Finding Hard Bottom

One of the things that can be key to finding fish is identifying the bottom content. Certain fish prefer certain types of bottoms at certain times. Which fish prefer what type when will be left up to you. This article will only help you understand what your depth finder is telling you about the bottom conditions below.

To be able to tell at a glance at your depth finder that you are on sand or gravel is asking a lot. If you get really good at reading your unit and you get to know a particular lake very well you may eventually be able to do this. Realistically, you will only be able to determine if the bottom is harder or softer than the area you have just moved over.

There are two ways in which your sonar will show you changes in bottom hardness. One way is by a difference in the appearance of the bottom signal. As the bottom gets harder below, your sonar will show the bottom band getting wider or stronger. If you know the bottom is relatively flat, you can use this information and feel fairly confident in what your unit is telling you. However, if you’re not sure how flat the bottom is, you can be mislead because a bottom with a steep slope will also show as a wider band than a flat bottom. The other, more reliable, method to use is to notice the changes in the second or even third echoes beyond the bottom.

An echo is created when the unit transmits a burst of sound and that sound is reflected back to the transducer. When this sound pulse reaches the transducer, it hears the sound, the unit calculates how long it took for the sound pulse to go down and back, and displays the result as bottom. But the actual sound doesn’t stop there. It is reflected by the surface of the water and the bottom of your boat back down towards the bottom. The sound is then reflected upwards again. When it reaches the transducer this time it is much weaker. The sonar also notices that it took this sound pulse twice as long to get back to the transducer. The result is a second echo. This repeats over and over. The sonar unit will display as many echoes as it sees. At some point, the return echoes strength will be so weak that the unit will not display it.

To read these multiple echoes as a bottom hardness indicator it is important that your sonar be set up properly. The first thing to do is to set the unit for manual range control. In order to see these echoes you need to have room on the sonar display for them. A second echo is always twice as deep as the first echo, or bottom. Say the depth is 25 feet and your depth finder is set to a 30 foot range. The second echo, at 50 feet, will not be displayed. However, if you manually change the range to 60 feet, the second echo will now be visible. To see a third echo, you will need to have the unit set to a 75 foot, or greater, range.

Now that you can see the echoes you are ready to determine the bottom hardness. A harder bottom will reflect sound much better than a softer bottom. The more echoes you see and the stronger they are the harder the bottom is. This hold true as long as the depth stay similar. If it gets quite a bit shallower or deeper, you’ll need to adjust the gain accordingly. The figure shows what you will see.

Another thing you need to do with your sonar set up is make sure that the gain, or sensitivity, control is set to manual. Automatic gain control allows the unit to adjust the gain level to keep a good signal of the bottom. As the bottom gets harder or softer the unit will make small adjustment in the gain level. This is not what you want it to do when you’re looking for bottom hardness. Set the gain on manual and turn it up enough so you can at see a second echo clearly.

Although the entire line of Vexilar Flashers can detect bottom composition, the Vexilar Edge3 is about the best there is for finding hard bottom. The Automatic Range Mode setting lets you tell the unit that you want to make sure that there is room for a second or third echo on the display. It will automatically adjust for depth changes, but will make sure the multiple echoes will fit on the screen. And with the full-color, dual sonar and split screen operation you can have one screen showing you echoes and the other set for the best view of the bottom. Also, the auto-gain and range buttons are right on the front of the unit. There are no menus to go through to turn these features on and off.

You can find hard bottom while ice fishing with your flasher too. You’ll need to have a number of holes drilled. Turn the flasher on to a deep range and set the gain up high. Now when you move from hole to hole, take notice of the strength of the multiple echoes.