How Sonar Works:
Greatly simplified, a sonar unit is just a combination of a speaker, microphone and stopwatch.
Every fish-finder is programmed to "know" that the speed of sound through water is about 4800 feet per second. Using a transducer, fish-finders transmit a sound pulse, or ping, and then measure the time it takes for echoes to return from the ping. Then they convert the elapsed time for each ping into distance. A built-in computer organizes all of this information and shows it on a display screen.
Conventional bottom-finding sonar uses a transducer to send ultrasonic sound waves, or pings, down through water. The pings bounce off the bottom, and objects such as fish, and are received back by the same transducer.
A sonar ping travels at a rate of approximately four thousand eight hundred feet per second. The Unit can monitor the time it takes for each ping to go out and come back, and accurately calculate the distance to underwater objects.
Once the distance is determined, it can be plotted on a screen, one ping at a time, to draw a historical picture of the bottom contour, and the location of fish and other objects that pass under your boat.
How Side-finding Works:
Side-finding sonar operates basically the same as down-looking sonar, with two important differences.
First a side-finder sends and receives sound waves out to the side of your boat rather than down. And second, a side-finder is specially programmed to ignore structure and display only fish on the screen. With a side-finder you're always fishing fish.
Side-finders are usually set to ignore fish smaller than seven inches, to avoid scattered baitfish clutter.
Side-finder's transducer emits a narrow nine degree cone to pinpoint the location of fish-meaning that when you see a fish on the screen, you know it is directly where your sidefinder is looking.
Side-finding Technology:
Side-finding is more than a sonar transducer pointed horizontally to look sideways.
Directing sound pulses sideways results in echoes from the underside of waves and from the bottom (especially in shallow water).
Bottom Line pioneered and patented the technology needed to separate the echoes returned from fishand those returning from the water's surface and the lake bottom. A considerable amount of sophisticated filtering is required.
Everyone who has searched for fish with sonar has wondered, "How many fish am I missing that are just outside my transducer's cone of coverage?" When trolling, a Side-finder can spot fish to your left and right, and help you determine the most productive track to take. When casting, a side-finder can search water not accessible with your boat.
A Side-finder with trolling motor mounted transducer can sweep the area 360 degrees around your boat for fish. When sweeping a circle around your boat, rotate the transducer slowly to give the unit time to work. The slower you move the transducer, the more time the Side-finder has to pick out the fish echoes.
Coverage:
In 30 feet of water, a bottom finding unit that emits a 20-degree cone angle gives you a search radius of approximately 10 feet. With a Bottom Line fishfinder that combines depth finding technology with Side-finder side-scanning technology, that search radius increases to more than 480 feet.
In terms of water volume, the standard 20-degree cone encompasses less than 1,000 cubic feet of water. With Side-finder, the locating zone increases to more than 4,000,000 cubic feet of water.
Sees Under Structure:
Side-finding technology lets you see fish hiding under protective structure like docks... cut-banks... and fallen timber. And, because side-finders ignore structure and show only fish, you can avoid barren areas, and spend your time casting to structure where you know fish are holding. Side-finder easily sees bass hiding under the dock, while ignoring the pilings. When you're fishing, if you don't see fish under a dock, don't waste your time... move on to the next one.
Sees Like a Flashlight:
Although slightly oversimplified, comparing a Side-finder to a flashlight is a good way to explain what a side-finder can see, and what it can not see. Basically, if you could go under water with a very powerful flashlight, you would be able to see the same fish as a side-finder will show.
While it can see all the way through most weed beds, it won't penetrate extremely dense weeds in shallow water and along shorelines. And, a rise, a bump, or a rock will cast a shadow, creating a small blind zone directly behind the structure. If there's a lone fish within a foot or so of the structure, it may not be marked. On the other hand, when Side-finder marks a fish, cast to the mark... the fish will be there.
Side Lobes:
Side-finders take advantage of side lobes to combine the coverage of an eighteen degree cone, with the ultra-high detail and spotlight focus of a nine degree cone. A side lobe is formed by the portion of a sound wave that extends beyond a transducer's main cone and effectively doubles the angle of the main cone in shallow water.
Display Reading:
The importance of seeing fish as perfect boomerang-shaped arches on the screen has greatly exaggerated over the years. It all has to do with how fish arches are created.
Imagine sitting in an anchored boat with your fish-finder turned on. Picture in your mind the transducer's cone-shaped scanning area under your boat. In order to print as a perfect arch, a fish will have to enter the edge of the cone, swim directly through the middle, and exit the cone. Let's say the fish holds a constant depth of 15 feet as he swims straight through the cone. The fishfinder measures the distance to the fish and starts to show it on the display.
As the fish enters the cone, it is 15 feet directly below the surface of the water, but because of the angle, probably about 16 feet from the transducer. As the fish swims through the center of the cone, it passes directly below the transducer (15 feet). When it reaches the edge of the cone again, it's 16 feet away just before it stops show on the screen.
This causes an arch to show on the screen-starting at 16 feet, curving up to 15 feet, then curving back down to 16 feet. The wider the cone angle, the more exaggerated the arch. If the fish changes depth, passes through only the edge of the cone, or wanders around under the boat before swimming off, it won't show as a perfect arch.
