Transducer Question

[Bluegill Bill]

My transducer is located under the rear of the boat and my question is, does that mean that whatever I'm seeing on the screen is located on the bottom at the back of the boat? I've always wondered about how these things work and where what their seeing is located, is there a web site or something that explains all this, thanks fellers.

"Shep"

[Jason Halfen #511]

I assume you're wondering about "traditional" 2D sonar.

Most units dedicated to 2D, down-looking sonar transmit at a frequency of 200 kHz, and have sonar cone angles of 20 degrees. A quick back-of-the-envelope calculation demonstrates that the sonar cone interrogates a circular portion of the bottom that has a diameter approximately equal to one-third of the water depth.

What does that mean? In water 20 feet deep, your sonar screen is showing returns from a circular portion of the bottom approximately 7 feet in diameter. In other words, your sonar is seeing objects on the bottom that are within 3.5 feet of your transducer (3.5 feet forward, behind, to the left, and to the right). A transom mounted transducer won't interrogate objects that are near the bow (of a 16' boat) until the water is close to 50 feet deep.

If that's as clear as mud, check out the graphic below, which illustrates a sonar cone of 20 degrees and the portion of the bottom (and the water column above) that it interrogates.

[Bluegill Bill]

Thanks Jason, I can understand how it works now. I didn't know how much area it was actually seeing and showing on the screen.

"Shep"

[CatFan]

It's also useful to understand that the cone isn't uniform in return strength and that it doesn't end sharply at the 20 degree mark.

The returns out at the edges of the cone come back with about half the power as those in the center. Returns from say, 24 degrees are going to be visible and pretty well indistinguishable from those at 20 degrees, and you may still get returns from very hard objects that are well outside the nominal cone of the transducer.

The reason this is useful to know, is that it explains why fish show the characteristic tapered arch as you approach them and then move away. The fish is initially out away from the cone's center, so the return is weak. As the boat comes closer to the fish, the fish is closer to the center of the cone so the return is stronger, making the center of the arch darker. Trigonometry tells you that a fish at the same depth but away from the transducer shows a deeper apparent depth, which causes the arch shape.