Repair of Electrical Cables for Sounder

[pcrussell50]

RATS! I had a mouse in the rigging tunnel of my Boston Whaler MONTAUK boat. It chewed a hole in at least one of the Baystar hydraulic lines, and it really did a number on the Garmin GPSmap transducer cable, where it was chewed clean through in at least three places. Does Garmin lubricate them with peanut butter?

So, going forward, I've ordered new Baystar hydraulic lines, and I have a plan to install 1/4-inch mesh to prevent recurrences.

Can I splice together cut transducer lines? Or, does [a spice] affect sensitive impedance values?

What are my odds that Garmin will have a replacement for a 10-year-old unit?

--Peter

[Jefecinco]

I have successfully spliced a transducer cable. I used solder rather than any clamp-on butt splice pieces. If I remember correctly I first soldered the conductor together and then wrapped the conductor cable with a high quality electrical tape used for termination kits for high voltage (4-KV) conductors. I then peeled back the outer conductor revealing the mesh steel shielding between the outer and inner insulated cable. The mesh construction of the shielding allowed it to be pulled together in such a way as to allow it to be soldered together. After soldering the shielding together I also wrapped the outer cable insulation in the same way as I wrapped the inner insulation. The cable was working satisfactorily when I sold the boat almost ten years later.

I'm not a highly experienced solderer so I believe anyone with very basic soldering skills can make a working splice to a transducer cable.

[porthole]

I have done multiple repairs like you are looking at. I would definitely try the repair before spending money, unless you find [a low-cost replacement].

I have had success with radar harnesses, my Northstar GPS antenna cable (10-12 wire bundle), and several other multi-conductor cables. Learn how to solder correctly and use adhesive lined heat shrink tubing to finish it up.

[jimh]

The cable between a sounder and the transducer is usually carrying very low frequency signals, in the range below 100-kHz. At that frequency, there are probably not any transmission line effects introduced by the surge-impedance of the cable. A small discontinuity in the impedance of the cable as would be introduced by a well-made splice will probably not be likely to cause any ill-effect on the operation of the SONAR.

For transmission line effects to occur, we look at the wavelength of the transmission line connecting the transducer and the sounder. When operating at 100-kHz, the wavelength is

300,000,000-meters/1-sec x 1-sec/100000-cycles = 3000-meters or 9842-feet

If the cable length is 25-feet, then the distance in wavelengths if 0.0025-wavelength

To put this in perspective, at a radio frequency of 156-MHz, a distance of 0.0025-wavelength is about 3/16th-inch. That is, a 25-foot cable at SONAR frequencies represents a length of about a 3/16th-inch cable at radio frequencies. An that basis, you can see that a splice in a 25-foot cable that is perhaps an inch long represents an equivalent distance at radio frequency of about 0.0006-inch. Did you ever loose sleep worrying that in your 50-Ohm coaxial transmission line for your radio there might be a small impedance discontinuity for 0.0006-inches of the cable?

If you had a really modern high-frequency SONAR that used 800-kHz signals, a splice might be of more concern, but still perhaps not likely to ruin everything.

In any case, there is little to be lost from making a splice and seeing how it works. FIrst, I would try to cut out all the chewed areas and make only one splice, if there is enough spare cable length to allow this. As for the technique, I would stagger the location of individual conductor splices, making the total length of the multiple splice connection several inches long. Use heat shrink tubing. Cover with more heat shrink tubing. The most import aspects of using heat shrink tubing is to slide it over one of the wires first, before soldering the splice, and to keep the heat of the solder iron away from the heat shrink so it does not start to shrink in the wrong location.

The most important aspect of the splice is to prevent any obvious shorts circuits or open circuits, and as best as possible to retain the continuity and coverage of the braided shield conductor to cover the splice. Usually the shield can be make larger in diameter, like a Chinese-handcuff, by compressing its length. If the shield is not suitable, you can splice in a larger shield. Again, remember to put this in place first, before you start making splices.

[fno]

A small note to add to Jimh's advise is to locate shrink tubing that has an adhesive on the inside. When that is not possible, buy some liquid tape (West Marine, others) and use that to totally waterproof your splices.

[Jefecinco]

I agree that heat seal tubing is superior to tape for the job in virtually every case. There are some specialty tapes that will do the job almost as well.

[jimh]

Re heat shrink with adhesive: to use this material properly you have to use proper heating techniques. The temperature of the heat shrink must be raised high enough to both shrink the tubing and to release the adhesive, but not too high that melting of the tubing or the wire insulation occurs. To properly use heat shrink, a heat gun with a reflector should be used. The reflector helps distribute the heat evenly around the conductor and also keep you from heating other components behind the wire. You cannot properly use heat shrink if you try to heat it with an open flame. A proper heat gun that produces an output with the temperature in the specified range for the tubing being used is necessary. Your wife's hair dryer won't do a proper job.

Personally, I do not own any heat shrink with adhesive. I just use regular irradiated heat shrink. Sometimes I put some dielectric grease inside the larger heat shrink that will cover the individual splices in a multi-connection splice.

Also, in my own boat, I have moved all the electrical cables out of the rigging tunnel, except for the SONAR transducer cable. That was done to keep as much isolation between the SONAR cable and other electrical cables to minimize signal ingress into the SONAR cable from the other electrical and electronic cable and vice-versa.

I have not tried this myself, but I have heard, read, and been told that scented dryer anti-static sheets make a good rodent repellent. Stuff a few in each end of the rigging tunnel.

[Jefecinco]

Re rodent damage: a few years ago I started my 2004 Titan pickup after letting it sit for several days. When I put it in gear to proceed it would not move. I had the truck taken to the Nissan dealer for repair. Eventually they found a mother squirrel and some young nested beneath the plastic engine cover under the hood. The total cost to my insurance carrier was over $7,000. My cost for the comprehensive deductible was $500. Most of the cost was labor as the entire wiring from the firewall forward had to be replaced.

[jimh]

staggeredSplice360x118.png (5.41 KiB) Viewed 6523 times

A staggered splice in a multi-conductor cable. If the cable was shielded, restore the enveloping shield.