Ecosystem Electronics Lab

Department of Marine Science


The tether is the long cable connecting the Pilot’s Console on the surface with the ROV, which operates under water. The tether supplies power down to the ROV and carries a video signal back up from the camera on the ROV to the Pilot’s TV monitor on the surface.

Tether in action.
Tether in action.

In the Catalina ROVs, the tether is about 50 feet (15 meters) long and contains the following:

  • 5 lengths of 16AWG 2-conductor speaker wire
  • 1 length of Cat 5e computer networking cable
  • 1 length of hollow, air-filled, flexible PVC aquarium tubing heat-sealed at both ends
  • Several sections of plastic spiral wrap
Closeup of tether
Tether close-up.

The speaker wires carry the electrical currents (up to several amps) needed to power the thrusters and lights. There is one 2-conductor speaker wire cable for each of the four thrusters and one more for the LED video lights. It’s helpful to label the ends of each power cable before you wrap the tether.

The Cat 5e computer networking cable carries the live video signal from the camera up to the Pilot’s TV monitor on the surface, so the Pilot can see what the ROV’s camera is looking at. It also carries a command signal the Pilot can use to turn the camera on or off while it’s under water. We’ve found this to be very helpful. By putting the camera in “one button mode” and turning off the camera briefly every 10 minutes or so keeps the size of the HD video files manageable. Using the wires in the tether to do this allows you do turn off the camera without having the haul the ROV back to the surface every 10 minutes.

The spiral wrap holds the wires together in a neat bundle, protects the wire insulation from cuts, and provides a relatively low-friction surface that helps the tether slide past many obstacles.

Assembling the tether

The Catalina ROVs have a 50-foot long tether. You can modify this length slightly up or down, if you’d like, but avoid the temptation to make it a lot longer. A longer tether will reduce power to the thrusters because of resistive losses in the longer wires. It may also allow the ROV to go deep enough force water past the seals on your thruster motors. (In our experience, 50 feet is about as deep as you can go routinely with these bilge pump motors before you start experiencing frequent motor leaks and failures.) Finally, a longer tether is more easily tangled and snagged on things deep under water where you can’t free it easily. The instructions provided below will give you a roughly 45-50 foot long tether. We’ve found this to be an optimal length, particularly for work in water 10-40 feet deep.

First, cut five 50-foot sections of paired speaker wire and one 55-foot section of Cat 5e. If you want a tether that’s exactly 50 feet long, cut somewhat longer sections for now and trim to length later. It's helpful to label both ends of each speaker wire pair while they are separate, so you can identify them more easily in the tether bundle later.

Before proceeding farther with the tether, you’ll need to assemble almost all of the underwater parts of the ROV including the thrusters, lights, camera housing, and frame. The Cat-5e cable needs to be routed through the ROV frame and into the waterproof camera housing as described on the Camera System page and Frame page before you proceed with the tether.

Assuming you’ve done that, you should have an ROV frame with 5 short pairs of 16AWG wires (1 pair for the lights and 4 pairs for the thrusters), plus a 50+ foot long Cat-5e cable, all emerging from a hole in the back of the ROV frame.

Now it’s time to connect the 50-foot-long speaker wire pairs that will form the bulk of the tether to the wires coming out of the ROV from the lights and thrusters. These connections must be robust — electrically sound, mechanically strong, and totally waterproofed — or your ROV will very soon cease to function once it hits the water.

For this reason, we’ll solder the wires together, embed them in epoxy, reinforce the epoxy with some heat-shrink tubing, and protect the entire set of ten such splices from flexing or other mechanical abuse by tucking the spliced wires snuggly up inside a section of 2-1/2" diameter PVC pipe used as a Junction Box.

As you can see in the photo below, the Junction Box pipe has two fairly large holes drilled through front and back sides for wires to pass through. After drilling these holes, and before putting any wires through them, round the edges of these holes with sandpaper or a round file, so they won't cut or damage the wires.

Attaching tether to ROV.

As you can see in the photo above, we’ve taken the bundle of wires emerging from the back of the ROV frame and run it into a second hole in the side of our Junction Box pipe. Once inside the junction box pipe, the thruster and light wires take a 90-degree turn and emerge from the end of the pipe for soldering. The Cat 5e cable passes more-or-less straight across the diameter of the pipe and emerges through a second hole drilled on what will be the back side of the Junction Box pipe.

The 5 paired speaker wire cables get pushed IN through this second hole and make a 90-degree turn to emerge at the same end of the pipe along with the thruster & light wires for soldering outside the pipe.

Before soldering, cut some 1" sections of heat shrink tubing and slide them onto each of the motor and light wires. Slide them far enough away from the end of the wires that soldering won’t heat the tubing enough to shrink it prematurely.

Next, strip about 3/8" of insulation off of the wires to be joined, tin each end with solder, and solder them together by placing the tinned ends side-by-side with the wires approaching from opposite directions. Make sure the solder joint is a good one. Repeat for the other 9 wire pairs to be joined.

After the last joint has cooled, mix up some 5 minute epoxy and use a toothpick or tiny disposable paintbrush (like those used to paint plastic models) to coat the solder joint, any exposed wire and some of the insulation on either side of the joint. Do this for only one joint at at time. Before the epoxy can harden, slide the heat shrink tubing over the epoxy and use a heat gun with the minimal heat necessary to shrink the tubing around the epoxy, entombing the solder joint in a water-tight epoxy casing inside the heat shrink tubing. Wipe away excess epoxy that may have oozed out of the joint as the heat shrink tubing shrank.

Repeat for the remaining joints. You may need to mix several batches of 5-minute epoxy to get through all the joints.

Slide the heat-shrink tubing over the joint and heat with a heat gun or hot hair dryer to protect the solder joint from shorting. At this point you should have a joint that is electrically sound, but it still needs to be waterproofed and mechanically reinforced.

You have 5 powered devices, each with 2 wires, for a total of 10 solder joints. You’ve just soldered one, now solder the other 9 joints in the same way before moving on to the waterproofing step. Be careful not to mix up wires. You want each thruster to have its own separate speaker wire line (a pair of wires), and the lights should have a pair, too.

We embed our joints in epoxy to seal out the water. Make sure the wire insulation is roughed up with some sandpaper and wiped with rubbing alcohol to remove any oils, so the epoxy will form a waterproof seal.

To prevent flexing or other movement that might eventually crack the epoxy or break loose the seal between the epoxy and wire insulation, we gently fold the wire bundle back up inside our Junction Box, taking care to ensure that the splices remain straight and unstressed inside the protective section of Junction Box pipe. You may need to push the Cat-5e cable to one side inside the Junction Box pipe to make room for the other wires.

Once all the wires are safely tucked inside, the drilled end-caps can be put on the Junction Box, and the Junction Box can be hose-clamped or otherwise anchored firmly to the ROV frame.

The bundle of tether wires emerging from the back end of the junction box should be wrapped in electrical tape and secured to the Junction Box with velcro-style cinch straps so that it leaves the ROV like an upward pointed tail. This is important to prevent it from flexing and eventually breaking wires right at the hole where the wires emerge from the junction box.

Now it’s time to wrap the tether in spiral wrap.

Image of tether being wrapped
Wrapping the tether.

This is a rather tedious, but important process. We found it was much easier if we cut the spiral wrap into sections about 5 feet long first and used waterproof vinyl tape to hold the ends of the sections together after wrapping. Start at the ROV end. Make sure the spiral wrap gets secured along with the wires to the Junction box.

When you finally reach the far end of the tether, roughly 50 feet from the ROV, you’ll probably discover that the wires aren’t all exactly the same length. No problem. You'll trim them to the same length shortly.

At the upper end, the tether wires terminate in two different connectors, both of which eventually plug into the electronics box on the Pilot’s Console (see photo).

Image of tether connectors
Tether connectors.

The five 16 AWG speaker wire pairs, which provide power for the ROV’s four thruster motors and the lights, all terminate in a 12-contact 0.093" (pin size) MOLEX connector with male pins. This large connector is the white plastic one that appears in the photo above. These connectors are available on-line from large electronics distributors, such as Digi-Key, Newark, and Mouser. Be sure to order both halves of the mating pair (plug and socket) and enough male and female metal pins for each. (Note that two of the 12 connections in this connector end up not being in this application, since there are only ten wires going to this connector.)

Image of Molex connector
This photo is looking straight into the Molex connector at the top end of the tether. If you look carefully, you can see that only 10 of the 12 holes actually have metal contacts in them.

To attach these connectors to the ends of the wire, follow standard instructions for using these MOLEX connectors. Basically, you trim the ends of the speaker wires to make them all the same length. (But do NOT the Cat-5e cable this short; see below.) Then you strip about 3/8" of insulation off the end of each wire and insert it into a 0.093" male MOLEX pin. You use a MOLEX crimping tool to crimp the pin onto the end of each wire before inserting the pin through the back of the MOLEX connector until it clicks into place.

The exact arrangement of the wires within the connector doesn’t matter, but it’s important to make sure that the pin positions on the tether side of the connection correspond to those used inside the electronics box. We've found it helpful to use a predictable pattern to reduce the chance of errors, for example, by keeping the motor pairs together and putting the light pair in its own row with the unused pair. It’s also helpful to label the wires and/or the connector itself with the function of each wire. (e.g., “vert” or “V” for vertical thruster wire pair, “LED” for lights, etc.)

Image of labeled wires
Two ways of labeling your wires: labeled tape on the wires themselves, or labels on the connector next to each pair of wires.

With the speaker wires terminated, the only remaining element in the tether that needs to be terminated is the Cat-5e cable. This is the cable that carries information to and from the ROV’s camera. In the photo above, it is the yellow cable. The connector used to terminate the Cat-5e cable is a standard RJ-45 networking cable connector — the usual connector for Cat-5e cables. These are widely available in electronics parts retailers and hardware stores. Instructions for putting these connectors on the end of the cable vary slightly depending on the manufacturer of the cable, but most require a crimping tool made specially for RJ-45 connectors. Note that it is cut several inches longer than the other wires in the tether, so that these other, sturdier wires and their sturdier connector are first in line to absorb the abuse, if the tether accidentally gets yanked or pulled, thereby providing some protection for the much more delicate RJ-45 connector.

Image of an RJ-45 plug
An RJ-45 plug crimped onto the prepared end of the Cat-5e cable from the tether. There are standard patterns dictating which wire colors go to which of the 8 pins on the connector. We have adopted the widely-used standard known as “TIA/EIA 568B,” which you can look up on the web.