The sonar unit, commonly known as a fish finder, is the most misunderstood piece of electronic equipment found on fishing boats. The average angler buys a sonar unit, follows the installation instructions and when the unit is powered up it works just fine. Set to auto mode, your sonar tells you the water depth, temperature and even shows fish under the boat every now and then, so everything is working. Right? Well, your sonar may be working, but you are definitely not getting the most out of it.
How Your Sonar Works
Your sonar unit is a precise piece of equipment. Understanding how it works will help you use it correctly. Sonar machines use an electrical impulse to generate a sound wave that is sent by the transducer. The sound wave travels through the water to the bottom, and is reflected back to the transducer, which converts the sound wave back into an electrical impulse.
The sonar unit is actually a specialized clock that measures the time it takes for the sound wave to go to the bottom of the ocean or lake and back to the transducer. That time is continuously displayed on your monitor in the form of pixels that show the bottom and any other objects in the water column.
Today's sonar units come with dual-frequency transducers. The lower the frequency, the greater the penetration. To read the bottom for structure, like wrecks and reefs, use the lower-frequency setting, commonly 50 kHz. To search the water column for fish or when fishing in shallow water, select 200 kHz.
Getting the Most From Your Transducer
Here's where the average angler comes up short. Set your sonar unit to manual mode. Turn the "gain" setting on your machine all the way down and then gradually turn it back up until you see two reflections of the bottom. The first bottom reflection is the actual depth of the water. The second reflection is from the sound wave traveling to the bottom and coming back to the transducer with so much strength that it bounces back down a second time. The second bottom should appear approximately halfway between the bottom and the surface. Slowly decrease the gain until the second echo disappears and you are left with a single bottom reflection. You are now seeing through the water with the maximum power, which gives you the most information possible.
Understanding the Screen
The thickness of the line at the bottom of your monitor indicates what type of material the bottom is made of. A thin line represents a hard bottom, usually consisting of rocks. Thicker bottom lines indicate soft bottom material, like sand, clay or mud. A wreck or rock pile that holds fish might just be a foot or two off the bottom. On your screen, that type of structure will show up as a thicker section of the bottom line.
The zoom feature on your sonar unit allows you to magnify any section of the water column you select. Using the zoom on the bottom is useful for getting an up close look at the wrecks and reef structure you are fishing. Individual fish are displayed as dots on the surface of the structure. You can picture them as the crumbs on your coffee cake.