'Morning, Marine. Thanks for your service. This old Sailor (radar tech) will tell you what little I know about depth finders. Maybe someone else can fill in the shady areas for both of us.
"Where does the sensor at the back of the boat read in the water" Generally speaking, directly under the sensor. The width of the beam depends on how deep the water is. The deeper the water, the wider the beam where it hits the bottom. Mine is a dual beam, so it shows solid color fish in the main beam; outlined (ghost) fish in the wider beam.
"how come the screen is always moving?" The screen appears to be moving because the depth finder is updating the information it is receiving from the signal it sends out.
Basically, a ping is sent out then recieved in an instant. The computer breaks down the returned signal as to depth and bottom contour as well as any obstacles or irregularities within the beam width. If you stay in the same spot, it refreshes what lies directly below the transponder. If your boat is moving, then each time it refreshes the presentation, it is showing the depth, bottom, and obstacles in the new location the boat is in.
That's the quick and dirty. If you want to read more about depth finders, Humminbird has a great site filled with information. Here's a great tutorial that will answer about any question you might have. Beams, beam width, theory of operation, etc.
http://www.humminbird.com/generic.asp?ID=403
I hope I was able to shed a little light on the mystery of depth finders. I'm going to go back to bed now and try to get some sleep. I will say a prayer for you in the meantime. Stay safe, Marine.
papa1
Looks like I need to modify my profile. Gender's wrong. Just what I needed. ;D