could be nicked, cut or a defect. Always my first check when there is a leak at a cable, check the whole length for any damage. Real fun when it’s the 300ft tether.

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Kelsey and Pierson,

Thanks for your advices! I will definitely check the outer cable for damage.


Think I found the problem!

For my very first version from ROV I have been using the A2212-outrunners. That means I have to solder the BR-cable onto the wires coming from the A2212-motor. To keep the water away from the connections/soldering I have used two sizes of heat tubing: a small size to protect the soldering and a larger one to cover part of the three wires and part of the BR-cable. I have used shrink tubing with glue.

I figured out the problem from adding a little pressure to the WTE while at the same time putting the leaking cable/motor into a bucket of water. What happened? Well, a few air bubbles quickly showed up in the bucket…

So, all I have to do is figure out a way to “seal” the end of the BR-cable to stop the water from entering… Potting in some way? I don´t know…

Have attached a few pictures below.
1st picture: cables before cut open
2nd picture: took away the large shrink tubing
3rd picture: must be here the air exits/the water enters


There is a product called liquid tape which is a drying rubber compound. I have used it before on wiring in wet environments but I’m not sure if it would deal with pressure.

Thanks! I will have a look at “liquid tape” to see if it could be a way forward.

Hi niclas

I would get a peice of heatsthink about double the size of the bluerobotics outer jacket, cut it long enough to cover about an inch past the splice and the outer jacket. Shrink the outer jacket end up about half an inch making a cup and fill the open end with the same epoxy you used on the penetrator. Make sure to completely cover the splice.
Leave a bit of room in the top of the potting so you can shrink the rest of heatshrink over the potting once it sets it makes it look much neater.
Also make sure to sand and clean all your wires with acetone before doing this it makes for a very good seal.

If you would like some more info or some pictures let me know and ill see what i can find.


Honestly At this point I don’t think applying liquid tape is going to cut it. Kelsey’s suggestion is a good solution. I would also consider just potting it in a mold, the saved frustration may be worth the additional effort. HDPE and a ball endmill will do wonders. Cover the entire junction in epoxy, how it is done professionally.


Thanks! I wanted to do some kind of epoxi-potting but I wasn´t sure on what to use as a “shell” to keep the epoxy in place - but thanks to Kelsey I found a smart and simple solution. I will definitely try using a heat shrink tube filled up with epoxy!

I´ll let you know how it goes.


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Hi Niclas,

You can use sealing heat shrink like the one used in OpenROV

Hi Enrique,

Thanks for your feedback!

Yes, I used those (black, with glue) for sealing the individual wires. The problem seems to be where the three wires enter the jacket of the BR-cable. Check out the picture a few posts above where the blue and green wires (the white wire is hidden behind…) enter the outer jacket of the BR-cable.


@tciii The thruster cable’s have a coating to help the epoxy stick to it. So, won’t sanding it make it worse?

I have Kickstarter BlueESC T100 so they may have changed the coating since I potted my cables. Also, I did not sand the jackets, just cleaned them with 91% alcohol. However I did sand and clean the individual wires as those wires have Teflon insulation I believe.