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Author Topic:   SONAR Cable Routing
jimh posted 07-05-2009 03:39 PM ET (US)   Profile for jimh   Send Email to jimh  
A few weeks ago my boat was available, the weather was decent, we weren't really inspired to go boating, and, as a result, I ended up spending most of the weekend working on the boat. I completed a little project that I had begun some time ago: I removed all the electrical cables from the rigging tunnel that connects the helm station to the stern and thence to the outboard motor.

In my boat, a Boston Whaler REVENGE 22 Walk-Through, the rigging tunnel terminates in the cockpit sump area. This design is quite common on Boston Whaler boats. The sump area has a through hull drain to the sea. You can almost guarantee that some water will be in the sump area. To put electrical cables into a location where you expect there to be water did not seem like the best design. Since the helm station on my boat is not a center console, but rather a side console, the cables could join other electrical cables running aft under the starboard gunwale.

I had already re-routed many electrical cables to the gunwale route, but there remained two engine cables: the main harness and the trim and tilt harness. I pulled them out of the tunnel. There was mud residue on them, so I cleaned them off with a bucket of soapy water and let them try. Then I re-ran them under the gunwale. Of course I had to re-run them to the engine, through the rigging hoses that traverse the transom, through the flexible nylon covers, through the cowling grommets, and finally to the engine. It was a lot of work. But I was happy. I had all the electrical harnesses out of the wet rigging tunnel. All that was left in the rigging tunnel was the hydraulic steering lines, the mechanical cables for throttle and shift linkage, and the pressure hose for the water pressure gauge--nothing with any electrical current.

Today we were out making a 25-mile run across Lake Michigan, and I noticed that on my SONAR I was getting some false echoes from electrical interference. The harness for the engine is probably too close to the SONAR transducer cable. The next step will be inspect my cable run under the gunwale and see if I can separate the SONAR transducer cable from the others in the bundle, so that it will have an inch or two of separation. If that does not work, then I will run the SONAR transducer cable back to the rigging tunnel! That should give it plenty of separation. As far as sitting in water, it will be much easier and much cheaper to replace the SONAR transducer than all the engine electrical cables. If the SONAR has to run in a wet location, it will have to tolerate it. The engine cables are probably 17 years old, so I would rather keep them dry.

jimh posted 07-06-2009 05:29 PM ET (US)     Profile for jimh  Send Email to jimh     
As we were cruising along and I was thinking about this SONAR interference problem, I thought of two influences. I did not notice the problem earlier because we were in shallow water. The problem only manifests in deeper water. This is probably because the gain of the SONAR is increased in deeper water in order to hear the weaker echoes. As the gain is increased, the SONAR becomes more sensitive to interference from the engine wiring harness.

The interference from the engine wire harness is probably from the KILL circuit. In an older engine like my 1992 Evinrude V6 225-HP, the KILL circuit conductor comes from the Power Pack, and it is rather high voltage. The KILL circuit can be as hot as 300-volts. That is a lot of signal. The SONAR is trying to amplify very small signals from the SONAR transducer. Also, the KILL circuit is at an AC frequency that is proportional to engine speed. If we have a multi-pole alternator, and the engine is running at 4,000-RPM, the frequency of the KILL signal can get rather high:

HZ = 4,000-Revoltions/1-minute x 1-minute/60-seconds x 6-pole/revolution

HZ = 400

The SONAR is listening for very weak signals at 200 kHz, but a 300-volt signal at 400-Hz is likely to cause some interference.

I think I will experiment with disconnecting the KILL circuit to see if it is the source of the interference.

jimh posted 07-09-2009 10:16 PM ET (US)     Profile for jimh  Send Email to jimh     
I abandoned the idea of disconnecting the KILL circuit at the engine. I was afraid I would not be able to shut off the engine. With my two-cycle classic motor, the ignition is powered by a magneto or permanent magnet alternator. If the KILL circuit is disconnected, there is no simple way to shut off the engine other than letting it run out of fuel.
Mr T posted 07-11-2009 02:05 AM ET (US)     Profile for Mr T  Send Email to Mr T     
Jim have you considered RF cores? I work with a good number of electronics and have need for them quite often. It is a pretty effective solution to noise from other cables. I'd suggest if you do so to put them near the head unit. Might be worth a look.
jimh posted 07-11-2009 07:00 AM ET (US)     Profile for jimh  Send Email to jimh     
I think you are suggesting using ferrite cores or beads to suppress radio frequency currents. I think it will be simpler to re-run the transducer line.
HAPPYJIM posted 07-11-2009 12:29 PM ET (US)     Profile for HAPPYJIM  Send Email to HAPPYJIM     
Back when I was an aviation electrician, we had access to the stainless steel braid like what is used in coaxial cable. It was fairly easy to install over wiring by just slipping it over the wire. We used it for abrasion problems also. Both ends were grounded after installation so I'm not sure if that would be possible with your installation. If your boat has the fuel tank grounding disc, I would think that would be a good route for induced EMF to be dissipated.
Mr T posted 07-11-2009 03:36 PM ET (US)     Profile for Mr T  Send Email to Mr T     
Yeah that is what I was talking about. They just clamp on to the cables at the connector, pretty easy to put on and take off to judge effectiveness. Rerouting cables is always another option

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