Hey Wargofcac,
Think of your screen as the left window of your truck as you drive down the road (safely, we hope). Everything that is visible behind the right side of the window is stuff you have already passed. If you are going real slow, what you see at the right side of the window is just behind your transom. If you are going fast, what you see is a long way back.
Something that is extremely important is understandind 'dead zones'. The machine sends down an inverted cone. It gets wider as it goes deeper. Say the center of the cone is exactly 20', transducer to bottom. On a flat bottom, the distance at the edges of the cone will be 22', transducer to bottom. The machine will NOT read farther than the closest thing that enters the cone,...the bottom at center.
The edge, from 20' to 22' will NOT be read. It is a dead zone, a 2' thick black line. By the same token, If you are running your boat parrallel to and over a sloping bottom, the machine will read NOTHING below the closest point that the beam touches. So... If the bottom slopes downward, from 20' to 30' as you travel along the slope, you will see a ten foot thick black bottom line from 20' to 30'. This is a dead zone. Everything deeper than the closest cone, contact point at 20' will be blacked out.
To read the slope you must run UP or DOWN it. As you travel up the slope the front of the beam 'reads' and what you see will be at the front of the cone(perhaps at the center of your boat in 30' of water). Every thing behind the front edge of the cone is in a dead zone. As you travel forward the front and back cone edges will , eventually, both be over the slope edge at equal depths and the bottom band will be very thin (remember the original scenario).
If you travel down the slope, only the back edge of the cone will 'read'. As you progress past the edge of the slope with the back cone edge at 20', all things in the cone angle AHEAD of the back edge is in a dead zone and the back edge(what is being read) is ten or so feet behind your boat. Again, there will be a ,10' thick, black bottom band that gets progressively narrower as the back and front of the cone get closer to touching the bottom at the same depth, 30'.
If this doesn't make sense, get a chalk board and draw the bottom configuration on it that I've talked about. Also draw the surface. Get a piece of cardboard and cut out a 20 deg. cone and a little, teensy-tinesey boat. Lay the boat/cone on the chalk board and it should become clear.
Remember, nothing, in the cone angle, that is below the closest contact to the transducer will be read. All machines, regardless of make or model,work in this manner
Oh. Oh. By all means order Iovino's book and, FOR SURE, his tape. Any one who buys a sonar should get these as a freebie.
Hope this helps
Bud