Just installed new Humminbird 747C at the concole of my Ranger 175VS. Ran Power to unit and all is well - Ran transducer abd temperature cable and connected to unit. All is still well. Took boat to lake on  8/05/07 and as soon as boat hit water unit read 1300 feet. sat in middle of lake in what I know is 20 foot of water and unit still read between 1100 and 1500 feet of water. Simulator runs just fine and it seemed that no matter what adjustments I made the unit would not give an accurate reading. I even cut all tie wraps and threw transducer over side of boat and still same readings. I exchanged the standard transom mount dual beam that came with the unit for a shoot thru the hull quadra beam. Is it possible I have a bad ground in the electrical system or have I gooten yat one mor bad transducer or head unit once again. I have owned boat for about a year and have not been able to fix issues with fish / depth finders hence the reason for just getting new one. Any help or suggestions would be greatly appreciated.

You said that you have the shoot thru? Right?

Then how did you throw the transducer over board? The shoot thru transducer should be glued down to the hull. If you don't have it installed right it will not work.