The more sophisticated side scans do in fact get clear targets down to multiple hundreds of feet depending on beam angle used. I installed the Furuno Dff3D multi-beam transducer last summer and the side scan is very useful for pinpointing bait off drop-offs and seeing the density of bait balls and where they are in relationship to bottom structure and the water column. Furuno uses the 165 Khz frequency which being lower then what is used by Garmin, penetrates deeper in the water... it's been said that you can generate clear side-scan pictures down to 100 fathoms...
I did have a magic moment last summer up in North Coast.... went by a steep cliff, saw no bait with my P66 or my Chirp transducer but the side scan clearly showed the face of the cliff off to starboard and a bait ball stuck to the side in about 30M of water.I punched in the waypoint which gave me a track line back to the ball and on the first pass I got the first "almost Tyee" fish of the trip...here are some screen caps of what you see on a Furuno DFF3D screen
First a bait ball on the multi beam. (The screen on the left is a B175W Chirp, the screen on the right is the DFF3D triple-beam but selected for single beam: please note the different way the bait ball is shown by the two different technologies): the B175W shows a "spiky" bait ball with 25 degree beam; the DFF3D shows a more "globular" ball with 40 degree beam
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Then the same bait ball on the side scan:
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More triple beam image and how it looks in side scan (please pardon the "noise" in the next two screen shots: I had just set it up the system and hadn't yet strung a grounding wire to the bracket of my outboard)
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This last pic shows another graphic cross-sectional display of side scan available on the DFF3D: The three-part cross section screen displays the real-time sea column echo in a 50-degree port, 20-degree look-down, 50-degree starboard mode, which adds up to a total of 120 degrees side-to-side coverage. This provides visuals to instantly understand the distribution of the bait in relationship to the hull of your boat and the density of the bait ball as it sits suspended in the water column
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The downside of this technology: a HUGE transducer which I hung from a bracket I had welded on my sponson: no way I wanted that sticking through my hull with a faring block, not with the amount of tailoring I do...
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