Thermocline on a Fish Finder (Sonar)

[Texasmark]

I hadn't been on a certain lake for half a dozen years. My outing Friday was on that lake. So I tooled around to see what changed making full use of the 2 ea 4" $99 free shipping, Hummingbird, color, sonar sets. One transducer was epoxied to the hull with JB Weld, right at the transom, inside the boat, and the other was under my bow seat where I run my ™ also attached with JB to the aluminum hull......reasoning is Sonar is sound transfer and metal is a perfect sound reproducer, so why not put it in your glue too. Sensitivity was set at 5 of 10 and was right for what I was doing. Just a little background info.

Surface water temp was 51 in the early AM and by early PM when I left it had moved up to 53. We had had several days of 30F lows with the fronts coming through.

Across my screen at 3 feet, on both scopes, was a blue line roughly 3-4 inches, scaling from the 3' depth and it followed me around in water above 3' and up to 16' while in protected water, the max depth there. When I moved out into open water, 24' max depth, it disappeared. Assuming that in open water, it means wind and waves and surface disturbances as you all know so I guess it dispersed the transition layer.

Bottom line, anybody have the their Sonar pick up a thermocline before?

 

[mr 88]

Need a stand alone FishHawk to pick up a thermocline/temp break and get down speed as well when trolling with underwater currents. That probe /transducer is attached just above the downrigger weight ,right where your lures are clipped on. They do not make a FF that has the ducer attached to the hull that is capable of showing you a thermocline/temp break , especially from where it may start and end. Even the FHawk just shows you surface temp [and speed ] then when you drop it just the temp at that depth is shown . So if its 70* at 20' when its lowered say 50' it now shows 55* you have a idea where the thermocline / temp break starts .... Odds are you where getting some sort of echo off your unit ,no way was it a thermocline showing up

 

[dingbat]

They do not make a FF that has the ducer attached to the hull that is capable of showing you a thermocline/temp break , especially from where it may start and end. Even the FHawk just shows you surface temp [and speed ] then when you drop it just the temp at that depth is shown . So if its 70* at 20' when its lowered say 50' it now shows 55* you have a idea where the thermocline / temp break starts .... Odds are you where getting some sort of echo off your unit ,no way was it a thermocline showing up

Scratching my head on this. Sounders have been capable of showing thermoclines for years.

My old Eagle unit did it back in the 90’s. I had to de-tune my 2001 Furuno unit not to pick it up. Obnoxious at times.

My CHIRP unit gives a very clear signature of the thermocline if I don’t tune it out.

However, I do agree that what OP is seeing isn’ the thermocline.

Aluminum is a horrible material to shoot thru because of the resonance of the material. Most sonar manufacturers recommend against it.

Given the signal disappeared in deeper water, pretty sure what your seeing is the secondary reflection of the signal off the hull.

 

[alldodge]

Have scuba in lakes and just a couple times in shallow water (25 feet or less), but most thermoclines are normally around 20 feet or so

 

[mr 88]

Scratching my head on this. Sounders have been capable of showing thermoclines for years.

My old Eagle unit did it back in the 90’s. I had to de-tune my 2001 Furuno unit not to pick it up. Obnoxious at times.

My CHIRP unit gives a very clear signature of the thermocline if I don’t tune it out.

However, I do agree that what OP is seeing isn’ the thermocline.

Aluminum is a horrible material to shoot thru because of the resonance of the material. Most sonar manufacturers recommend against it.

Given the signal disappeared in deeper water, pretty sure what your seeing is the secondary reflection of the signal off the hull.

I stand corrected , they will show a thermocline or difference in water density but will not give you a temperature at any depth but where the transducer is located. Everyone that I know and then ,that seriously fish on Lake Ontario and target Kings use the FishHawk to get to the preferred temps that Kings like to be in No one relies on what shows up on the FF as a thermocline or density change to set there gear up at.

 

[Old Ironmaker]

I have run an old Eagle Ultra in the 90's and Garmin, Lowrance and Humminbirds today. I have found the thermocline on Lakes Ontario and Erie with the newer units only in fall only for some reason. Maybe because I was looking for it to flip then. It's not a thing in my bag of fishing tricks. Is there a reason you want to know where it is? If you are fishing this late I'm thinking I know why but you may have a different reason.

Also I tell guys there is no reason to shoot through the hull when a more accurate signal will come directly through the water on the transom of a boat as long as the prop wash doesn't interfere. Mine are close to the 115 and 15 kicker but the transducer at the bow tells me that all 3 are accurate.

 

[Texasmark]

Scratching my head on this. Sounders have been capable of showing thermoclines for years.

My old Eagle unit did it back in the 90’s. I had to de-tune my 2001 Furuno unit not to pick it up. Obnoxious at times.

My CHIRP unit gives a very clear signature of the thermocline if I don’t tune it out.

However, I do agree that what OP is seeing isn’ the thermocline.

Aluminum is a horrible material to shoot thru because of the resonance of the material. Most sonar manufacturers recommend against it.

Given the signal disappeared in deeper water, pretty sure what your seeing is the secondary reflection of the signal off the hull.

This is my 3rd alum boat with it mounted to the bottom on the inside....the same transducer that is used on a bracket on the transom, or mounted to the bottom of a TM. Have had Hummingbirds before. Never saw a line like this on any of them. Other thing is that it remained at 3 ft. regardless of what scale I was on and was picking up bottoms clearly at varying depths as mentioned above so if it was the second time around or something of the sort, it would have changed with depth change.

Just curious. If I had a thermometer or a ducer that was loose, like if I had it attached to my ™ and took it off and lowered it, I could have come up with more info. just found it odd.....and have been out in this boat at least a dozen times and never was out this cold and never had it before.

No biggie, just an aquatic anomaly and was curious.

 

[Texasmark]

Have scuba in lakes and just a couple times in shallow water (25 feet or less), but most thermoclines are normally around 20 feet or so

Neighbor was over and I was discussing it with him and he mentioned while SCUBA diving he found one at 30'.....said it was mighty cold down there, come up 10 feet and nice and comfy.

 

[Texasmark]

Is there a reason you want to know where it is?

No sir, just curious as to what I might be seeing. I check the 2 bird readouts frequently and they are dead nuts on each other in temp and displays. Course in deeper water I need to turn one off as both run at 200kHz. Not a problem as I use the transom for running and the bow for ™ fishing. Just takes about 2 seconds to turn one off and hold the button for 2 seconds coming back on and it's ready to go. No fiddling.

 

[alldodge]

Neighbor was over and I was discussing it with him and he mentioned while SCUBA diving he found one at 30'.....said it was mighty cold down there, come up 10 feet and nice and comfy.

Agree no way to not notice when one is passed thru, it gets real cold

 

[Old Ironmaker]

Course in deeper water I need to turn one off as both run at 200kHz.

I haven't gotten the bottom of the boat wet this year and only a few times last. So please remind me why it's necessary to turn 1 off if both on same kHz? I don't remember what mine are even running at. An old Lorance and a newer Humminbird combo at the helm.

 

[dingbat]

Never saw a line like this on any of them. Other thing is that it remained at 3 ft. regardless of what scale I was on and was picking up bottoms clearly at varying depths as mentioned above so if it was the second time around or something of the sort, it would have changed with depth change.

The “scale” is a software function. Selects which part of the signal to display on the screen.

It’s not a “second time around signal”. Reflections generate a very different signature from what you describe.

What your describing is some sort of resonance of the metallic hull.

Without the ability to play with settings to rule out scenarios, I surmise one of these is to blame.

1. Your getting a reflection (bounce back) off the inside of the hull.

2. Transducer is initiating a secondary “ring” in the metallic hull causing interference.

3. The aluminum is causing a frequency shift in the signal, although this usually manifests as a loss of sensitivity

Does the return stay the same when switching to 50 kHZ

 

[Old Ironmaker]

I have dived for years. A few years ago just snorkeling, no SCUBA. Often I find a sudden cold section of water, a very cold section. It isstrong current and not the thermocline in my experience as I will find it at various depths not 1 continuous stream as the thermocline would be located in. Also the thermoclines we see here in Lakes Ontario and Erie are much deeper than 20' before it turns over.

I've attended a few Marine electronic seminars up here and it is never recommended to shoot through the hull of an aluminum or steel hull. To me it's amazing how much more accurate info we can get out of less expensive sonar units if we are dialed in correctly. I am not apperently, I don't watch cartoons, I fish. My units do what I need them to do. Accurate depth, tweeked a bit to find bait balls and solid hooks. I know guys that are constantly playing with their units and miss more than a few fish concentrating on the wrong thing, like the screen and not fishing.

 

[Texasmark]

That's why I like the bird. Just turn it on, auto does it all for you, or turn it off. On both running on 200kHz, both receiver's bandwidth are tuned to that frequency and they know not nor care where the signal originated, like any receiving instrument subject to a deliberate "jammer". The way to prevent that is to change frequency on one of them but these are single frequency SONAR, like a single frequency Magnetron in a pulsed RADAR system. The deeper the water the bigger the problem as the cones overlap better. No biggie to just turn one off....like I move from the helm to the bow seat and dump the ™ over the side, no biggie to just turn the units on or off as an additional action.

The resonant frequency of a 17' boat hull has to be in something like the single Hz range. 200 kHz is a decade higher than a good human hear can recognize. What's the resonant frequency of water? Clear or tainted, how tainted, with what? You could set a beaker of water on top of a woofer, hooked to a good amp, and a signal generator and run the dial till you saw the water go nuts. No offense to the seminars, but one has more control over the consumer (ultimate satisfaction with the product) if something like in-hull mounting on who knows what by whom doesn't have to be considered. Transom or ™ mounting takes the vessel out of the picture other than on the transom mount, getting the right angle and height setup. On ™ mounting, as we all know, they make a special bracket for attachment.

If metal didn't retransmit sound better than air or water, why stethoscopes for listening to machinery in trying to locate a problem? Seems to have a pretty wide bandwidth otherwise the usage wouldn't be an advantage.

Nothing to do this morning. Chores done, nothing to fix, news crap is a bore, maybe I'll go out and rake some leaves.

 

[H20Rat]

If metal didn't retransmit sound better than air or water, why stethoscopes for listening to machinery in trying to locate a problem? Seems to have a pretty wide bandwidth otherwise the usage wouldn't be an advantage.

That is exactly the problem, it TRANSMITS sound really well. Any time you have any energy wave, be it sound or light, going from one material to a different one that has different transmission qualities, the original signal is going to be different. Think of diffraction looking into water, the same thing happens to a transducer shooting through metal.

Also, because metal vibrates and transmits sound well, the energy leaving the transducer is also going to make the metal hull ring in other areas. Just because it isn't at the resonance frequency doesn't mean it isn't going to ring somewhat. Take a hammer and tap the side of your hull near the transom while your buddy is holding his ear to the bow. He of course will hear it loud and clear. The transducer is doing the exact same thing. And the quality of the return single is highly reliant on a single pulse of energy from a single source, NOT the entire boat.

 

[Old Ironmaker]

That's why I like the bird. Just turn it on, auto does it all for you, or turn it off.

Exactly what I do. I will play with them a bit depending on the clarity of the water, often Erie is crystal clear often algae floats in it that is enough to give you false readings. Then I go back to factory settings and tweek depending on water conditions. When my Garmin and Bird read almost identical I'm happy. If I turn on the bow mount Lowrance which isn't often and it reads the same I'm really happy.

 

[Texasmark]

The “scale” is a software function. Selects which part of the signal to display on the screen.

It’s not a “second time around signal”. Reflections generate a very different signature from what you describe.

What your describing is some sort of resonance of the metallic hull.

Without the ability to play with settings to rule out scenarios, I surmise one of these is to blame.

1. Your getting a reflection (bounce back) off the inside of the hull.

2. Transducer is initiating a secondary “ring” in the metallic hull causing interference.

3. The aluminum is causing a frequency shift in the signal, although this usually manifests as a loss of sensitivity

Does the return stay the same when switching to 50 kHZ

I don't think my $99 unit has anything but 200k. Gotta spend another 100 bucks for dual frequency. That was going to be my first test......checking at different frequency which would upset all the speculation about what the hull is and isn't doing with the signal....other than making for a great antenna.

The ducers are pushed down in a pool of JB weld kneaded stick till they touch the bottom of the boat. If there is a problem between the piezo device and the hull it would be the epoxy of the ducer surrounding the piezo crystal....and one would expect (I'd assume) the same interference to be observed if immersed in water.

Part of the confusion on what's going on is whether or not water temp dropping from roughly 85F the previous outing without the phenomenon to upper 50's with it.

On the hull and a frequency shift, no doubt as "Lambda" for 200kHz is a lot shorter than 17 ½'.......but that variance was there the last time out without the phantom return.