Sonar Mysteries

[Paul Roberts]

OK...What do you make of these?

It appears that my Skimmer transducer (Lowrance) is somewhat directional. Since I'm in a float tube I can see the difference rapidly. In this shot it's esp noticeable. I am rotating while over a gentle drop-off. Look at the difference in density of returns.Those are not boulders but caused by me simply rotating in my tube! It looks like a hard spot I was at the edge of. But I noticed I got a hard return one direction and a soft the opposite. So...is this a hard or soft bottom? It's along the same shoreline as the following screen shot.

This is a complex bottom area I haven't figured out yet. In this shot I am traveling in a straight line. Haven't fished it so I haven't sent a jig down yet. See the overlapping returns in the valley? What do you think?

This is the edge of a large isolated weed clump off a really nice sunken hump. I carefully marked it's dimensions and then buoyed it. I then snuck back later to fish it. I hooked two fish but both came unpinned quickly and felt smallish. I also caught a 2lb LM.

What was weird was I couldn't feel the weeds! I finally put on a heavy exposed hook jig trying to foul weeds, wanting to know what kind they were, being so deep -running 13 to 19ft deep. Nada! So I went over it again and my carefully marked weed bed was gone! It was a pile of fish -I'm guessing crappies. Felt kinda foolish. I use a voice recorder for journaling. When I reviewed what I had said when I first found the mass I said, "Looks like a pile of fish!"

[Tin]

You're in a float tube in 42* water? 

I would widen the cone that hits the bottom. Seems like you are just shooting down a straight line despite being in 14-19' of water.  :-?

[Catt]

Asking me to interpt one screen shot without knowing the previous or following screen shots is next to impossible and farther complicating it by the fact you're moving.

Just a suggestion; zoom in

[WRB]

The TD mount is subject to the movements of the flexible float tube. When you make a radial turn, nearly impossible with a boat, the signal return is eccentric.

Interpreting signal returns can be difficult; brush, bait, trees, fish close to the structure or cover, suspended debris all can look alike.

I have been looking at sonar signal returns for a very long time and sometimes get fooled thinking a bush is bait etc., although not on a float tube. The new StructureScan helps to eliminate most of the signal interpretation issues, however this isn't available as float tube technology for several years.

The fact you are fishing from a float tube indicates a small return signal area about 1/3rd the depth; a 7' foot area @ 20' could easily be widened to 10' as you spin around, plus the water turbulence you make will changes the signal density (color) slightly. Hope this helps.

WRB

[senile1]

Paul, I like it when you start sonar threads.  I think these can be very helpful to a number of people.

[J Francho]

I guess you'd have to get in shallow, and duplicate the 1st set of returns with a known entity.

I can relate to the pack of fish looking like weeds. Best to switch to 200 kHz narrow bean to get the best definition possible. I had a similar occurrence yesterday. What looked like big clumps of dying milfoil were actually moving, LOL. You can see more details in my Other Species post.

[EDIT]Meant to say switch out of 200 kHz mode.....to wide beam.  I always screw that up.[/edit]

[WRB]

  Quote

Asking me to interpt one screen shot without knowing the previous or following screen shots is next to impossible and farther complicating it by the fact you're moving.

Just a suggestion; zoom in

+1, don't switch, just zoom in, you are only working 20' of water depth.

WRB

[Paul Roberts]

It's a single beam 200kHZ transducer. And it's pretty narrow at such depth. I'll try zooming in on that crevice next time.

I'll have to play with this directional idea further shouldn't be the case. Maybe I was on the edge of a hard lip. I sure hope so. Don't need a direction variable added to things.

The second shot looks like a drop with a huge boulder at the bottom. But there's a tree growing out of it! or very close to it. I don't think it's a boulder as it's longer then that running some unknown distance along the bottom of the drop. That overlap of returns runs a ways. It was most probably formed during construction work before the pond was filled. The surrounding topography (surrounding the pond) is pretty darn flat. Must be the work of a dozer.

These are preliminary shots. As I decipher things I'll post it.

  Quote

Paul, I like it when you start sonar threads. I think these can be very helpful to a number of people.

I'm trying NOT to start threads lately. I've got too much work to do and too little play time as it is. Someday

  Quote

You're in a float tube in 42* water?

5 hours of it. Neoprene, fleece, and excellent circulation (a gift from my Dad). A Ranger appeared as I was pulling out (thought he was going to lecture me on hypothermia, or worse County liability) but he didn't. I told him it was downright cozy out there. I have a good PFD, tight fitting waders, and figure if I fell in I'd have just enough time to swim to shore before I stiffen and sink lol.

[J Francho]

42° is bath water to me, LOL.

[WRB]

The TD sonar signal cone is round, not directional. However if the cone isn't exactly perpendicular (90 degrees) from the water surface, it becomes elliptical instead of a round conical signal return. This happens every time you approach a sloping bank or point; the furthest and weakest return signal is at the outer edge of the signal cone, the strongest signals are in the center area.

You can intentionally angle the TD cone forward, backwards or to one side. I believe this is happening unintentionally with the float tube. The signal returns are difficult to interpret because we (todays Lowrance users) are conditioned to interpret a straight down pointing signal return.

The new side and down pointing sonar systems have complex operating systems to display the images so we can read them.

WRB

[Wayne P.]

Paul, to eliminate the movement variable, try mounting the transducer to another separate floating device so it remains parallel with the surface no matter how much you move.

[Paul Roberts]

WRB, that's interesting. My transducer appears to be relatively stable, not much movement really. But it MIGHT be enough to alter density readings. That is something I can play with. I have tried moving the TD around a bit to see what I get and haven't noticed anything, but I'll play around some more.

Wayne, a good idea, but I think my tube would be more stable than something smaller pulled behind, esp considering I throw a wake as I "motor".

[WRB]

It not so much as bobbing up and down, that happens all the time with bow mounted trolling motors with TD's mounted on them. What is important is the TD is pointed nearly straight down. If you use a flash light for example and point it straight down and rotate it around, the spot stays on the target. If you point the spot light off to one side at an angle, then turn it around the spot is egg shaped makes an arc. That is what causes the distortion in return signals. If the trolling motor shaft, for example isn't straight down, then same distortion occurs, we just don't realize it.

WRB

[J Francho]

  Quote

What is important is the TD is pointed nearly straight down.

That is a very good suggestion.  I run into that very issue when mounting my TD to a tin can rental boat.  As much as I tried to account for transom angle, you just can't count on getting the same boat over and over.  Though, the owner of the marina knows that I have y faves, LOL.

[Paul Roberts]

It hangs pretty straight, but I'll keep that in mind as I go. Thanks WRB.

A friend sent me a screen shot of a huge dense school of crappies, and it had the exact same shape as my third pic: a big rectangle! Flat on top with a vertical edge, looking like a weed wall. Pretty neat. I've heard of crappie schools commonly being vertical. But they can be square too! It's not related to temperature as his showed a surface temp reading of 80F.

[FD.]

  Quote

Paul, to eliminate the movement variable, try mounting the transducer to another separate floating device so it remains parallel with the surface no matter how much you move.

This also eliminates any charting errors caused by you "radial turns".  Seems to me if you were towing your TD, you should be able to achieve the same stability as a trolling motor mounted TD.

[Catt]

Set your upper & lower limits narrower, slow your chart speed, and zoom in

[Crestliner2008]

Tough situation being in a float tube with a sonar! Stability of the transducer is paramount to receiving accurate readings. However, what's you've shown here seems to be O.K.. I can almost guarantee that the "weed line" in the third picture is either baitfish or a school of crappie. I've seen it too many times. The proof in the pudding is when you go over the same spot and it's vacant!

Also, I agree with what has previously been said about your settings. Make sure you slow your chart speed down to about 60%. And if adjustable, raise your ping speed to maximum. Your sensitivity setting appears to be just fine.

The second photo shows either a tree or a vertical column of bait. Sometimes you will get a similar response if you move quickly; i.e.: gunning the engine when you leave a spot. I don't think you're too worried about that happening in a tube however!

When trying to get a visual of what the bottom is truly like, try to do it the same way each time; moving laterally in one direction vs. turning. Going back & forth over a given structure will define it much more accurately than turning around and around.

Can't comment on the hard vs. soft bottom of your colored unit though; mine is an X-135 black & white!

[Paul Roberts]

I think I'm doing OK with settings. I'll look into the effects of play in my TD mount to solve the question in pic#1.

Pic#2 is going to yield fish somewhere down the line. There is a nearby hump with a finger bar that is all rock. When I first found it I thought the 'dozer operator had fish on his mind!

#3 will have me much more mindful of the shape and appearance of fish on my sonar.