Identifying Hard Bottom on Sonar – By Scott Chapman

Identifying Hard Bottom on Sonar - By Scott Chapman

I can’t count the number of times that I’ve read or heard “take your fish-finder off of automatic”! ” If you really want to know what kind of bottom you’re over, use manual mode”. Well that’s all well and good, but what am I looking for? Various vendor tutorials talk about thickness of the bottom line or intensity of the red line on the bottom. Thick relative to what? Red relative to what? The demo pictures showed the theory, but when I tried to duplicate this out on the water, nothing jumped out at me as screaming HARD BOTTOM. I kept searching online, determined to find an explanation that would give me the skills to know what kind of bottom I was over. The theory turned out to be pretty simple. When the bottom is hard, the sonar echo bounces back up with so much strength that it bounces back down a second time. That second echo hits the hard bottom and bounces back up to create a SECOND ECHO. On really hard bottom, it’s not unusual to even get a third echo. And that’s where manual mode comes in. In automatic, the range of your machine will set itself to just below the real water depth. You’ll never see a second echo at twice the “real” depth because the screen doesn’t show the additional depth when you’re in automatic. Here’s a picture to help:

… here’s what you do: when the digital display shows an actual water depth (in this case) of around 6 ft., you set the manual range of your sonar somewhere in the area of 20 ft. You’ll be amazed. You cruise along with a single echo and a lot of empty space below it. But when you get over hard bottom, just like in the picture above, you’ll see that second echo start creeping across the screen. Thicker the second echo, harder the bottom. Get a third echo? Harder bottom yet.

Scott Chapman
Montpelier, VA

Your thoughts here
  1. Scott, thanks for the contribution. Much appreciated.

    Capt Mike

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