i bought a depthfinder late last summer, but I'm not getting it.I fish out of the back of my jon boat, and my transducer is mounted on the left side of the boat ( i clamp it there). I know you are supposed to look at structure and humps, dips etc. I still dont get why you would want to see whats directly under your boat.You cant fish under your boat. Please fill me in a bit. Thanks

Well as you probably know, the fishfinder is more of a structure finder. What you are looking for is not necessarily fish but habitat that could support fish. If you see fish or baitfish that's a plus but often times you will catch fish where you might not necessarily see them.

So, when you pass over a spot that looks good and has all the right attributes you simply mark the spot, by throwing a buoy or just make a mental note of it. You can then back up and target that area.

The fish are usually sitting there holding on or off the structure, relating to it. They usually won't straight up move by the time you get to them. 

Gotarheelz14 hit the nail on the head.

I fish directly under my boat all the time.  So do many, many others.  I watched the fish in my avatar come up to my bait, and take it, all on the graph.  Its called "fishing in the cone."

Other times, you move in a search grid pattern to define a section of the bottom, and possibly locate fish.

Here's a shot of my graph as I checkout a deep basin area:

I catch many of my fish right under the boat but I fish in clear water and it normally has to be at least 15-20' deep.  Just because you see something on the screen doesn't mean it is right under the boat.  The signal goes out in a cone shape and it bounces back to the unit.  If something like a suspended fish shows up at say 25' that means it is 25' from your transducer and can be anywhere in the cone.

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I catch many of my fish right under the boat but I fish in clear water and it normally has to be at least 15-20' deep. Just because you see something on the screen doesn't mean it is right under the boat. The signal goes out in a cone shape and it bounces back to the unit. If something like a suspended fish shows up at say 25' that means it is 25' from your transducer and can be anywhere in the cone.

Use a narrow angle beam, like 200mHz. I think its like a 10* beam, or something close. 

I catch many of my fish right under the boat but I fish in clear water and it normally has to be at least 15-20' deep. Just because you see something on the screen doesn't mean it is right under the boat. The signal goes out in a cone shape and it bounces back to the unit. If something like a suspended fish shows up at say 25' that means it is 25' from your transducer and can be anywhere in the cone.

Use a narrow angle beam, like 200mHz. I think its like a 10* beam, or something close.

Yep. I have a custom built 8° xducer just of deep ledge fishing in the winter.  But you have to be in slick water to keep the bait in the cone.