Rudiger,
It is recommended from Humminbird using standard equipment that you use a maximum of 1 - 30ft extension cable. So 20ft on the transdcuer cable and 30ft on the extension is 50ft.
But some users have spliced low-impedance cable to make towfish and have had great results at 150ft plus. I belive even a few guys have gone to 100 meters, but you have to jack the sensitivity way up.
Is that what you were asking for?
Robert