si/di question

Just getting started in the concept of s/i and d/i of sonar units and have not been able to determine the position of the boat in relation to the screen shots. in a side imaging screen is the boat considered to be in the center of the screen shot???? I understand that d/i screens are scrolling right to left as the boat moves forward and the area coverage is determined by the cone degree and the depth of the water but where is the boat in relationship to the screen shot. Any info or links to videos to understanding this is appreciated.

It's not the boat position, per se, but the transducer position on the boat that determines the leading edge of the screenshot. In both cases, the information coming on the screen is what the leading edge of the sonar ping is receiving. On the SI screen ... that's the top of the screen -- and on a DI screen ... it's the right edge of the screen. Everything else showing on the screen is "history" .. as in what you have already moved the transducer past. So if your transducer is on the bow ... the boat is right behind the leading edge of the sonar ping. If it's on the transom ... the boat is (it's length) ahead of the leading edge of the sonar ping.
Both are narrow fan shaped slices (approx 6deg thick), with SI being almost 180deg (left/right) coverage and DI being 45deg @ 800kHz and 75deg @ 455kHz settings (depending on brand of unit).

Lowrance angles :

http://i826.photobucket.com/albums/z...newDIbeams.png



Humminbird angles :
http://www.sportfishingmag.com/sites...I7pPY&fc=50,50

Doug vahrenberg has a lot of great info on using sonar check out his YouTube channel. Here is a video on side scan.

https://youtu.be/3xe6pLPdMKc

Thanks for the replys. Spent some time on the water today and beginning to get a feel for the operation of my unit. Still got a lot to learn!!!!!