TRANSDUCER FREQUENCY QUESTIONS

[johnny striper]

Hi all...I run & fish in moderately turbid, shallow water with an average depth of 8' - 16'. Sometimes as shallow as 2' sometimes as deep as 50'.

My FF is a H-Bird 581i HD DI. I really get lost in all the numbers and was hoping that maybe one of the gurus out there could help.

Specs for my transom mounted transducer: DI is 455 kHz / 75* cone or 800 kHz / 45* cone. Traditional is 200 kHz / 25* cone or 455 kHz / 16* cone. I'm confused!!

What settings are best for:
1 Cruising and WOT on the flats (2' - 4') where depth accuracy is most critical?
2 Cruising and/or drifting in average depth looking for fish?
3 Cruising and/or drifting in deeper water looking for fish.
4 Watching your jig move up & down thru the water column.

I am familiar with the term RTFM...which I have done a few times...I also know that one must experiment...

...which I have also been doing.

Thanks for any & all replies.....Mike

 

[gm280]

WOW, a lot of interesting questions there. I can't answer all of them, but running at WOT in 2' to 4' of water is really scary. And here is why. If you are relying on any depth locator to see a possible problem in such shallow water, by the time it showed on the locater screen, your screwed. You literally can't react fast enough to keep from running over or into any problem encountered. No such depth locator can save you from that scenario. It is like over driving your headlights at night. You have to remember how a depth locator actually works. Your transducer sends a ping sound out, then it hits objects or the bottom and bounces back to that same transducer that is now waiting for the return ping. Then the head unit calculates the depth of the object and/or the bottom and displayed it on the screen. Now think about that running WOT in 2 to 4 foot of water and you can easily see how you will see that results AFTER you have passed the problem... As far as all the other questions about cone size and frequency, those issues are subjective to what YOU want to do or see. The wider the cone, the less specific you will be able to discern the results. But the narrower the cone the better individual targets will be but anything outside of that narrower cone will be missed. And remember that you are trying to visualize a three dimensional cone view in to a two dimensional screen representation. So everything takes time and experience to achieve the best capability from any chosen locator. I suggest you read the manufacturer's info on the selected locators you're looking at for the best answers to your questions and needs. After all they will know more about their products then anybody else in the world... Of course this is just my opinion and others will chime in here with have their opinions as well...

 

[johnny striper]

Thanks gm280...I appreciate your input. Yes, running in 2' - 4' can get a little hairy. Fortunately I know the soft bottomed area "fairly" well. My unit seems to be quite accurate and usually reads down to like 1.8' when cruising and like .8 while drifting. But, sometimes (not always) at WOT while on the flats the reading may change from 3' to like 65' when I know I'm in 3'. I thought that maybe by changing the freq would help to eliminate the issue....My shallow water alarm setting is at 3'

 

[gm280]

johnny striper, it is okay when you're running WOT and the depth goes deeper, but if, for chance, there would be something submerged just under that water line and you're running along, you will hit that object before your depth alarm ever goes off... That's all I'm talking about in shallow waters. As for the issue of the depth changing supposedly erratic, that could be the position of where you have installed the transducer. And slight change in the boat tilt could cause erroneous readings. So maybe look at that location and see if it could be changed... Just another idea.

 

[dingbat]

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[johnny striper]

Thank you gentlemen.... Changing the angle of the transducer, slightly, does make sense. The airmar info is great but, unfortunately, it's the kinda stuff I get lost in. My "old man" brain just doesn't process info like it used to. Give me some nails and I'll build a house... Tell me to build a house and I'll forget the nails

 

[UncleWillie]

I like the car analogy.. but it is more like having your headlight on the rear bumper and pointing down at the pavement.
You will get a great view of whatever it was that you just ran over.

Keep in mind that with down imaging the cone may be 75? wide left to right but it is less than 2? wide front to back.
It is similar to side imaging only without the 180? field of view.

At 4 feet the 75? DI cone will be 15 feet wide on the bottom.
But there is no information to discriminate if the fish are 3 feet down or 3 feet left or right. Just that they are 3 feet from the transducer.

In order to see your jig, it will need to be within the sonar cone. Traditional sonar will be the preferred mode.
But you will want to drop your line right next to the transducer in water less than 20-30 feet.
A 16? cone is only 7 feet in diameter 20 feet down.
If you drop the line more than 3 feet from the transducer it will not be in the sonar beam until you drop it more than 20 feet.

The transducer mounting location is also critical.
If mounted on the transom, it needs to be mounted Below the bottom of the hull.
If mounted above the hull bottom, the air bubbles in the turbulent water behind the transom, will give you nothing but grief.

Sonar is a useful tool, but you need to be able to work within its limitations.

 

[johnny striper]

Thanks for the info, UncleWillie... good stuff. (funny, I got a bud with that nickname)...My ducer is mounted correctly and reads correctly 99% of the time. It's only when running WOT in the shallows do I get the occasional erratic readings...possibly just the nature of the beast. I don't choose to run in 2' - 3' sometimes it's just what ya gotta do to get from A to B. I generally fish the uppermost areas of the Chessie Bay.


PS...I use my GPS to show the way to carry the sleigh and do have a few courses plotted across those flats. I know, something "new" can be there today that wasn't there yesterday...but in 2' - 3' it would be rather small and probably visible when scanning with binos.

 

[gm280]

Great post there UncleWillie, Exactly what I was trying badly to convey... Much clearer description though.

Some times you can get too technical and have no idea what the understanding could be to the question poster. So I try to go middle of the road with explanations and even that gets pretty technical. Hard to convey what is true going on without turning into a total math exhibition...

 

[Texasmark]

at WOT while on the flats the reading may change from 3' to like 65' when I know I'm in 3'. '

I have this kind of response when I raise my trolling motor out of the water which has the forward system transducer mounted to the bottom of the TM housing. It's caused by loss of signal and apparently over the flats at high speed you are getting some "second time around" signal information (very strong returns...the previous return bounced off the boat, hit the bottom and came back again along with the current return) which is confusing your computer......this happens in pulsed radar frequently when you have lots of power output, sensitive receiver front end, and a target giving a good return too close to you.

If you have a good solid hull at the transom, be it glass or alum, you might move your ducer inside. If your hull is not solid glass, like some boats used to have a blasa wood filler between the outside glass and inside glass it won't work satisfactorily. On my glass or alum boats this is how I mounted my main ducer (not the TM ducers when I had 2 systems) and it worked fine to 50 mph....fast as I could go. Lot less cavitation interference (not prop cavitation).

Since bubbles are small, a higher frequency can give you a better return (meaning better bubble return which you don't want) due to the shorter "wavelength". Not familiar with your system, but if you could, run with the lowest transmitted frequency when running at WOT. Lower frequencies would work better at the deeper depths for the same reason. If you want to see your fishing lure, then use your highest frequency with the shortest wavelength for the same reason.

HTH,
Mark

 

[johnny striper]

Thanks Mark...More really good info. Been fishing & boating since the 60s. Been fishing the flats since '08 in a brand new deep v tin boat. without 1 scratch on the hull to date. My painted aluminum prop does however have some "shiny spots". The problem is that the high tec FFs are new to me and I'm confused with the settings. I'm truly not worried about running aground, if I do, I do....just want to use my electronics to the best of their abilities...maybe this is as good as it gets....I just don't know.
Thanks again for the great input.

Mike

 

[dingbat]

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[johnny striper]

Thanks dingbat...that's kinda what I was thinkin. Just wanted some advice from those more experienced, than I, with electronics. I've had no issues running WOT in deeper water (50' is deep to me). I appreciate your input. T.Y.

Tight lines....Mike

 

[Texasmark]

For a pulsed system the "ping rate" is the rate at which the pulsed transmissions are transmitted.....aka, 1 per second, 1 per milli second, 1 per microsecond, etc.

Has nothing to do with the target detection per se, just the intensity of the return.....aka if you have a detectible target that you pulse once per whatever you will have a certain intensity on your screen. If it pulses 10x that amount, depending on the luminance of your viewer it may be 10x brighter and so on.

However, frequency still plays a part in the detectability of the target and is as I said. So yes it is a combination of both. Which is more important? Depends on the system parameters.

HTH,
Mark

 

[Texasmark]

Thanks Mark...My painted aluminum prop does however have some "shiny spots". T

Mike

SS props have their pluses and minuses. You have to determine which type suits your situation best. SS will tolerate the dings and sand and shale and the kind of things you encounter in tidal waters. SS will saw through a submerged, rotted, tree stump. However, if you hit a 3 cubic foot concrete block which is mooring docks at a marina, it may or may not survive and if it does the gear train may or may not survive. Your decision. I boated since the mid 1950's. Got my first SS prop for my1972 125 Johnson when the OMC SST came out in 1973. Have never lost any engine parts as a result and have never had to take one to the shop to get fixed. However, if you boat around large boulders, you need to rethink your objectives. They are non-forgiving.

Mark

 

[johnny striper]

No doubt SS props are superior and give better performance...I'm just too cheap to buy one...lol. I have a newer motor - 2013 Yamaha F70 on a 16 1/2' G3 deep V. Came with a huge alum prop. Dealer "threw in" a spare, just in case...I'm sure I paid for it somewhere along the way. 90% of my fishing is on the upper Chesapeake for shallow water stripers in non-rocky areas. Nothing like hookin into a 40 pounder in 3' of water Only ever had very minor nicks in the prop, which so far, I've been able to sand out by hand. If repainted it would look brand new. If I ever need a new prop I'd replace it with SS and keep the other alum prop as a spare.

Thanks for the input :fish2:

Mike

 

[dingbat]

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[Texasmark]

Huh.....the ping rate has nothing to do with signal strength. The ping rate is the units "sampling rate". The ping rate affects a units performance in shallow water (increased sampling rate minimizes signal saturation), high speed operation (out running the signal) and target definition/detection (less time between "snap shoots", fewer missed targets)

Everyone has seen a fish finder searching for the bottom after losing it. When it does, it sometimes reports absurdly deep or shallow numbers for a few seconds. That is because it is searching through the ranges for one that will give it a return with a logical signal strength relative to its time of flight.

"The ping rate has nothing to do with signal strength." Agreed but does have an effect on the display intensity which could make a weak target more detectable in a luminance display system. On a digital system don't know; never worked on/studied them.

"(increased sampling rate minimizes signal saturation)" My training/experience is just the opposite. The more times you hit a target with a "given power level signal" in a given time, the more signals the receiver has to process (higher average signal level) and for a given received signal intensity the higher the level of Automatic Gain Control which is the mechanism that prevents saturation in AM, FM, and pulsed applications. Don't know about digital receivers. Never trained nor worked on them.

On outrunning the signal, sound travels a whole lot faster than his boat and he's not about to outrun it, even in water.

I have seen the searching you are referring to when changing depths and the unit is adjusting to the new depth. Course I'm not up on the electronics required to monitor and display information on current digital display depth finders where the range automatically changes and all the new technologies involved in equipment available today. My background is more of the old school RADAR with Planned Position Indicators and such. Same technology applies to piezoelectric crystals that get the signal out and back. What the instrumentation does with it is something else.

My 2c,
Mark

 

[UncleWillie]

Pinging it fast would increase the signal level only if some part of energy from the previous ping were still present.
The ping rate slows down as the depth increases to allow the previous ping to get back to the boat before the next one is sent.
You can ping faster in shallower water because the round trip time is so short.
Pinging faster also makes the display scroll faster. Each ping add a new line to the right side of the display.
Multiple pings might be averaged together but the principle remains the same.

The round trip time to a depth of 100 feet is just under 50ms (1/20 of a second).
Practical constraints allow pinging no fast than 5-10 time a second at this depth.

 

[johnny striper]

WOW...quite a conversation...from which I've learned a lot more than anticipated. Again, thank you gentlemen for all your help. It's been a pleasure.

Tight Lines....Mike