GRAPH QUESTION. Where are the fish?

Discussion in 'Fishing Electronics Review' started by SeedTick, Mar 20, 2006.

  1. SeedTick

    SeedTick New Member

    Messages:
    1,414
    State:
    Conway Arkansas
    Guys, I've never had a good sonar before so I really don't know much about them. I could tell the depth of the water and that was about it. What I'm having trouble wrapping my pea brain around is; where are the fish in relationship to the boat. My new graph (Lowrance X107C) uses a dual cone angle transducer at 35 and 12 so I've kinda got an idea about the size of the area I'm looking at. The graph speed is faster than the boat speed so does it show the same fish more than one time if I'm trolling really slow? Here is another thread with a picture of a graph that I am keeping an eye on.

    http://www.catfish1.com/forums/showthread.php?t=10364


    Here is the picture.
    http://www.catfish1.com/forums/attachment.php?attachmentid=6331&d=1142141080

    :0a36:
    Thanks for your help.

    ST
     
  2. jim

    jim New Member

    Messages:
    2,579
    State:
    Jacksonville NC
    Keep one thing in mind when using any graph.THE ONLY thing that is current on that screen is the right hand edge.The rest is history.Any fish arches you see and your picture shows some dandies around the baitfish, are directly under the transducer cone.So if the transducer is on the rear of the boat by the time you "read" the graph the fish are behind you.Also keep in mind that good arches are a function of the boats speed,higher speed more defined arch.If you are sitting still and a fish slowly swims thru the cone it will look much more like a straight line.The scrolling speed of the graph doesn't relate to boat speed per se.just increases the definition or amount of pixels turned on as the fish passes thru the cone.Go to the Lowrance website and get on the product emulator for your unit and practice practice.;) :) Or take the tutorial which is the best explanation of how sonar works and all the Lowrance features that I have ever seen.:)
     

  3. SeedTick

    SeedTick New Member

    Messages:
    1,414
    State:
    Conway Arkansas
    That's the stuff I need to hear, the left side being like history. I've played with the simulator and it really helps. I need to get out and try it on the water. We've had a drought here in Arkansas for the past year and since I got my boat and got it rigged up it ain't done nothing but rain. I'm not complaining though because we sure needed the water. Thanks for your help.

    Anybody else? When you don't know anything even the simplest stuff helps.

    :0a23:

    ST
     
  4. Catfishhunter

    Catfishhunter New Member

    Messages:
    332
    State:
    Lanesville, In.
    Seedtick. In you pic the D fish are showing at 1-14' and 2-18'. Those fish are not 14' & 18' off the bottom. They are 14' & 18' from the center line of your transducer. They could be anywhere in relation to your boat, front,back,left, or right. Hope this helps. I used to have a tutorial on how to read a depth finder, I'll see if I can find it and pm you with it...
     
  5. SeedTick

    SeedTick New Member

    Messages:
    1,414
    State:
    Conway Arkansas
    Thanks Catfishhunter, I need all the help I can get.

    ST
     
  6. Mr.T

    Mr.T Active Member

    Messages:
    2,554
    State:
    MO
    Well, sort of...

    When the depth finder paints a fish arch at 14 ft, it's 14 feet below the transducer, but not necessarily *directly* below the transducer at 14 ft.

    With a 35 degree cone angle, the fish finder is seeing a circle with a roughly 4' radius (8' diameter) at 14 feet deep (depth * tan (cone angle/2) if you want to do the math). A fish *anywhere* within that circle will be painted as being 14 ft deep.

    So while the fish arch may not be *directly* under the transducer, it can't be that far away from the transducer, even in the worst case. As you get deeper, the size of the circle grows, but even at 50 feet, a 35 degree transducer is looking at a circle with a 15' radius.

    Most depth finders intended for fresh water use a cone angle of 20 degrees, which makes for an even smaller radius -- at 15 ft, it's just 2.6 feet in radius or about 5 ft diameter and about 17 ft diameter at 50 ft.

    The sensitivity setting on your depth finder also affects the effective cone angle that the transducer is seeing -- higher sensitivity means it's looking at a larger angle; reduce the sensitivity so that you don't see much on the screen and you're basically looking at fish directly beneath the transducer.

    The most important thing to remember though is as Jim says -- by the time you see the fish arch on the right edge of your screen, you're already past the fish -- the depth finder shows you history only, not "right now". Some depth finders have an optional "flasher" display where they show real-time sonar results as little bars at various depths - that's the closest you'll get to seeing what's directly under the boat right now.

    When you're sitting still, you won't see fish arches. All you'll see is a horizontal line, representing "something" that's under the transducer at that depth. If it disappears, the line represented a fish that swam through the transducer signal; if it stays for a long time, it may be a branch or some other submerged "thing".
     
  7. ShilohRed

    ShilohRed New Member

    Messages:
    4,339
    State:
    West Tn
  8. SeedTick

    SeedTick New Member

    Messages:
    1,414
    State:
    Conway Arkansas
    Mr. T, I was hoping someone would post the formula. So, have I interpreted it right?

    depth x (tangent cone angle) = diameter of coverage

    ST
     
  9. SeedTick

    SeedTick New Member

    Messages:
    1,414
    State:
    Conway Arkansas
    Pete, I don't really understand. Will you elaborate? Remember...pea brain..

    ST
     
  10. Mr.T

    Mr.T Active Member

    Messages:
    2,554
    State:
    MO

    Almost. Divide the cone angle in half before calculating the tangent. The result you get from the formula is the radius of the circle -- double the radius for the diameter.

    The cone angle is the total angle of the cone, not just the angle from the center of the cone to a side. A 20 degree cone angle looks 10 degrees either side of center. And tan(20) isn't the same as tan(10) * 2 -- it's close but not quite.

    A reasonably accurate "rule of thumb" for the typical 20 degree transducer is that the DIAMETER of the circle is about 1/3 the depth -- so you're looknig at a roughly 10' wide circle at 30 ft or a 20' wide circle at 60 ft. -- the exact number is actually a little more than 1/3 but that's close enough.
     
  11. SeedTick

    SeedTick New Member

    Messages:
    1,414
    State:
    Conway Arkansas
    Okay, that's what I get for trying to cut corners. So it would be this

    depth x tangent (cone angle/2) = radius of coverage
    radius of coverage x 2 = diameter of coverage


    Boy, I sure appreciate the help. Keep it coming.

    ST
     
  12. ShilohRed

    ShilohRed New Member

    Messages:
    4,339
    State:
    West Tn
    It will just take time on the water. Also change the factory setting. That is the only way to get the best out of a depth finder.
    I can watch a catfish coming up from 40+ ft of water on my old one. And sure the new one will do it better. You see a line start at the bottom and it works its way all the way to the top.
    Also I will save a few pictures this year on the new one and post them.

    So get out on the water and play with it. Best to try it out somewhere you know good and that will help a lot. Learn how to read what is what on one. Like this is a tree see the branches or tree top.
    Also drop your bait under the transducer and see how it shows up on screen. It just takes a while
    On the arches from what I have been told, The thicker the middle of the arch is the bigger the fish is. But then again it has to hit the transducer right to show that.
    Pete
     
  13. SeedTick

    SeedTick New Member

    Messages:
    1,414
    State:
    Conway Arkansas
    Thank you guys for all your help. After I have a chance to get out on the water and try it out I will probably have a lot more questions for ya.

    :0a31:

    ST