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Author Topic:   Splice In Cable Between SONAR Transducer and Control Head
WestBayWhaler posted 10-27-2007 04:16 PM ET (US)   Profile for WestBayWhaler  
After my local yard installed a new four-stroke engine to replace my dying Yamie, I found they had cut and then spliced the cable from the control head to the transducer of my SONAR. And not long afterwards, I started having trouble with the SONAR. Re-reading the installation instructions, I found that the manufacturer specifically stated that you should not splice the cable. But before I demand that the yard buy me a new cable, I thought I would ask the CW crowd their opinion: can you splice the cable between the transducer and control head? Or must it be replaced?


Whale1 posted 10-27-2007 04:25 PM ET (US)     Profile for Whale1    
<I found that the manufacturer specifically stated that you should not splice the wire.>

Seems pretty definitive to me:-)


WestBayWhaler posted 10-27-2007 04:35 PM ET (US)     Profile for WestBayWhaler    
Thanks ONHO2 But often times what the manufacturer says has more to do with selling you something and not what is actually necessary. That's why I thought I would tap into the collective wisdom of CW before I made a nuances of myself insisting that my yard replace the connector.
Casco Bay Outrage posted 10-27-2007 08:02 PM ET (US)     Profile for Casco Bay Outrage  Send Email to Casco Bay Outrage     

Get the yard to pay for it. Just plain stupidity on their part.

Chuck Tribolet posted 10-27-2007 09:26 PM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
The Field Replaceable Unit (FRU) is the transducer with cable
and connector, not just the cable or connector.

run at frequencies high enough that just plain electrical
connectivity isn't enough.

The yard owes you a new transducer, installed by somebody
else of your choosing.


Henry posted 10-27-2007 11:06 PM ET (US)     Profile for Henry  Send Email to Henry     
This is one reason I do everything on my boat myself. You hear about these types of things all the time. This type of thing is not only the mistake of a slip shod hack it is the attempted cover up of a crook. An honest business would have told you what happened and offered to replace the transducer. I would not be surprised if they adamantly denied doing it.

How did they splice it? Did they twist the wires together and wrap some electrical tape around it?

These guys installed your new motor, huh. Wonder what other stupid mistakes they made. Good luck.

kwik_wurk posted 10-31-2007 12:09 AM ET (US)     Profile for kwik_wurk  Send Email to kwik_wurk     
Any splice in any high frequency transducer cable can cause funny impedance problems. And it is a known problem on sonar devices; so yeah get a new transducer and cable assembly.

Get the boat yard to pony up.

jimh posted 10-31-2007 08:30 AM ET (US)     Profile for jimh  Send Email to jimh     
Most SONAR devices operate at very low frequencies and the cable which connects the transducer to the control head is not operating as a transmission line. In other words, the cable does not have a characteristic impedance and there is not a conjugate impedance match between the surge impedance of the cable and the source impedance and terminating impedance at each end of the cable. Generally the connectors which are used on these cables are not a constant impedance connector, and they represent a change of impedance or discontinuity to the signal flow.

If the spice of the conductors in the cable was reasonably well made so that it did not represent an impedance discontinuity any greater than a connector, it should not disturb the performance of the SONAR device. This would be particularly true if the SONAR were just a low-price non-sophisticated device operating at 50-kHz. A well-made splice would be one in which each conductor was carefully joined and insulated, with the splice section staggered along a length of the cable, the overall size and lay of the cable was not significantly disturbed, and if shielded cable were used the shield must be carefully restored and maintained.

Now all that said, a crudely made, poorly done, kludge splice could very well affect the operation. And the higher the frequency of the SONAR--some units are now operating at 800-kHz--the more important the cable becomes. But I suspect that most manufacturers prohibit splices because they are just an invitation to introduce problems. In some cases the length of the cable may also be a factor, as some tuning may be done to optimize performance which takes into account the cable length. On the other hand, many SONAR devices are sold which offer extension cables with connectors to length the cable between transducer and control head.

There is no question that the installer who cut your SONAR unit's cable was not doing you any favor, and you are justified in seeking a remedy which would return you to your original condition: an intact cable.

bigjohn1 posted 11-02-2007 07:44 AM ET (US)     Profile for bigjohn1  Send Email to bigjohn1     

A randon data point regarding your points above - the lionshare of commonly-available single frequency sonars for pleasure craft utilize 200kHz. Only the more pricey dual-freuency units utilize both 200kHz (for shallow water) and 50kHz (for deeper waters). Navy submarines utilize both MUCH lower and MUCH higher frequencies, with the precise values being classified.

jimh posted 11-02-2007 09:18 PM ET (US)     Profile for jimh  Send Email to jimh     
The formula for wavelength is

FEET = 984/MHz

The wavelength at 50-kHz (0.050-MHz) is therefore 19,680 feet.

The typical SONAR cable connecting the transducer to the control head unit is about 20-feet long. In terms of a wavelength at 50-kHz that cable is therefore

20/19680 = 0.001 wavelength

It is hard to imagine that there can be serious transmission line effects caused by an impedance discontinuity in a transmission line which is only 1/1,000-th of a wavelength long. The splice would be only a matter of an inch long. How long is an inch in terms of wavelength? This works out to

0.0833/19680 = 0.000004

or about 4-millionths of a wavelength

Let's compare this to a VHF Marine Radio operating at 156-MHz. The length of an electrical wave at that frequency

984/156 = 6.3 FEET

So a similar discontinuity in transmission line at that frequency would be on the order of

6.3 X 0.00000423 = 0.0000266 Feet

or about 4/10,000ths of an inch.

In other words, a small splice in the cable in a 50-kHz SONAR will be about the equivalent of a few extra molecules of solder on your VHF radio connector joint. Even scaling up to 200-KHz only increases this by a factor of four. A splice in a 200-KHz cable is like about 1.6/1,000-ths of an inch change in the coax of your radio. This is not much of an impedance bump or discontinuity.

The harm to the SONAR is not from an impedance bump but just from lousy connections, ingress of water, wear and tear. But if the spice is well made it should not affect the operation of the SONAR. The frequency is so low that the physical change in the conductors is not significant.

But, again, I do not find it reasonable or normal to have the engine installer cut the cable. He should have worked around it or properly disconnected it and re-routed it.

an86carrera posted 11-02-2007 11:06 PM ET (US)     Profile for an86carrera  Send Email to an86carrera     
A joint in the rigging tunnel IS a problem. As Jim has stated, water ingress etc... At 200 Mhz outside of the tunnel, hmmm...have one working for years.

However, does the manufacturer approve of a splice... NO, you pay a service facility to do indisputable work, is the work disputable?... YES, did they discuss the risk factor with you vs. the cost? if you took it to another reputable yard, would they warrantee any other work they performed to that system?...NO, not with that cable!

Safe bet for the service company, make it like new.

I would push for a NEW cable, will you get it? ...probably not ...will you go back? will you suggest that service to a friend?...I would not...

What is going on here? If you want it done right.. there is only one solution, oops, maybe A LOT.

I do not understand how anyone can own a toy (boat) and trust a mechanic to your maintenance unless you have unlimited funds.

.02, yes, that is the value of my opinion.

I hope that splice works.

This is what I do when I buy a used boat: ... rewire the whole thing all new. A pleasure boat is the most bullshit wired transportation unit in the universe, can.

Unfortunatly, we have more to risk here than in our cars that, when the smoke starts leaking out we can get out and watch it burn. try that in boat even four miles from shore, you will be praying sooner than you ever imagined, even us agnostics.


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