Topic: Connecting cables for towfish

[Kennyscott]

Thinking of building a towfish. Seems cable connections could be a problem. Could this work?
Towfish tranducer comes with 15 ft or 20 ft cable then use  two or three 30 ft extensions plugged together. To keep connections water tight wrap with fixit tape " f-4 silicon self-fusing tape".  Any suggestions???

[Jolly Roger]

Hello Kenny and welcome to the forum!

I had the same question when I built my fish last year and decided to go for a "one cable" solution with 50 meters of CAT 6 PIMF cable.

I thought that each extra connection might be another possibility where water might get in and shortcut the cable. So I decided that two are enough and cut the original transducer cable and spliced the CAT 6 in between. I used heat shrink tubes in various diameters to insulate and watertighten the soldered spots.
I admit that a "plug and play" solution would be much more convenient, but I couldn't find heat shrink tubes which would fit over the connector and at the same time would shrink enough to sit watertight. Another thing was that the original Humminbird extension cables had a relatively high resistance compared to the CAT 6 cable.

I don't know your suggested "f-4 silicon self-fusing tape". But since the DIY world is much about trial and error, it might be worth a try. But before you go and hook up your unit to such a prepared cable, I would do some tightness tests if the connections are really watertight. Just set up a cable like you want and measure the resistance of each pin/wire. Write down the measurements and then make a water test by dragging the cable like it would be dragged when the fish is deployed. After this test recheck the resistance of the pins / wires. If they are still the same, the connections should be water tight and you should be good to go for a "hot" run with the unit (I said a prayer before I turned on my unit the first time with my fish attached to it   ).

The "plug and play" solution has the advantage that you still can use the transducer and extension cables after they are dried if the tests didn't work.

I wish you good luck with your project. Let us know what you are doing and how it turns out.


Best wishes from Germany

Harry

[kron]

Hi Kenny.

As Harry wrote, the humminbid cable has high resistance. Anyway I haven't neither tested to connect more extension cables, but it would be easy and hopefully perfect. I heard "someone" told me that there could be more resistance in the connections( It seems logical to me). About the sealing, I used shrink tube with glue to seal the cable on my towfish. The glue inside is, I guess, the same as melting glue that is used in melting glue guns. So that's why I also used that kind of melting glue to seal some of the cable. The shrink tube wasn't so long.   

[abraquelebout]

i use 3m rubber mastic tape, good result
for the transducer, i cut cable and soldering it to a low impedance cable (60 m), every small conductor are put into a termoductable gird to isolate them (2 thermo ind transducer part because in depht there's more pressure) and all of them are seal whith the 3m mastic tape for water isolation, it's working well.
for cable resitance i use dyneema cable.

[Dale]

We have used a combination of heatshrink tube, self fusing tape and a gel to rejoin a 150m cable with a 25kg load on the end. The deepest the join went was 110m for a couple of hours. The fusing tape http://www.f4tape.com/ was amazing stuff. It provided the outer watertight join, plus the strain relief of the payload!

I'd never quite trust heatshrink by itself, but that fusion tape is really something.

We all must get a cable join stickie up here amongst other things!

[bell47]

I read above that someone used "CAT 6 pimf" cable. Is this what I would need to use to make a long cable? What other cable options are there? I'm pretty confident I can make waterproof soldered joints/splices. Does the display unit need a certain amount of resistance to operate?  What about the Transducer? I think I'm gonna make about a 400-500 foot cable to try and get the towfish down about 250 feet. What is the maximum resistance the cable can have and the unit/transducer still operate? I realize that long of a cable maybe wishful thinking. Are you guys towing the towfish with the electrical cable or do you run it parallel to a "tow" cable? I was thinking run it along side of a coated 1/8 inch steel cable that supported the towing strain.

[Jolly Roger]

@ Bell47,

I choose the CAT 5 cable, because it's basically the same what Humminbird uses for the transducer cables (PiMF = Pairs in Metal Foil).



I found an article in Wikipedia which describes the different categories. Very informative!

This one is in German:
http://de.wikipedia.org/wiki/Twisted-Pair-Kabel

This one is in English:
http://en.wikipedia.org/wiki/Category_6_cable

You can click on the different categories at the end of the page.

As far as I understood the difference between the Categories have something to do with crosstalk and interference with other network cables. That would not be the case when just having one cable in use (which is the case when using the cable for extending the transducer cable for tow fish use).

For the 100m cable I'm looking to find a PiMF cable with a big diameter (of the wires inside) to reduce the electrical resistance in the cable.


I use a 7 mm steel cable as a tow cable along with the transducer cable to keep away the strain from the transducer cable when the fish is deployed. My cables are attached to each other with quick ties, which work so so....
For the next cable I will use shrink tubes to attach the two cables to each other, because that way the cables will be more "hand / finger friendly"  .

The 7 mm are a bit oversized I admit, but the cable was cheap and since I designed my fish quite heavy, I thought the bigger the better.
Another thing I was thinking about was that a thicker cable will provide better grip when hauling the cable in or let it out. I guess hauling in a 2 mm cable would cut more into the hand or glove than a 7 mm cable.  That proved to be true so far.

As for using other cables, I think as long as you would use a shielded twisted pair cable and get the right wires connected to each others you will be good to go. Rickard is the best Point of Contact for such a question. I hope he will comment on this as well.

If I can find a PiMF cable that would have a real robust coating or strain releave, I would rather go for that for my 100 m cable, but I just haven't found a shop to get it  .


@ Dale: Thanks for the link! Sounds like the real stuff to use for water tightening....

Best regards / Harry

[Rickard]

Not sure...

about the best wiring of extensions. I did like Kron, I spliced in a braided patch PIMF Cat-6 ethernet cable after cutting the original transducer cable. Then the hot and ground wires in each SI-circuit forms a twisted pair. I thought this was important until I saw Abra's design with the space-industry cable. He has joined the plus wires in the SI channels in the same twisted pair. I would have thought this would cause alot of crosstalk between left and right SI-channel, but it doesn't! Perhaps the only critical feature is a good outer shield?

When I have used my towfish I have noticed that most of the drag comes from the cable and the steel wire (I use a 4 mm PVC-coated steel wire for towing), not the towfish. Thus, I think it's important to select the thinnest possible line/wire/cable to bring down drag. The towfish itself doesn't need to be that streamlined. The cable I use is probably strong enough to take normal towing forces using a 10 kg towfish at 30 m depth and a speed of 3 knots, but it will probably break in the event of a bottom contact. I will try to measure it's strength some day. If I can do without the steel wire drag will be reduced alot.

Rickard

[Jolly Roger]

Good point about the drag there Rickard!

I think too that most of the drag is caused by the cables. The more cable is deployed the more drag is produced by it. Actually it should be possible to to calculate the drag. Diameter of cable x cable length = surface which produces the drag. Then counter measures can be taken to compensate some of the force that keeps the fish up when dragged by attaching a depressor wing.

Then less cable must be deployed to reach a desired depth.

Regards / Harry