The owner of this boat called me and said his depth finder wasn't working correctly on a short "Shake Down" cruise from Sarasota, to Egmont Key and back. This was about a 70 mile trip up and back, and he said the Furuno depth finder acted weird on the whole trip. Since we were in about 10 feet of water, the Furuno RD30's depth of about 176 feet, did seem to be a wee bit on the high side. The Nobeltec sounder right next door does have the correct depth. Look at the picture closely, and above the depth number on the Nobeltec system, you can see the the wording "200 khz "Cruising."
So playing the game of "What's different between these two pictures?", we can now see the Furuno RD30 is reporting the correct depth, and the Nobeltec sounder is also. The difference is the Nobeltec sounder is now using the "50 khz Cruising" setting. So by now, a lot of boaters have figured what has happened, but for those who need some enlightenment, this is what is going on, in Americanized English.
The piezoceramic devices in the two transducers are pinging out sound waves, and listening to the returning sound waves as they echo back from the bottom. They are designed to send out a certain frequency sound wave, and hear that frequency when the sound wave bounces back from the bottom. In this case, since both transducers are using the same frequency, they are hearing each other, and the Furuno unit is getting confused. I suspect, but I'm not dead sure, that the Nobeltec sounder module, has a more powerful transducer (I did not climb into the engine room to verify this, it is what it is), and the lower power Furuno transducer just gets overwhelmed. In other words, the Nobeltec transducer is not adversely affected by the Furuno's transducer, but the Nobeltec's transducer bugs the heck out of the Furuno's transducer, if they are operating at the same frequency. Both transducers are made by Airmar, and I hope I got this dialog generally correct Ms. Robb.
The owner now knows, that if both sounder systems are operating at the same time, they must be set to different frequencies, if you don't want to be in 176.2 feet of water all of the time. There are transducer technologies, that can overcome this issue available, and the link below to Airmar will provide wealth of transducer information.