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Author Topic:   Cutting and Splicing SONAR Transducer Cables
diveguy posted 03-25-2005 12:42 AM ET (US)   Profile for diveguy  
I'm mounting a new transducer on the transom of my Whaler, and obviously need to run the cable to the console. The transducer cable is quite small, except for the large plug which connects to the back of the fish finder. I really hate to drill a 3/4-inch or larger hole to fit the plug and wire though the transom, only to have to fill the space when I'm done. I'd really rather drill a much smaller hole for just the cable to fit through.

Can I cut the cable, feed it through a much smaller hole, then reattach the part of the cable I cut with the connector plug on it? Will I get bad (or no) readings on my fishfinder when I'm done?

Any thoughts, tips, and experiences are helpful. Thanks!

andygere posted 03-25-2005 01:10 AM ET (US)     Profile for andygere  Send Email to andygere     
My Garmin unit said do not cut the cable. I ended up drilling the larger hole, sealing the inside of the hole with epoxy, and finishing it inside and out with Perko clam shell vents. I bedded the wire in marine silicone inside the hole, so it could be removed easily if needed. Do not bed it in 3M 5200 or 4200. Been there, done that.
Liteamorn posted 03-25-2005 04:28 AM ET (US)     Profile for Liteamorn  Send Email to Liteamorn     
I never drill a 3/4-inch hole in my transom I have run it thru the drain on the splash well or run it over the top of the transom .
brisboats posted 03-25-2005 07:26 AM ET (US)     Profile for brisboats  Send Email to brisboats     
diveguy, Faced with a similar situation I once called Humminbird, and I was told do not cut the cable. However, since then I have dismantled several boats in the yard that have had spliced cables, with splices ranging from a nice coupler to twisted wire and electrical tape. My take is although the manufacturer will advise against it, it is possible to cut and splice a transducer cable. A narrow profile connector midway along a transducer cable sure would solve a lot of rigging headaches.


Legobusier posted 03-25-2005 07:51 AM ET (US)     Profile for Legobusier  Send Email to Legobusier     
I would advise against it. My Humminbird unit was on the blink last year - it would power itself on-off and just generally act strangely. I was about to replace it when I found there was a splice in the cable. I cleaned it up, reconnected it, soldered the connections, wrapped it in good electrical tape, coated that with liquid electrical tape, and the whole 9 yards.

It's BETTER now, but still erratic. I get the feeling it's losing just enough signal due to the splice to confuse the unit. I'll be replacing it real soon and will NOT be slicing the cable. Just my experience of course.


jimh posted 03-25-2005 08:23 AM ET (US)     Profile for jimh  Send Email to jimh     
I look at it this way. Somebody installed that connector. You should be able to re-install it. One caution: investigate disassembling the connector first. Many use a molded-on strain relief that makes re-use very difficult if not impossible.

I recommend running the cables over the top of the transom. I do not recommend running the cable through the drain holes of the splash well.

davej14 posted 03-26-2005 08:44 PM ET (US)     Profile for davej14  Send Email to davej14     
I believe transducer cables are coaxial. There is no problem replacing the connector on a coaxial cable providing that the replacement connector is designed for the impedence of the cable being used. Don't just cut the cable and splice it back together without proper connectors because you will degrade the signal and you will introduce reflections into the signal path that may "confuse" the head end of the system.
scaleplane posted 03-28-2005 09:15 AM ET (US)     Profile for scaleplane  Send Email to scaleplane     

I'm curious why you don't recommend running the transducer cable through the drain hole in the transom. I installed my cables (transducer and speedometer cable) for my Garmin 240 Blue through the drain holes in my 15' Sport, and it seems to work just fine.


gss036 posted 03-28-2005 12:30 PM ET (US)     Profile for gss036  Send Email to gss036     
This is AirMar's answer to the question of cutting the transducer cable. They make most of the transducers that [are used in recreational SONAR units]:

Can I cut my transducer cable?

Yes, the transducer cable can be cut. However, if the transducer came with a connector do not cut it off. The molded on connector is waterproof. You need to cut and splice the cable away from the connector using Airmar's splash-proof Junction Box. The connections will not corrode and the strain relief grommets are water resistant and have excellent cable retention. Please note that cutting the cable or removing the connector, except when using Airmar's junction box, will void the sensors warranty. You can buy a junction box and splice kit for our distributor Gem Electronics.

Gem Electronics

Phone: 843-394-3565 Fax: 843-394-3736.

Note: Carefully follow the instructions provided with your junction box.

***If you have a TRIDUCER multi-sensor, the ceramic element inside builds up a charge. When the cable is cut, it can discharge possible disabling the speed sensor.

drumbeater posted 03-28-2005 01:49 PM ET (US)     Profile for drumbeater  Send Email to drumbeater     
I spliced mine on a Humminbird once. I did solder back the wire connection using a Western-Union-type splice [possibly a SCOTCH-LOK connector--jimh]. I put heat shrink tubing around the splices and then a liberal coating of liquid electrical tape. That was seven years ago and the current owner told me the other day that it is still working fine.
towboater posted 12-14-2008 01:50 AM ET (US)     Profile for towboater  Send Email to towboater     
[This discussion was dormant for several years, then it was revived and the topic changed. The new topic of discussion has been removed from this discussion of cable splicing.--jimh]

I prefer the larger holes, a seal of the exposed wood, use of a grommet, SS clam shell cover, and HD marine calk.

On the other hand, a former US Army Radio Section Chief who knows just enough about electricity to get into trouble. I don't understand why one couldn't spice any cable or wire that conducts electricity. Actually, I would fear a junction box with connectors that just might have a little too much resistance far more than a straight ol' fashioned wire to wire field splice. Some of today's spray-on electrical sealers, connectors, shrink wrap, solvents and solder are far superior to the many layers of electrical tape I've used with great success over the years.

I've even field spliced co-ax cable to lengthen RCA plugs between fairly high dollar amp and pre-amps with GREAT results. But, I've never cut a transducer cable primarily because the SONAR unit cost over a grand and it is not much harder to make a larger hole than to properly splice the many wires involved between the sender and unit. Appreciate the reports from those who have.


Jerry Townsend posted 12-14-2008 02:25 AM ET (US)     Profile for Jerry Townsend  Send Email to Jerry Townsend     
The transducers and electronics have to be matched, and that match includes the impedance (think of it as resistance) in the cable. Now, how sensitive the match is depends on the electronics, but I would not cut any transducer or antenna cable. The decision is really dependent on the electronics and not the transducer.

Re the transom holes: I just used the drain hole on my Outrage - and one or two (too darned cold to go outside and look) stainless cable clips - each held to the transom with two small stainless screws and sealant. --- Jerry/Idaho

swist posted 12-14-2008 07:29 AM ET (US)     Profile for swist  Send Email to swist     
As a general rule, I would not cut cables because of the reason given above by Jerry, but depth sounders run on relatively lower frequencies and the cable impedance and concerns such as reflection are likely to not be as significant as if cutting a RADAR or radio cable. If you can make a decent splice, it should work.

Obviously the manufacturers tell you not to, as there are more wrong ways than right ways to do it, and they don't need that warranty exposure.

By the way, I agree this is a very annoying characteristic of a lot of marine electronics. Huge (relatively) connectors on thin cables forcing large holes to be cut. Some of those fancy feed-through bushings they sell can help in smaller cases. For bigger connectors, another common technique is to use a clam-shell vent (stuffed with silicone) to hide a big hole.

jimh posted 12-14-2008 10:01 AM ET (US)     Profile for jimh  Send Email to jimh     
[This discussion has been moved to the SMALL BOAT ELECTRICAL from another discussion area.]

Regarding whether or not one should use the drain holes in a transom splash well as a conduit for routing electrical cables, my reasoning for this is simple: the hole in the transom is intended as a drain for the transom splash well and running electrical cable through it reduces the area of the drain opening. It also clutters up the splash well with the cable.

Regarding splicing SONAR cables, I suspect that in most instances this is possible providing the splice is well made, that is, it maintains the electrical continuity, prevents water from reaching the conductors, and does not place a physical strain on the cable.

Regarding whether or not there is an impedance match between the cable, the transducer, and the SONAR device, I have some doubt. Except in the latest and newest technology of broad band sonar devices in which special attention has been paid to handling of the SONAR transducer's impedance, the impedance of a SONAR transducer is probably not well matched to the characteristic impedance of the cable connecting it to the SONAR unit, and one should not think of the cable as acting like a transmission line conductor. There are several reasons for this.

First, the wavelengths involved are very long. At a typical SONAR frequency of 50-kHz, an electrical wavelength is 19,680 feet. If we allow for a propagation velocity of 0.66, a wavelength is still almost 13,000-feet. This means that the typical 20-foot electrical cable between the transducer and the unit has an effective length of only 0.0015 wavelengths. Consider this distance in wavelengths for a moment in terms of the VHF Marine Band radio. If talking about an equivalent electrical distance for 156-MHz, we'd be discussing a length of cable that was 0.1-inch long. Any impedance change in a transmission line of such small dimension is clearly not likely to have much effect on the transmission line.

Next, the transducer is a typically a resonant piezoelectric device, and its excitation voltage is often simply a strong DC pulse, and not a pulse of ultrasonic alternating current traveling on a transmission line. DC current does not care much about impedances and impedance matching.

When the transducer is acting as a receiver, we do not have to worry about minute signal loss due to impedance mismatch because we have a great deal of reserve gain available in the receiver. If a splice caused a slight increase in attenuation, say a loss of 1-dB of signal, the SONAR unit would just increase its gain. Now if we were at the ultimate gain of the unit, trying to pick up the most faint echo from say 5,000-feet below, a tiny signal loss in the cable might begin to have an influence. But most of the time the SONAR is operating it will have plenty of reserve gain available.

For these reasons, attempts to apply transmission line theory to the cable connecting a SONAR transducer to its control unit are, in my opinion, probably invalid.

All that said, I would avoid making splices if possible mainly to maintain the components in their original state which should improve reliability. It is unfortunate that many of the connectors used in these devices are molded-on connectors, and field replacement or repair is almost impossible.

Jefecinco posted 12-14-2008 10:13 AM ET (US)     Profile for Jefecinco  Send Email to Jefecinco     
Is there some problem with AirMar's junction box solution?

It seems pretty elegant to me. The box could be mounted in the console thus allowing the cable to be pulled through the rigging tube with minimum drama. Having pulled one of those BIG connectors for my xducer through a rigging tunnel I'll say that it can be a bit of a job.

Next time I'll use a junction box.


jimh posted 12-14-2008 11:44 AM ET (US)     Profile for jimh  Send Email to jimh     
Coaxial cable has been used to connect a SONAR transducer to its control head, but I suspect that in most cases it was chosen more for its ready availability and good shielding than for any consideration of its transmission line characteristic impedance. SONAR transducer assemblies now often contain other sensors (such as water temperature or water velocity sensors), and the connecting cable must then contain multiple conductors. This has led to a decline in the use of coaxial 50-ohm or 75-ohm transmission line and growth in use of multi-conductor shielded cable for a connecting cable

Some really modern SONAR transducers are self-contained instruments and only send serial data in digital form to a network or control head, and there is no base band ultrasonic signal sent on the cable that comes from the transducer.

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