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How To: DIY Waterproof Epoxy Cable Splices

Hey, I’ve been wanting for a while to show a field repair tip that has saved our bacon on a few cruises.

Our main business is side scan sonar survey, and due to wear and tear on the wet end of our tow cables, it’s fairly common to need to cut and reterminate them in the field. We’ve field terminated at least a dozen sonar cables over the years, and we’ve worked out a process that’s straightforward enough to do offshore with minimal tools, if needed.

We needed to make a custom battery cable for our BlueROV2, the other day, so we documented the process to share here.

Here’s what you need:

  1. 15 minute epoxy
  2. Solder/flux or butt splices.
  3. A couple of medium-sized syringes
  4. Small torch.
  5. Some solvent.
  6. Large and small shrink wrap.
  7. A helping hands or a couple clamps to help hold the cables straight. (optional)

Take one of your syringes and pull out the plunger. Snip off the narrow end and use a drill bit, pocket knife or other tool to ream out the opening until the cable will just barely fit through it. You want a snug fit against the cable to keep any epoxy from leaking through.

After pulling the cable though the hole, slide the syringe about a foot down the cable. The wide end of the syringe should be facing toward the cable splice.

Strip your cables and expose the wires to be spliced. Use a paper towel and some solvent to clean the insulation along the last 12 inches of the cable.

Now it’s time to connect your wires with solder or splices and test your connections for functionality. I prefer to solder if possible, because soldered joints are the most streamlined, but splices will work also. Beware that you might need to use a larger sized syringe if your splices are too bulky. (Note, if there is any way your solder joints can touch, each one should be insulated with heat-shrink. If you offset the solder joints, as I did here, you can skip the heat shrink because there’s no way they can touch.)

Test all your connections with a multimeter. Once you’re sure everything is functioning properly, slide the syringe up over the joint. Make sure it is large enough to cover all of your exposed wires with some overlap over the outer cable sheath on both sides.

Now mix up a small batch of your preferred epoxy. I use 15-minute epoxy for quick field repairs and 24-hour flex cure epoxy if I’m in the shop. Draw the epoxy up into your spare syringe, and inject it into the top of the wire splice. It helps if you rig up some contraption to hold your cable splice vertical, so the epoxy can’t spill out.

Tap the splice if needed to help the bubbles rise to the surface of the mold. Keep holding the cable vertical until the epoxy cures. You should then have a functional waterproof splice that looks something like this.

I’d prefer if there was a little bit more overlap of the main cable sheath, but this will work as long as there is not too much load or bending force on the cable. At this point I like to reinforce the splices with glue-walled heat-shrink tube. This makes the splice look more professional and adds a little extra water resistance and rigidity to the splice.

For your first few splices, I recommend using a larger syringe as an epoxy mold. The 10 cc syringe used above is about as small as is practical for field repairs. We’ve used syringes as big as 60 cc to make splices for sidescan tow cables.

We based this process on the 3M Scotchcast resin splices that come with the plastic molds and the little funnels for pouring in the epoxy. https://www.amazon.com/3M-Scotchcast-Inline-Voltage-Conductor/dp/B00O3P93I8

To be honest, we prefer our DIY splices to the 3M ones. The 3M mold is really hard to seal around the cables, and it usually leaks everywhere after you pour in the epoxy. The 3M kits also make large bulky connections that are difficult to keep from flopping around in the water currents.

Good luck!

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Rather than the hard epoxy ScotchCast kits, I mostly rely on the 82-F1 urethane kit from 3M. $50-$60 each, and they make a flexible and durable splice. You can actually use the resin from these kits to fill syringe splices like yours, and they end up working really well. If you’ve got a couple of splices to do, you can usually get multiple pours out of the 2130 resin before it starts to cure. I’ve spliced hundreds of cables for oceanographic work, and the 82-F1 or 82-F2 kits are my go-to. In the field, you can even fill the included mold, wrap the whole thing in self-fusing tape, and toss it over the side. It’ll cure underwater.

The epoxy you’re using is at least semi-flex, so the cable is less likely to fatigue or pull away from the potting.