It would, otherwise what is there to create the shadow?
What he may be seeing is the shadow while the primary return from the fish is still far enough away to be over the bottom. It’s there but he may not be able to see it yet until it gets close enough that it is now in the water column and easier to see.
The 455kHz Humminbird Si sonar should be showing the entire school as it covers 180 degrees (unless it is being physically blocked), I don’t know what the other brand covers.
10 feet from each side of the boat.Quote:
Originally Posted by wilbur
The width of the dark center would be double the water depth which in this case would be 20 feet wide.
Remember that the distance show is always the distance from the transducer.
Yes that's exactly what I was seeing the shadow before the primary return, the fish were shallow and the water was fairly deep so the shadows came first. But why the entire school didn't show on the graphs I still don't know unless maybe because the lake bottom was too hard....Or I thought the 455 had a cone angle for the transducer beam too until I saw the above pictures. I don't know how well the anchoring and watching SI would work at a regular lake. This was a power plant lake the fish concentrate on the warmest windblown side of the lake in the winter, along with the bait, Some schools are huge as you can tell by how long it takes them to pass both transducers. Jan, Feb we were fishing 55 degree water while the regular lakes were frozen solid. It's kind of a strange lake how the black crappie school up on cloudy days and roam the warmest water. Catch 3-5 foot deep. It's pretty clear water, on Sunny days you can pick up a few dragging bottom 12-15ft or out of grass edge, but the schools split up or go to 30ft on Sunny days. It probably wouldn't work this time of year, haven't had a chance to try, they've probably split up to find spawning beds now. Lake of Egypt, IL where they had the Crappie USA Megatournament is where I was.
Following is how I explain it in my seminars...:Quote:
Originally Posted by wilbur
If we tie a string to the xducer and extend the string straight down to the lake bottom...this is the water depth ("distance x")....
If we now swing this string in a complete semi-circle from the "water's surface left to the water's surface right"...any target in this semi-circle will show in the dark water column ....due to the fact that the target is closer to the xducer than "distance x"....
http://i477.photobucket.com/albums/r...psm7jtyyhn.jpg
Rickie
I guess the school its self could have been blocking the further returns when the big schools were close, I hadn't thought of this, they were pretty tight schools, I don't know. But I had trouble seeing fish past the water column at this particular lake anyway, they just didn't show up no matter how I set my unit. A crappie isn't a very big target though.... Anyways we'll see how they do at my local lake with a soft bottom when I get a chance to go this spring. I do now understand that the fish that I saw in the water column were very close to me. Thank you all for all the reply's.
The angles Rickie is showing represents the HB SI beam angles. The structure scan angles aren't that wide......What did you have the SI range set to on both units?
I usually use 60 feet on both units because I'm usually in 12-16 foot of water. I think unless looking for major structure this seems to be about the limit in that depth or for looking for a stake bed or Christmas tree. I was in 24 fow though at a different lake and the shad schools showed very good on the hds out to 100 foot. Even individual fish showed very good returns in a creek channel at 24 foot...I could see the creek channel with the 998, but no fish returns unless the size of a football.... I don't mean to be pro Lowrance but this is what my initial views are. It may change.... I haven't had time to really use either unit much yet I got the 998 last Dec and hds in Feb. So I have a bit of getting used to and learning to do. I've only had the hds out 3 times. 998 about a dozen or more and it helped me find fish, but fish returns were weak. I don't know if there is actually a functional difference as far as one finding more fish/ structure than the other, athough the down image is actually real on the hds and fish returns are much larger and brighter as well as is the screen. I may keep the 998 for shallow stake bed scouting and the hds on front for DI and closer SI views of the structure I find and when fishing. I had planned to pick a brand, but I may very well find that a combination of both will work best for me. And yes I know I always write too much....
I still catch myself viewing the screen sometimes like a tv of what is around me instead of concentrating on the first few inches and thinking of that thin slice to the left and right when looking for fish to cast at. I'm just now getting over this. And when a school enters that slice it would be very easy for the returns to be blocked before they got far out if you think about it, thus blocking the further part of the school from being recorded on the chart. I think I'm right in theory on this anyway. This could be my answer, and I think likely is.
Anyways this "school chasing" is over for this winter and now I will go to the traditional use for what the SI was meant for, finding structure or structure with fish on it.
If you think about sonar shadows, they are no different than normal shadows cast by any light source. The "players" must line up appropriately, that is, in a direct line. First and foremost, the light source, then the object and finally, the shadow. Your shadow from the sun, for example, will be cast in a line directly behind you when you are facing the sun. In a sonar shadow, the players are first and foremost, the transducer, then the fish and finally, the sonar shadow.