When I fish in my Alumacraft Yukon, I see a consistent return on the depthfinder (Lowrance 334i with transom-mounted transducer) at 15.5’. It draws a continuous line at that depth, calm water or rough. Turns out, it’s marking my bow-mount trolling motor. Stop the TM and width of the line changes. Lift the TM out of the water and the line goes away. FWIW, the TM has a 54” shaft and it’s nearly all the way down when deployed.

I know a “20 degree” transducer cone radiates sound over a wider angle than 20 degrees, but I didn’t expect to see a return from an object 90 degrees from the transducer face (but, then, I can't find any Lowrance documentation on the 334C transducer cone angle). This is my first boat with a bow-mount TM- I wonder if others see the same thing.

I think you are either seeing the thermocline (98% sure) or possibably some interference.... I'm pretty doubtful it's your trolling motor.

At first, I thought the same thing you did- thermocline or interference. There is absolutely no doubt I'm seeing the trolling motor. I won't bore you with the list of experiments to prove that.

I have seen the same thing with my kicker. The line is at 5 foot which is the distance from my starboard transducer to the port kicker. It disappears when I raise the kicker. I haven't tried to see the TM. I will check it out the next time I am on the water. The transducer cones have side lobes that “see” my kicker. Sonar “sees” the distance of the target from the transducer not the depth of the target. Nice observation.

Bruce:

Thanks for the reply. Since you know a thing or two about Lowrance electronics...

As I understand the physics of transducers, the "cone angle" gets larger as the transducer piezoelectric element gets smaller. Lowrance Skimmer transducers for my older units are clearly labeled "20 degree" at LEI-Extras. I have been unable to find a spec for the cone angle for the transducer that came with my 334C. The best I could find was a vague statement in the owners manual for the 334C and 339C that the transducers have cone angles up to 60 degrees. So, I wonder if Lowrance used a smaller element in those transducers, leading to a much wider cone.

The cone angle of the skimmer 200 kHz transducer is 20 degrees. The cone angle of the 200 in the 200/50 kHz skimmer transducer is 12 degrees. They measure it as the edge having ½ the power as the center so there is sound past the 20 degrees. This is why Lowrance says the 20 degree transducer has coverage up to 60 degrees depending on the conditions. The side lobes are different and “see” to the side so using the cone angle to define why you see the TM is not pertinent. The Vexilar site has a nice diagram of the side lobes. http://vexilar.com/pages/support/support_tips/article_006.html

Bruce:

Thanks- nice explanation.

If it is any use to you in your Lowrance classes, I can capture a sonar log showing the TM marks. Let me know.

If I can have a bit more of your attention, I have another observation from something I tried this morning.

My boat has an Eagle 320 in the bow and a Lowrance 334C on the console. They do not respond the same when I adjust the sonar sensitivity.
- When I increase the Eagle's sensitivity, the new elements drawn on the screen show stronger returns, more noise, more surface clutter. What's already been drawn on the screen does not change.
- When I increase the Lowrance sensitivity, the entire display shows stronger returns, more noise and more surface clutter.

It's as if the Eagle is adjusting the strength of the sonar pulse or the amplifier gain, while the Lowrance is only adjusting the screen display. Have you observed this?

I don’t use Eagle but I suspect that the Eagle is just different. There are gremlins working on the inside and I don’t know all what they do. It sounds like when you make settings changes the Eagle display is only showing changes since you changed the settings and the Lowrance is applying the changes to the history.
I am going to try and capture what you did with my equipment as I have to test everything myself.

Question for Bruce, the Lowrance 200 khz 20 degree ducer views about 1/3 of the depth, so in 18 FOW I am looking at 6' of bottom. What about the 200/50 ducer, what kind of area are they looking at?

Thanks

At first, I thought the same thing you did- thermocline or interference. There is absolutely no doubt I'm seeing the trolling motor. I won't bore you with the list of experiments to prove that.

Does this unit maybe have a depth line feature that is turned on?

icontact:

The depth line feature is not what is causing the signal I see. The sonar is marking the bow-mount trolling motor. I was surprised when I figured out where that signal was coming from, but the evidence was clear.

The area under the boat is not exact but an approximate since there is sound outside the cone. I was told the 12 degree cone is a liitle more than 1/2 the 20 degree cone. How is that for not being exact? Actually close counts as most people just want a general idea and with sonar it is general not exact.

My old boat with a 20 degree eagle did the same thing. Line was much more evident with the motor running, turbulence from the prop made for a larger return. On my new boat, my bird has a much wider angle, and a deeper trolling motor, but I don't get that return on this setup. He's not crazy, it's definitely the trolling motor. Annoying, but true.

I have a 20 foot Crestliner 202TS and I put in my 72 inch shaft Minn Kota maxxun (custom made) adn my transum transducer does not pick it up and does show a line for the kicker which is 4 feet from the transducer. That is my test results.

When I fish in my Alumacraft Yukon, I see a consistent return on the depthfinder (Lowrance 334i with transom-mounted transducer) at 15.5’. It draws a continuous line at that depth, calm water or rough. Turns out, it’s marking my bow-mount trolling motor. Stop the TM and width of the line changes. Lift the TM out of the water and the line goes away. FWIW, the TM has a 54” shaft and it’s nearly all the way down when deployed.

I know a “20 degree” transducer cone radiates sound over a wider angle than 20 degrees, but I didn’t expect to see a return from an object 90 degrees from the transducer face (but, then, I can't find any Lowrance documentation on the 334C transducer cone angle). This is my first boat with a bow-mount TM- I wonder if others see the same thing.

I had this same issue with a 332c. The only difference is it didn't make the line for a year, then all of a suddent the line appeared. Same thing, I could change the width with the trolling motor, everything. Only difference is my line was at 18 foot.

Replacing the transducer fixed my problem. I've had more tranducer issues with these newer small color Lowrances than I had the entire life of my old LMS-350a.

Wheels:

I think seeing the TM means our transducers have a undesireable large transmission side-lobe.

I'm leery of replacing my current transducer with another Lowrance-brand, due to many reports of problems with their transducers since moving production to Mexico. Seems one could easily get 2-3 bad units in a row and at the current price, that's not an experiment I want to do.

I'm looking at buying either Vexilar or Airmar transducers to replace my Lowrance transducers. As I understand, Airmar used to make Lowrance's transducers. Both companies have an excellent reputation for their products, which cost 50-100% more than Lowrance transducers, but I'd pay that for a reliable, well-performing product.

I will look into these options and report back on what I learn.