Building a Side Scan Sonar.....
Well my friend, I have been down that road! All I can say is your time would be more well spent working at a job to pay for one to buy! If you have a full time job and can't afford say the Humminbird 987 or New 997 at $1900, get a 2nd job to foot the bill!
I have wasted so many hours working on projects like this and unless you are a complete electronics genius and mechanical engineer, I would hate to see you go down the same road I did.
Don't get me wrong, its doable! I built Dan's side scan and it works, but the images are nothing compared to what you get from a real side scan or the Humminbird. I have also built a fluxgate mag successfully, but I spend more time fixing it than I have had it in the water.
If you feel like saving a little money compared to the Humminbird, there is a fella named Gary Burton who has built a nice unit and sells a semi-kit form where you can build the towfish and buy the depth finder separate. It works off a Lowrance depthfinder and the data can be recorded and played back on your computer. The only, only, only downside is it looks to just one side not both. The resolution seems to be great. I think his packages start around $600.
www.burtonelectronics.com
You also might want to check out the Geotech forum and see some of the projects there. Its a great resource if you are serious about building mag, sidescan, metal detectors, etc.
http://thunting.com/geotech/forums/
Good Luck and keep us up to date with what you end up doing.
Oh, and welcome to the forum. Let us know if we can help with anything!
Robert in SC