How do I choose a sonar unit?
Response by Dave A. - Date unknown
The scope of this article is to explain the various factors that are important when deciding on which sonar unit will best serve your needs. This article will not address concerns of how to use or interpret the sonar readings.
The five main criteria that need to be considered when selecting the right sonar are:
What is really boils down to is this: The larger the cone angle, the more bottom area you will see. For sonars with an 8-10 degree cone angle you can compute the bottom area that is seen by the sonar by dividing the depth by 6. If you are in 60 feet of water, these sonars see a 10 foot area on the bottom and about 5 feet in the middle of the water column at 30 feet. For sonars with an 18-21 degree cone angle divide the depth by 3. If you are in 60 feet of water, these sonars see a 20 foot area on bottom and a 10 foot area in the middle of the water column at 30 feet. Wide angle sonars sporting a 45-60 degree cone angle see an area equal to the depth. In 60 feet of water, the sonar sees 60 feet of bottom area and 30 feet in the middle of the water column at 30 feet.
When choosing which cone angle is right for you, it is important to consider the depth you spend most of your time in and what you will use your sonar for. If you spend most of your time trolling deep water, you will want a narrow cone angle. Wider angles will show you more information than you want to see. You might mark fish on your sonar, but they could be 50 feet or more away. If you spend most of your time in 20 feet of water or less, you should select a wide angle transducer or you will not see enough to easily understand the bottom contour or detect the presence of fish. With a 9 degree cone angle in 15 feet of water you will be viewing an area that is smaller in diameter than your boat is wide. If you fish a variety of depths you might consider a medium cone angle model as a compromise or you could select a model that offers multiple cone angles that can be selected from the unit.
Software and adjustments
Some sonar units have a feature that places a fish icon on the display whenever it detects a sonar signature it interprets as being a fish. This feature is indispensible if your sonars resolution is incapable of displaying the true sonar echo of the fish on the display. Most units are powerful and sensitive enough to detect fish that cannot be displayed in the limited room the screen offers. Suppose you are in 40 feet of water and a big fish is suspended 10 feet off the bottom. If you have a low resolution unit that displays 128 vertical pixels then each pixel is representing 4 inches. If the bass is 6 inches tall by 18 inches long, then only 3-4 pixels will be used to display the fish (horizontal resolution is usually less than or equal to vertical resolution). This is not nearly enough to put the distinctive fish arch on the screen. The fish will look like noise in the display. The problem gets worse as you get into deeper water. While this feature is wonderful in identifying fish once your screen resolutions limits have been reached, you will lose a lot of detail when this feature is on. You will not see baitfish, thermoclines etc.
Another feature is the ability to select the water depth range you want to see on the display. You may be in 40 feet of water, but perhaps you notice that most of the fish are down 20-30 feet. Some units will allow you to omit displaying the area above and/or below this zone. This increases the number of pixels that can be used to display fish. In other words, you tell your sonar not to waste valuable pixels displaying empty water. Some units allow you to control the range that is displayed, most have factory presets that focus on the common fish holding ranges.