My dad installed a Humminbird 798c fishfinder towards the end of this summer.  We followed the directions in where to place the transducer, correct distance from the outboard, where to place it, etc.  Everything seemed like it was working until we took it on its maiden voyage.  The GPS was fine, Side Imaging didn't seem like it had any problems but the graphing image with depth is what is giving us problems.  The biggest issue is with depth.  We fish the tidal Potomac River so it is a generally shallow body of water.  I would know we were in ~4 ft of water but the readings coming back might read 4.3, 4.7, 3.2, 5.1, 4, 7 or something along those lines.  Essentially the readings were all over the place.  My first thoughts were that something with the settings needed to be changed.  I fiddled with all those but nothing got rid of the incorrect readings.  Another problem was that in shallower water, maybe 5 ft or less, the entire screen was just a mass of different colors.  There is a lot of vegetation in the Potomac but there is no way that accounts for the clutter on the image.  The fishfinder we had on previously was an older black and white Lowrance which had none of these problems. 

    When in deeper water, the screen clutter was gone for the most part.  However the depth readings are still jumpy.  If the boat is in 12 ft of water, the majority of the readings will be close to that, with an occasional one jumping to 19 or 7.  So something is still wrong.  I'm out of ideas on what could be wrong so hopefully someone here can offer some help.  Does the transducer need to be moved up a bit, maybe we got a faulty transducer, is there an obvious thing we're overlooking?  Thanks in advance for any help you guys can offer

A couple of things to try.  Even though it is new get the newest software installed to it.  Register the unit or Humminbird.com and download the software update to a blank sd card.  Put the card in the unit and follow the instructions.  You will want to do this again in about a month to get the new Down Imaging software and revised waypoint management as well.  Also, use the systems sonar menu to adjust the sensitivity and surface clutter settings.  In shallow water make sure the TVG is ON (SwitchFire in new software) is set to clear mode.  In dirty water I have had similar issues and by making some adjustments you can clear up the screen.

I will be installing a 798ci SI this weekend. I have a Matrix 97 and it gives depths in tenths also. With the wave/wake action on the Potomac, certainly you will get varying depth changes constantly and noticeably in shallow depths.

This is a screen shot I did at Briery Creek when passing over vegetation, then a deeper open space, on to vegetation again, notice the depth reading.

Thanks guys.  Acar I'll have to try out what you suggested next time I'm on the water.  I played around with the sensitivity some but was still getting the funky depth readings.  I'll have to see if I can get rid of the surface clutter.  These are things I'll have to play around with some I reckon.  Wayne, the problem is most of the time I am nowhere near waves or wakes.  The water was smooth as glass many times.  And at a slow idle or trolling speed I will get readings anywhere from 3-9 ft when I know in reality the depth is 4-5 ft.  That just isn't right.  On my older fishfinders, I saw exactly the same things in regards to vegetation as your picture shows.  You could see individual clumps, etc.  But now if I am in shallow water I just get a screen full of color when I know full well there isn't grass everywhere.   :-/  Anybody else got suggestions? 

Make sure your transducer is pointing perpendicular to the bottom regardless of boat attitude (the way the boat sits on the water).

Lower the sensitivity setting to about 30%. The TVG may do that automatically.

I rode around in the cold/cloudy conditions for a couple of hours with the 798 and got these shallow readings. I had the Switchfire set on "Clear" and the sensitivity set to 5.

Another shot

Hmm, so did everything seem to be functioning properly.  The big thing being were you getting consistent/realistic depth readings?  I.E. knowing you are in 4 ft of water but getting readings all over place

Yes, I was getting a consistant depth reading in those screen shots. The only inconsistant readings I got was in less than 2' depths because even at the lowest sensitivity setting it would get echos. There are some live and dead weeds that may have contributed to the inconsistant depth readings though. The water was real muddy with less than 2" visibility. At least the depth display did not go into the hundreds of feet like the X series Lowrance units do.

I haven't permanently mounted the transducer yet and have it placed 7" above the keel.

Alright thanks for the info.  Over Christmas break I'm gonna get out on the water and play around with it and see if I can't get it functioning in a way that satisfies me.  I'll shoot you a PM if I've got any questions