BRM... you're right on sonar I believe. Rule of thumb ive always used is a 200 is covering 1/3 of the depth you're fishing ( meaning if you're looking n 9' of water you're seeing 3' of bottom) and 83 is showing 1:1( 20' of water/20' of bottom) if that's not right... it's been working for years now! LOL

My understanding of the DI/SI is it is not a cone. It prejects out left and right but only a few degrees front to back.

That's about as far as I can explain it. The rest is magic.

We need doublegobble to chime in.. he's pretty knowledgeable on this stuff.