June 26, 2023
By Shane Beilue
When side-imaging sonar was first introduced almost 20 years ago, it was revolutionary in that anglers were no longer limited to searching with sonar directly below the boat. The ability to look out as far as 200 feet to either side of the boat has dramatically shortened the amount of time spent looking for fish-holding cover, contour changes, baitfish and game fish away from the shoreline.
Side-imaging sonar offers picture-like images of the water column and lake bottom by emitting a very thin, high-frequency signal from each side of the transducer that covers the areas perpendicular to the boat as well as the water column directly under the transducer. The data is displayed onscreen from a bird's-eye view. Therefore, objects such as rocks and trees appear as though you were looking down upon the lake bottom from an airplane. The screen view with side imaging is unique in that it refreshes from top to bottom, with the newest data nearest the boat displayed at the top of the screen and all the history behind the boat scrolling toward the bottom of the screen.
The water column directly under the transducer is represented as a black band down the center of the display screen. The deeper the water, the wider this black column will appear. Objects suspended within the water column directly under the transducer, such as standing timber, baitfish or gamefish will appear within this black column on the display screen.
How it Works
A great analogy for understanding how side-imaging sonar works is to imagine two powerful flashlight beams directed to either side of the transducer. Objects such as rock piles or brush will be clearly illuminated by the flashlight beams (or, in this case, the sonar beam). However, these same objects will also cast shadows behind them. These shadows are what give the objects definition when displayed on the side-imaging screen. For example, a school of bass located 50 feet to the left side of the boat will cast a shadow on the lake bottom, making them easily recognizable. Similarly, a tree limb angling up from the lake bottom will cast a "shadow" onto the lake bottom, which orients the direction the limb is facing on the display screen. However, consider that anything positioned directly behind any major object will be hidden from view. For this reason, game fish hanging behind large boulders or buried behind the limbs of a large brush pile may be hidden from view with side-imaging sonar.
Advertisement
Side imaging is also an excellent tool for determining various bottom compositions. Bass and other gamefish often feed on "hard spots" along an otherwise silty bottom, making gravel, shell beds or hard clay bottoms bass magnets. These hard spots are easily recognized on side imaging as bright patches on the screen, in contrast to the softer mud bottoms that appear darker.
Depth changes occurring to either side of the boat can also be easily found with side imaging, which is a tremendous shortcut for finding creek channels or high spots. Descending depths will create an increasingly darker return, while ascending contour changes like a point or hump will be brightly illuminated.
Implementation
Side-imaging sonar is best used from the console of the boat when idling 3 to 5 mph in search of cover, depth changes and game fish offshore. While the side-imaging transducer could certainly be mounted to the bottom of the trolling motor, the image is distorted with the turning of the trolling motor shaft or when the boat is stationary. The technology works well in both shallow or deep water; however, extreme depths of 40 to 50 or more mean the water column directly under the boat will be excessively wide on the sonar screen, which limits its effectiveness at displaying the bottom at those depths.
Advertisement
Side-imaging sonar is a tremendous asset for allowing you to seek out large swaths of the lake bottom and water column in a single pass. When in "search mode" over likely fish-holding areas, idling in a back-and-forth pattern as if mowing the lawn will cover every square inch of water and give you excellent insights into whether to stop and fish the area or go seek out another location.
Getting the best imagery from your side-imaging unit starts with the range selection on both sides of the boat. Whatever distance you’re "searching" must be condensed to fit within the confines of the sonar screen. Therefore, maximum search distances of up to 200 feet to either side of the boat mean small but important objects may be compressed and easily overlooked. For this reason, reducing your search range to around 80 feet to either side of the boat still offers a total search area of 160 feet, yet allows improved resolution and imagery.
Also, when searching an area with side-imaging sonar, slower idle speeds of 3 to 5 mph offer the best resolution on screen. Utilizing side imaging while the boat is on plane will cause skips or gaps in sonar coverage, making it rather ineffective for finding detailed features at high speeds.
If there’s one word that summarizes the benefits of side-imaging sonar, it’s "efficiency." The ability to cover large swaths of the lake bottom and water column in a single pass is a huge timesaver when fishing offshore, providing tremendous shortcuts for locating fish.
